"A good 70-mm refractor..."

Discuss your refractor type scopes here.
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mikemarotta
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"A good 70-mm refractor..."

#1

Post by mikemarotta »


So, still trying to split the epsilon Lyrae companions, I decided to find out more about the range of 70-mm refractors.
Meade series 6000 Quad Agena.jpg
Williams Optics Gran Turismo 71.jpg
Astro-Tech 70mm Astro-Tech.jpg
National Geo 3 Explore Sci.jpg
Meade 70 Celestron 70 Mile High.jpg

SvBony 501 70mm Picture1.jpg

Evostar 72 mm Mile High.jpg
Last edited by mikemarotta on Fri Oct 29, 2021 4:23 pm, edited 1 time in total.
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Michael E. Marotta
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: "A good 70-mm refractor..."

#2

Post by mikemarotta »


The Airy Disk
Astronomer Royal George Biddle Airy (1801-1892) modeled the visual telescopic image of a star in the course of his investigations of optics the so-called “Airy disc” or diffraction disc is the small central portion of the false image of a star formed by a telescope at focus. Light not contained in the disc forms neat, concentric diffraction rings, or Fresnel rings surrounding the disc. The size of the star image is proportional to the wavelength of light, and inversely proportional to the aperture of a particular optical system. Thus, the larger the aperture, the smaller the Airy disc in stars of the same color.

This ideal representation has confused some observers, who assume that their optics are flawed when stat images on the Airy model do not appear during routine sessions. … Due to atmospheric turbulence and “local seeing” disturbances in the air in and around the telescope, the model appearance is rarely glimpsed in the field. Typically, one sees an amorphous central discoid surrounded by a series of broken, shifting ring segments. […]

In years of observing, the author has only experienced a handful of occasions when the atmosphere rendered a perfect Airy model visible—all in the wee hours of still, humid, subtropical mid-summer nights of marginal transparency. At times like this, planetary details stand out like the lines on a banknote, and doubles generally seen as barely split seem to have widened to admit an extra measure of black space between their components.” Care of Astronomical Telescopes and Accessories by M. Barlow Pepin, Patrick Moore’s Practical Astronomy Series, Springer-Verlag, London, 2005, page 24.

RESOLUTION AND DAWES' CRITERION
As stargazers, let us look at practical criteria, since there is no "ideal sky," and assume that all things being equal, resolution is dependent primarily upon D, or aperture. Under the Dawes Criterion, sometimes referred to as the Dawes Limit, the resolution theoretically attainable is succinctly defined for practical purposes. Pre-eminent double star observer The Reverend William Rutter Dawes (1799-1868) worked it out based on the separation of "white" double stars of comparable magnitude (-6 visual). Dawes' rule, in practice, defines the resolution attainable with an excellent 6-inch achromatic objective. He arrived at it by testing various excellent telescopes of different aperture, including his 6.5-inch Merz, and 8.25-inch Clarks, and an 8-inch Cooke with which he pursued double star measurement and cataloguing from 1831 on.

The simple equation for Dawes' limit is: a = 11.6 / D
...
An excellent 4-inch Apochromat refractor does have the ability of resolving 1.1 arc second double stars under careful scrutiny. Thus, Dawes' criterion holds for excellent seeing, while many other observations confirm that image degradation by atmospheric turbulence and turbidity due to suspended water vapor or dust particles limit resolution under most circumstances, regardless of telescope quality. Care of Astronomical Telescopes and Accessories by M. Barlow Pepin, Patrick Moore’s Practical Astronomy Series, Springer-Verlag, London, 2005, page 25-26.
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Michael E. Marotta
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: "A good 70-mm refractor..."

#3

Post by Ylem »


That Astrotech has an attractive price!
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Re: "A good 70-mm refractor..."

#4

Post by notFritzArgelander »


Some small points.

The Dawes criterion formula is correct only for aperture measured in centimeters, units need to be specified for D for the formula to work.

I gave my grandson an Orion Observer II 70mm refractor and a set of BCO eyepieces. The optics are fine but the mount needed some tweaking to become stable. epsilon Lyrae is split on average (1-2 arc seconds seeing) about half the time. It's similar to some of the other offerings with a price at ~$110.
Last edited by notFritzArgelander on Sat Oct 30, 2021 12:16 am, edited 1 time 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: "A good 70-mm refractor..."

#5

Post by pakarinen »


Ylem wrote: Fri Oct 29, 2021 6:30 pm That Astrotech has an attractive price!
I believe prices go up early 2022 if it matters to anyone.
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Re: "A good 70-mm refractor..."

#6

Post by mikemarotta »


notFritzArgelander wrote: Fri Oct 29, 2021 9:44 pm. The Dawes criterion formula is correct only for aperture measured in centimeters, units need to be specified for D for the formula to work.
It is so stated in the book. Note the ellipses.
---------------------------------------
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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: "A good 70-mm refractor..."

#7

Post by notFritzArgelander »


mikemarotta wrote: Sat Oct 30, 2021 6:40 am
notFritzArgelander wrote: Fri Oct 29, 2021 9:44 pm. The Dawes criterion formula is correct only for aperture measured in centimeters, units need to be specified for D for the formula to work.
It is so stated in the book. Note the ellipses.
The ellipses were noted and aren't informative without the book.
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: "A good 70-mm refractor..."

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Post by mikemarotta »


The formula for resolution has several expressions, depending on the chosen units, English versus metric, linear or circular.

Angular resolution in micrometers = 0.25 times [wavelength in micrometers / aperture in meters] (bringing the orders of magnitude into conformance.)
Angular resolution in radians = 1.22 times [wavelength in micrometers / aperture in micrometers]
Angular resolution in arc-seconds = 0.25 * [wavelength in micrometers / aperture in micrometers]
For D=70 mm
Green = 1.9 arc-seconds
Violet = 1.38 arc-seconds
Red = 2.43 arc-seconds
Mean = 1.9 arc-seconds
A Student’s Guide to the Mathematics of Astronomy by Daniel Fleisch and Julia Kregenow, Cambridge University Press, 2013, 2020.

Resolution = 5.45 / D inches
[from which I calculate the Resolution of a 70-mm = 1.98 arc-seconds, and checked by interpolating the table provided in the citation which gives 51 and 76-mm (2-inch and 3-inch) apertures.)
Star Ware: The Amateur Astronomer’s Guide to Choosing, Buying, and Using Telescopes and Accessories, 4th Edition, by Philip S. