70mm F6 doublet airy disk and rings question

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mikemarotta
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Re: 70mm F6 doublet airy disk and rings question

#21

Post by mikemarotta »


RealFlow100, I have enjoyed reading your posts as you have progressed here over the past year. I know a little bit about your environment having lived in Charleston for two years. I point out that even though you live in a nominally small town, far from the bright lights of the big city, every neighborhood is different and all problems are local.
realflow100 wrote: Thu Oct 21, 2021 12:59 am ... stable jupiter was a clear disk with 2 easily identifiable bands. and looked very nice. fleeting moments I saw maybe 1 or 2 additional bands.
Saturn had very clear ring separation from the planet. and maybe a faint band just above the rings of the planet too.
and I'm almost certain I saw cassini division for a few brief split-second moments in moments of exceptional atmosphere seeing.
Does this sound like typical results for a 70mm achromat doublet telescope? (non-ED)
Yes. Unless you are like at 5000 feet on a desert mountaintop or deep in federal lands in Wyoming or some other godforsaken place, your seeing will vary from moment to moment and you have to let your eyes and brain sort out the shifting images. You seem to have done that. I also have a 70-mm refractor and I live in a big city. So, I can relate to your observations. I tip my hat: congratulations on your observing!
:observer:

I took a suggestion from Ronald Stoyan’s “The Visual Astronomer” webpages (here: http://visualastronomer.com/ -- but not https secure and not updated recently) who says that magnification is never wasted and that you need to let your eyes continue to look while the shifting atmosphere is opening and closing for better and worse seeing.
John Baars wrote: Thu Oct 21, 2021 8:17 am
realflow100 wrote: Thu Oct 21, 2021 12:59 am (...)
Does this sound like typical results for a 70mm achromat doublet telescope? (non-ED)
Sorry to dissapoint you. To me It doesn't sound fully like a 70mm achromat.
I know that in many reviews the double double is put forward as a kind of accuracy test. However, they are not, especially for larger instruments. With 2.3 and 2.4 arcsec separation, they are at most a good test for a 50mm instrument. With a good 70mm instrument they are already separable well below 100X. At 70X or 80X you already get the impressions as you describe above.
Sorry, John, but I have to disagree. I have tried to split the double-double many times with my 70-mm and never did it because it takes good sky seeing. I know that others do this with 60mm or smaller, but "seeing" is everything and neighborhoods are local. For me, it takes the 102-mm F=660 mm for f/6.47 and it takes 4mm (8mm with 2x Barlow) for maxing out at 165X which is the limit for my instrument.

Also, you need to consider his eyes versus yours versus mine. A person can function quite well with nominally "poor" vision because we adapt to our environments. I discovered that I needed glasses for distance only when my employer sent me to a strange city for a week and I could not read the street signs in the dark while driving. In my home town, the fuzzy clues were enough, given that I usually knew pretty well where I was anyway.
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Astro-Tech 115 mm APO Refractor Explore Scientific 102 mm f/6.47 Refractor Explore Scientific 102 mm f/9.8 Refractor Bresser 8-inch Newtonian Reflector Plössls from 40 to 6 mm Nagler Series-1 7mm. nonMeade 14 mm. Mounts: Celestron AVX, Explore Twilight I Alt-Az, Explore EXOS German Equatorial
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Re: 70mm F6 doublet airy disk and rings question

#22

Post by John Baars »


@mikemarotta
Hi Mike,
Realflow asked a question about the performance of a typical 70mm achromat and my answer is "no". I can't ignore optical performance with respect to diffraction rings, whatever personal/environmental conditions one wants to consider around them. I live in one of the most light polluted area's in Europe, observe from my backyard under a street lantern, so I am quite used to those negative circumstances. Sharper eyes do not make fewer rings, bad seeing only makes them fall apart, good seeing does not make them disappear. Optical laws don't care about light pollution.

I am not discounting observations made with this or similar instruments and am purely giving an optically correct answer based on observations. I am separating the appreciation for the observations from the question on the optics.
Environmental and personal circumstances may count for the appreciation of a report, I do not detract from that, but not for the optical properties of an instrument.A good 70 mm should separate Epsilon Lyrea with ease since it is only at 75% of its resolving power.
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
Amateur astronomer since 1970.
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Re: 70mm F6 doublet airy disk and rings question

#23

Post by notFritzArgelander »


John Baars wrote: Wed Oct 20, 2021 8:52 am
realflow100 wrote: Sun Oct 17, 2021 10:50 pm Whats a typical amount of rings and size of airy disk you would see?
Does star brightness matter for how many rings you see?
Or should it always be a max of 1-2 rings or something?

When I test my telescope dim stars have 1 diffraction ring.
moderately dim stars have 2 rings and a faint third ring.
and brighter and very bright stars have many rings. usually quite rainbowy rings too. looks colorful.
I dont see any evidence of coma or pinched optics.
Seeing isnt affecting my result too much
Using 140x magnification

if I go inside and out of focus just a tiny bit enough to notice a difference
one side looks almost the same as in-focus
and the other side the diffraction rings immediately disappear and a sharp bokeh ball/disk forms
if I defocus a bit further. the diffraction rings on one side of focus fade away and a faint fuzzy bokeh ball/disk begins to gradually appear
and on the other side of focus the sharp bokeh ball just gets larger
if I defocus quite a bit more (so the out of focus disk is at least 10 times the diameter of the in-focus star
in-focus shows a soft bokeh ball/disk with EXTREMELY faint rings inside it
and out-focus shows a sharp crisp bokeh ball/disk with moderately faint rings inside it
Good optics show:
-no rings on dim stars.
-no rings on moderately dim stars, maybe one very faint and thin one depending on the brightness.
-one faint and thin ring on bright stars and the faintest of all as a second ring if seeing permits.

More rings mean spherical aberration. There is no way of encountering top-optics in a departement-store lens. Individual testing, correcting, retesting and recorrecting takes a lot of time of the optician working on it. His wages count. In a factory which produces the cheapest telescopes, usually only one of lenses from an entire batch is tested. If correction is needed, it is applied to the whole batch at once.

The startest you describe sounds like spherical aberration 0.5 wave or bigger. There is no way of correcting this by distance between lenses, only refiguring.

Resolving point sources is different from linear resolvement on objects with a certain surface area; this is expressed in lines/mm, and always outruns resolving pointsources in the way nFA already said. ( this is why we see the Cassini division and cloud belts on Jupiter) If you don't have comparison material in the form of top optics, the daytime image in a telescope will quickly appear sharp. An observation from which you would quickly reconsider if given the same conditions you were looking through top optics at high magnification.

What I know from previous threads of yours about this telescope, you've already taken it to the limit. It couldn't be better, given the circumstances. Time to just start enjoying it?
I was going easy on the claimed error of 0.3 wave or better. Your guess at 0.5 is likely more realistic. I think revisiting Aberrator with a more critical eye would be interesting. First use estimates of aberrations are often biased. Practice, if done critically, makes better, never perfect.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: 70mm F6 doublet airy disk and rings question

#24

Post by notFritzArgelander »


John Baars wrote: Thu Oct 21, 2021 10:12 am @mikemarotta
Hi Mike,
Realflow asked a question about the performance of a typical 70mm achromat and my answer is "no". I can't ignore optical performance with respect to diffraction rings, whatever personal/environmental conditions one wants to consider around them. I live in one of the most light polluted area's in Europe, observe from my backyard under a street lantern, so I am quite used to those negative circumstances. Sharper eyes do not make fewer rings, bad seeing only makes them fall apart, good seeing does not make them disappear. Optical laws don't care about light pollution.

I am not discounting observations made with this or similar instruments and am purely giving an optically correct answer based on observations. I am separating the appreciation for the observations from the question on the optics.
Environmental and personal circumstances may count for the appreciation of a report, I do not detract from that, but not for the optical properties of an instrument.A good 70 mm should separate Epsilon Lyrea with ease since it is only at 75% of its resolving power.
Agreed on the split. I've split epsilon Lyrae with smaller scopes in a light polluted city. A well made finder or guiding scope at 40-50 mm can do it. Good seeing is required but good seeing (absence of scintillation is required, not absence of light pollution) can happen in cities as well.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: 70mm F6 doublet airy disk and rings question

#25

Post by mikemarotta »


So, is it your considered opinion that the Svbony 501 70 mm f/6 refractor doublet telescope is not a good instrument?

https://www.svbony.com/sv501/

Are you warning people here that it is a "hobby killer" like a Bird-Jones reflector?
Are you declaring that this is not a well-made instrument?
John Baars wrote: Thu Oct 21, 2021 10:12 am A good 70 mm should separate Epsilon Lyrea with ease since it is only at 75% of its resolving power.
notFritzArgelander wrote: Thu Oct 21, 2021 1:03 pm Agreed on the split. I've split epsilon Lyrae with smaller scopes in a light polluted city. A well made finder or guiding scope at 40-50 mm can do it.
I only know my own experience. Finding and resolving the double-double epsilon Lyrae took some work. I tried the 70-mm and failed. I used the 102-mm at high power and succeeded. Then, I set up both at once and interchanged the oculars. I can only report my own results. You report yours. That is as far as it can go.

I do note, also, the subtle dodge about the quality of "seeing."
notFritzArgelander wrote: Thu Oct 21, 2021 1:03 pm Good seeing is required but good seeing (absence of scintillation is required, not absence of light pollution) can happen in cities as well.
I agree that a telescope can cut through some light pollution. I have done it night after night for years. However, I note here and now, also, that you are tossing in (and tossing out) "scintillation." In other words, atmospheric distortion from heating and cooling, both in layers and in large bubbles. I know from flying that at noon the plane rocks as large bubbles of hot air rise off the ground. At night, as the Earth cools that diminishes. In the morning, it is minimal. In the city, the waviness that affects your viewing can come from a source some kilometers away. It goes on and goes off without your cognizance.
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Astro-Tech 115 mm APO Refractor Explore Scientific 102 mm f/6.47 Refractor Explore Scientific 102 mm f/9.8 Refractor Bresser 8-inch Newtonian Reflector Plössls from 40 to 6 mm Nagler Series-1 7mm. nonMeade 14 mm. Mounts: Celestron AVX, Explore Twilight I Alt-Az, Explore EXOS German Equatorial
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Re: 70mm F6 doublet airy disk and rings question

#26

Post by notFritzArgelander »


mikemarotta wrote: Thu Oct 21, 2021 3:03 pm
.....

John Baars wrote: Thu Oct 21, 2021 10:12 am A good 70 mm should separate Epsilon Lyrea with ease since it is only at 75% of its resolving power.
notFritzArgelander wrote: Thu Oct 21, 2021 1:03 pm Agreed on the split. I've split epsilon Lyrae with smaller scopes in a light polluted city. A well made finder or guiding scope at 40-50 mm can do it.
I only know my own experience. Finding and resolving the double-double epsilon Lyrae took some work. I tried the 70-mm and failed. I used the 102-mm at high power and succeeded. Then, I set up both at once and interchanged the oculars. I can only report my own results. You report yours. That is as far as it can go.
No it isn't. Optics is a science. Any instrument of 70 mm that cannot split epsilon Lyrae is not up to standard which is 0.25 wave rms wavefront error.
I do note, also, the subtle dodge about the quality of "seeing."
notFritzArgelander wrote: Thu Oct 21, 2021 1:03 pm Good seeing is required but good seeing (absence of scintillation is required, not absence of light pollution) can happen in cities as well.
I agree that a telescope can cut through some light pollution. I have done it night after night for years. However, I note here and now, also, that you are tossing in (and tossing out) "scintillation." In other words, atmospheric distortion from heating and cooling, both in layers and in large bubbles. I know from flying that at noon the plane rocks as large bubbles of hot air rise off the ground. At night, as the Earth cools that diminishes. In the morning, it is minimal. In the city, the waviness that affects your viewing can come from a source some kilometers away. It goes on and goes off without your cognizance.
There is no dodge about seeing at all. Scintillation = seeing. There can be good seeing in a city. I've had wonderful multiple band Jupiter and great double star splitting in large urban centers. Light pollution is not the same as seeing = scintillation. You are trying to make a rhetorical case without the right science. If you can't even recognize that "seeing" is physically synonymous with "scintillation".....
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: 70mm F6 doublet airy disk and rings question

#27

Post by Bigzmey »


Mass produced refractors went a long way. Even cheapest are certainly usable and can't be applied to observing. I don't believe nFA and John are saying that Svbony 501 is a bad scope. From what I see in the discussion OP is getting a good return on his investment. Still it would be silly to expect that a couple of simple tweaks would bring it close to the performance of hand-configured premium optics.
Scopes: Stellarvue: SV102ED; Celestron: 9.25" EdgeHD, 8" SCT, 150ST, Onyx 80ED; iOptron: Hankmeister 6" Mak; SW: 7" Mak; Meade: 80ST.
Mounts: SW: SkyTee2, AzGTi; iOptron: AZMP; ES: Twilight I; Bresser: EXOS2; UA: MicroStar.
Binos: APM: 100-90 APO; Canon: IS 15x50; Orion: Binoviewer, LG II 15x70, WV 10x50, Nikon: AE 16x50, 10x50, 8x40.
EPs: Pentax: XWs & XFs; TeleVue: Delites, Panoptic & Plossls; ES: 68, 62; Vixen: SLVs; Baader: BCOs, Aspherics, Mark IV.
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Re: 70mm F6 doublet airy disk and rings question

#28

Post by mikemarotta »


Bigzmey wrote: Thu Oct 21, 2021 8:40 pm I don't believe nFA and John are saying that Svbony 501 is a bad scope.

There is no reply on that point.

So, is it your considered opinion that the Svbony 501 70 mm f/6 refractor doublet telescope is not a good instrument?
https://www.svbony.com/sv501/
Are you warning people here that it is a "hobby killer" like a Bird-Jones reflector?
Are you declaring that this is not a well-made instrument?

Again and again: "... any good 70 mm..." So, is the Svbony 501 a "hobby killer" like a Bird-Jones?


Myself, I deny the label. The best telescope is the one that gets used. I said that about the Bird-Jones. I have a new post on my blog about my National Geographic 70 mm with which I have not split the double-double epsilon Lyrae, but with which I have sketched Mars, photographed the Moon and much more. It is based on my post on this board about my homebuilt Vixen mount.
(https://necessaryfacts.blogspot.com/202 ... mount.html)

It is true that you get what you pay for. That is how the market works and amateur astronomy is an open and free market. So, yes, a $129 beginner scope will likely be noticeably better than a $79 beginner scope. But if all you can afford is the latter, then what counts is getting out an using it. Personally, based on my experiences in numismatics, I recommend saving your money over time to buy the best instrument you can afford when you can afford it. However, it is also true that if you really save over time, you can afford not just a $300 telescope but a $3000 telescope. In the meantime, of course, you go without any telescope and do no observing. .

It is up to the individual to make their own economic decisions. And once made, we can only be supportive of their involvement in the hobby. Too many here and on other discussion boards sniff with derision at anyone who does not invest the same money that they did.
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Astro-Tech 115 mm APO Refractor Explore Scientific 102 mm f/6.47 Refractor Explore Scientific 102 mm f/9.8 Refractor Bresser 8-inch Newtonian Reflector Plössls from 40 to 6 mm Nagler Series-1 7mm. nonMeade 14 mm. Mounts: Celestron AVX, Explore Twilight I Alt-Az, Explore EXOS German Equatorial
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Re: 70mm F6 doublet airy disk and rings question

#29

Post by mikemarotta »


realflow100 wrote: Thu Oct 21, 2021 12:59 am The image is flipped left-right since im using a refractor. so they are the other way around compared to stellarium.
One pair was more vertical and the other pair was more horizontal.
and I'm pretty sure one pair was very slightly closer together
The left pair was roughly horizontal orientation. and the right pair was roughly vertical. though both are slightly tilted. and it took a minute or so to really see them.
Again, to reply on point: Congratulations!

If you can do that, there's a lot more out there, depending on the view in your immediate area, which you have written about at length in the past.
If you can see the Zodiac, then there is the double star Algedi alpha capricorni and the triple Dabih beta capricorni. On the other side of the sky is the double eta cassiopeiai. Try those out.

Since you do well with Jupiter, you might make a table of your views of the Four Moons over time. It is possible to record truly remarkable changes in position just over the course of a few hours. Come back and look again if you can.
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Astro-Tech 115 mm APO Refractor Explore Scientific 102 mm f/6.47 Refractor Explore Scientific 102 mm f/9.8 Refractor Bresser 8-inch Newtonian Reflector Plössls from 40 to 6 mm Nagler Series-1 7mm. nonMeade 14 mm. Mounts: Celestron AVX, Explore Twilight I Alt-Az, Explore EXOS German Equatorial
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Re: 70mm F6 doublet airy disk and rings question

#30

Post by realflow100 »


It was 30$ on amazon when I ordered it. I got it without the tripod since I already have one. so it was like half the price.
Naked eye with my glasses I can barely see magnitude 4.8 stars on a good night with no moon.
magnitude 5 stars are out of my reach naked eye.
ive very faintly seen andromeda naked eye in averted vision 1 or 2 times. but it took over 20 minutes of looking in that area back and forth over and over before I finally confirmed it. and I had to look quite far away from it before it was even detectable for brief moments
Never seen orion nebula once though naked eye. not even a tiny hint of it.
its very hard to see orion nebula even in my telescope with a UHC filter. just a faint fuzzy featurless glowy fuzz with some stars in and around it. usually i can't even detect the wings in the nebula. and the dust piller in the core is also very hard to notice.
I can only detect those things in averted vision for brief fleeting moments over a long observing session.
binoculars show a lot of stars in the orions belt area. but orion nebula is not visible. its too faint at 7x50. I only see some stars. however andromeda galaxy is detectable with my 7x50 binoculars for some reason.
I need at least 40x with my telescope before it even begins to show the wings or dust piller.

I also tried splitting polaris but wasn't able to see its companion star visually. Too dim I think. could be light pollution. it was detected in a photograph though.
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Re: 70mm F6 doublet airy disk and rings question

#31

Post by John Baars »


@mikemarotta
51 years ago a fifteen year old boy started out with a carton tube, a simple spectacle glass in front and a simple magnifier at the other end. He saw the rings of Saturn with it and Mars, the way Huygens did. It was the only one he had and used it. This Lipperhey-like f/50 telescope didn't meet by far
modern standard optical terms. He knew that there were better ones out there. The boy was me and I am still an amateur, so the scope surely was not a hobby-killer. The real hobby killers are hidden between the human ears and are called false expectations , disappointment, demotivation, spoilage, lack of imagination, lack of......you name it.

Have you noticed that you are the first one who has dropped the label "hobby killer scope" into this discussion? And keep on trying to elicit us to say the same? And we don't do that? So, hobby killler? Your words , label and exemplar horse, not ours.
we can only be supportive of their involvement in the hobby. Too many here (...) sniff with derision at anyone who does not invest the same money that they did.
I heartily agree on the first part, but I disagree with you on the second part. I consider it an accusation. That is not the case as far as TSS is concerned. I have seen Fora in my time, and this forum is quite a relief compared to others. No bragging about own equipment or derision about others. You'd be surprised at how close our opinions are, if you really looked into it. Instead, in my opinion you are exaggerating, going too far and taking hostage this topic that simply started with the questions of topicstarter if what he saw was typical for a 70mm f/6 refractor. He got an answer and moreover encouragements to continue.

What else is there to say, fulminating here is barking up the wrong tree.
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
Amateur astronomer since 1970.
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Re: 70mm F6 doublet airy disk and rings question

#32

Post by Bigzmey »


John Baars wrote: Fri Oct 22, 2021 9:00 am 51 years ago a fifteen year old boy started out with a carton tube, a simple spectacle glass in front and a simple magnifier at the other end. He saw the rings of Saturn with it and Mars, the way Huygens did. It was the only one he had and used it.
This made my day John! :) I made my first refractor the same way and at about the same age. There was an eyeglasses lens in front and microscope EP at the back. The only thing I have managed to find with it was Venus, but I did resolve the phase and was ecstatic about it. The rest of my observing as a kid was done with 7x50 monocular and boy did I push it to the limits! :lol:
Scopes: Stellarvue: SV102ED; Celestron: 9.25" EdgeHD, 8" SCT, 150ST, Onyx 80ED; iOptron: Hankmeister 6" Mak; SW: 7" Mak; Meade: 80ST.
Mounts: SW: SkyTee2, AzGTi; iOptron: AZMP; ES: Twilight I; Bresser: EXOS2; UA: MicroStar.
Binos: APM: 100-90 APO; Canon: IS 15x50; Orion: Binoviewer, LG II 15x70, WV 10x50, Nikon: AE 16x50, 10x50, 8x40.
EPs: Pentax: XWs & XFs; TeleVue: Delites, Panoptic & Plossls; ES: 68, 62; Vixen: SLVs; Baader: BCOs, Aspherics, Mark IV.
Diagonals: Baader: BBHS mirror, Zeiss Spec T2 prism, Clicklock dielectric; TeleVue: Evebrite dielectric; AltairAstro: 2" prism.
Filters: Lumicon: DeepSky, UHC, OIII, H-beta; Baader: Moon & SkyGlow, Contrast Booster, UHC-S, 6-color set; Astronomik: UHC.

Observing: DSOs: 3106 (Completed: Messier, Herschel 1, 2, 3. In progress: H2,500: 2180, S110: 77). Doubles: 2382, Comets: 34, Asteroids: 255
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Re: 70mm F6 doublet airy disk and rings question

#33

Post by notFritzArgelander »


mikemarotta wrote: Thu Oct 21, 2021 9:09 pm
Bigzmey wrote: Thu Oct 21, 2021 8:40 pm I don't believe nFA and John are saying that Svbony 501 is a bad scope.

There is no reply on that point.

So, is it your considered opinion that the Svbony 501 70 mm f/6 refractor doublet telescope is not a good instrument?
https://www.svbony.com/sv501/
Are you warning people here that it is a "hobby killer" like a Bird-Jones reflector?
Are you declaring that this is not a well-made instrument?

Again and again: "... any good 70 mm..." So, is the Svbony 501 a "hobby killer" like a Bird-Jones?
OK since you insist. The reason for my personal non response to the question is that the question comes close to being a counterexample to the truth of the slogan "there is no such thing as a stupid question". The reason being that answering "yes" or "no" would be a stupid answer. The better question would be "if a particular Svbony 501 70 mm f/6 refractor doublet telescope cannot split epsilon Lyrae is it a bad example?" That can be answered with a simple "yes". The generalization to the whole class of examples cannot be justifiably made without a larger sample in the discussion.

Furthermore, your posing of the question seemed to me to be a bit of rhetoric and sophistry that was tendentious and argumentative. So I initially declined to reply.
Myself, I deny the label. The best telescope is the one that gets used. I said that about the Bird-Jones. I have a new post on my blog about my National Geographic 70 mm with which I have not split the double-double epsilon Lyrae, but with which I have sketched Mars, photographed the Moon and much more. It is based on my post on this board about my homebuilt Vixen mount.
(https://necessaryfacts.blogspot.com/202 ... mount.html)
The phrase "the best telescope is the one that gets used" is just a slogan. You are brandishing slogans about as though they were rational arguments. They aren't. Your Nat Geo scope is not in good condition if it can't split epsilon Lyrae. Whether or not you want to do anything about it is your business. We would be happy to help you and we have been happy to help realflow100, should you choose to work on it, if working on it is indeed possible.

No slogans aside, the best telescope is the one with the highest Strehl and the smallest rms wavefront error. The telescope that gets used is a pleasurable one, not the best. Sorry. I have little patience with sloganeers. If you do too much cable news you pick up bad habits so I gave up listening to sloganeers and other sophists.
It is true that you get what you pay for. That is how the market works and amateur astronomy is an open and free market. ...
That is an economic question that is open to debate. The number of "open and free" markets in the sense of Adam Smith has plummeted as nation states become intertwined with industrial production. That is what Adam Smith inveighed against. Another time perhaps.....
It is up to the individual to make their own economic decisions. And once made, we can only be supportive of their involvement in the hobby. Too many here and on other discussion boards sniff with derision at anyone who does not invest the same money that they did.
We (have you really read this thread?) have been supportive of @realflow100 in his efforts to squeeze every bit of performance possible out of his investment. He is certainly to be commended for his efforts which have been rewarded with considerable success. What have you done to help?
Last edited by notFritzArgelander on Fri Oct 22, 2021 9:36 pm, edited 5 times in total.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: 70mm F6 doublet airy disk and rings question

#34

Post by notFritzArgelander »


realflow100 wrote: Wed Oct 20, 2021 11:22 am I was testing it at night though
and running through aberrator the wave error doesnt resemble 0.5 at all. way too different looking
it resembles closer to 0.1 to 0.2 waves of error fairly closely
I havent been able to see cassini division yet. but maybe that could just be seeing/atmosphere needs to be extremely good?
on jupiter i see very rare over the course of a few minutes periods of fleeting sharpness where I see slightl detail in the cloud belts. but I havent located the red spot yet. though it should be visible just need to look harder and longer for it
this is very good news indeed! i strongly urge you to rest from your labors to improve the scope. you've likely achieved all you can. i'm very glad to think that our suggestions helped. use it for a while before considering tweaking it more. very good seeing is rare.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: 70mm F6 doublet airy disk and rings question

#35

Post by notFritzArgelander »


@realflow100

I want to emphasize very strongly that the responses in this thread do not disparage your scope or your efforts. you have our admiration for your ingenuity and willingness to learn how to improve your scope. The suggestion that we are making fun of your efforts are totally out of line and to be blunt, false!

Again, you've done well. If you can split epsilon Lyrae you can rest on your achievements with some pride. If you can't and you want to play more with your scope to further optimize it, I'd advise against unless you are mostly interested in optics. I think you've done the rotation of the lenses with respect to each other, if I recall correctly? I know you've played with the spacing. I don't know how (and I doubt anyone does know) how to improve it more.

If you decide to take this further mark the edges of the lenses so you can recover the present configuration. That will minimize risk of losing the fruits of your labors thus far.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: 70mm F6 doublet airy disk and rings question

#36

Post by realflow100 »


I dont think any lens rotation matters on my particular scope (No astigmatism as far as I can tell. doesnt seem to make any difference. coma doesnt change or anything. airy disks are round and diffraction rings dont have astigmatism visible either. they are pretty clean in good seeing)

The lens edges are blackened with a sharpie marker. so i'm not sure how I could mark them again. unless I had a white sharpie.

They are kept held in collimation as perfectly as possible by using 2 layers of 3 pieces of scotch tape stuck around the edges of the lens elements. to keep them from coming out of alignment. and keep them from rattling around too much.
The spacing of this scope (svbony SV501 model) 70mm F6 achromat is close enough out of the box I decided to leave it as-is. so at least the lens element spacing is close enough. its probably off by 0.1mm in one direction or the other. but i'm not gonna fiddle with it anymore.
splitting epsilon lyrae (both doubles) is good enough for me.
and seeing the bands on jupiter and faint hints of cassini division on saturns rings is probably as good as it will get without a professional putting it through extensive testing and finding the perfect lens spacing adjustment. which is not worth the money lol.

I think my only last thing left is to find the perfect balance of baffle spacing for the telescope tube and focuser. to minimize vignetting/aperture loss. as well as minimize glare to as little as possible.
Its only got one tube baffle. and one focuser baffle (I made the focuser baffle myself)
Svbony SV503 70mm ED F6 420mm FL refractor telescope (New)
Canon EOS 100D/SL1
Tamron 18-200mm F3.5-F6.3 II VC lens
canon 50mm STM F1.8
svbony 8-24mm zoom eyepiece
svbony goldline 66 degree 9mm and 6mm + 40mm plossl + 2x barlow.
svbony UHC 1.25 filter + astromania 1.25" O-3 filter + also an svbony H-B filter.
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Re: 70mm F6 doublet airy disk and rings question

#37

Post by notFritzArgelander »


Good to hear that you are reaching the point of satisfaction. That you can split epsilon Lyrae is good indicator that you've been doing well.

It sounds like it could benefit from additional baffling. I believe the more baffles the better. This is especially true when observing from a bright urban/suburban environment. Here is a good discussion for doing the geometry for a baffle design.

https://www.cloudynights.com/topic/4934 ... tting-out/

Now, this diagram has only one baffle. I'd prefer more in the tube, as many as 4-6 depending on your patience with the task. The diameters can be calculated for different spacings from the objective, the x in the formula. With that many baffles you might be able to do away with the focuser baffle. The more baffles you have, the more you'll be immune to stray light from some crazy angle.

Some folks like to simplify the task by making evenly spaced baffles in the optical tube. There's nothing wrong with that. You wind up with something like this:

https://www.cloudynights.com/topic/6710 ... ted-field/
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: 70mm F6 doublet airy disk and rings question

#38

Post by realflow100 »


My focuser has really really bad glare/reflections without a baffle. totally unusable.
its extremely bad compared to the inner part of the telescope tube.
Whatever paint they used is not matte enough. too reflective. especially during the daytime!

is there any easy way to guess if my baffles are cutting into the light path without any calculations or other equipment?
by looking through the eyepiece from a distance a certain way. with it pointed at the sky in the day. or something
or just looking through the diagonal without an eyepiece at a certain angle?
Any convenient simple way to check roughly that its not making my exit pupil smaller?
Does it not cut into the light path as long as my exit pupil is still "full size" in the eyepiece?

theres 2 baffles in my telescope
One in the main telescope tube assembly.
and one in the focuser. somewhat in the middle but closer to the objective than the eyepiece side.

I tried spacing them by adjusting their position until when i looked into the eyepiece its JUST blocking the glare at the edges of the exit pupil (when your very close to it but not 100% you can see the glow around the edge of the focuser)
and with the current spacing it's JUST right so I dont see the glare in the focuser. or in the telescope tube itself. while at infinity focus with my largest eyepiece. 40mm plossl.
Svbony SV503 70mm ED F6 420mm FL refractor telescope (New)
Canon EOS 100D/SL1
Tamron 18-200mm F3.5-F6.3 II VC lens
canon 50mm STM F1.8
svbony 8-24mm zoom eyepiece
svbony goldline 66 degree 9mm and 6mm + 40mm plossl + 2x barlow.
svbony UHC 1.25 filter + astromania 1.25" O-3 filter + also an svbony H-B filter.
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Re: 70mm F6 doublet airy disk and rings question

#39

Post by notFritzArgelander »


I think that some calculations are the best path forward for baffling the tube. Light undergoes diffraction so you want to increase the inner diameter of the hole i=of the baffle a little bit.

For the focuser.... I'd suggest repainting or flocking it.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: 70mm F6 doublet airy disk and rings question

#40

Post by John Baars »


Flocking the focuser and perhaps the inside of the zenith mirror / prism are recommended anyway. Like nFA already said. The glare from that is closest to your eyepiece and you should definitely get rid of that.

When I made my own refractors, I always drew out the beam path 1:1 on paper. That way I could determine on paper where and how big the baffles needed to be.

In practice it comes down to the fact that from the focuser (without eyepiece) you should not see the tube wall anywhere. But you should still be able to see the spacings in the lens at the beginning of the tube. That way you can be sure that you are using the full aperture.

Looking straight ahead like that, it seems to be pretty good. But you shouldn't be satisfied too soon. Even looking in at an angle, you should no longer see a tube wall.

The placement and size of the baffles is easiest to determine with thin black cardboard rings, which you can move from the focuser position. You then keep the focusser unscrewed so you can reach it more easily. Homemade baffles should be properly sharp. (So no wide rings as you sometimes see in some models) A wide ring reflects unwanted light to the eyepiece anyway. So make them sharp.
baffles in Vixen (640x480).jpg

Baffles in my Vixen. Narrow tube, so many baffles. You shouldn't think what the image would be like if the baffles were wide: one big glow, the effect would be completely gone. Photo taken from eyepiece position.

You can make them from thin tin or cardboard and make four tabs on them to be able to glue them. Paint them dark matt black on both sides. You can also use the paint to seal any light leaks between the baffle and tube wall.

If the tube is narrow, you need more baffles than with a very wide tube.

Flocking the whole tube with Protostar is also a possibility. https://www.fpi-protostar.com/flock.htm
Good stuff. It works great. Absorbs more than 99% of angled light. The hairs don't come off, the stuff has been in my larger Maksutov for over 20 years and is still attached.
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
Amateur astronomer since 1970.
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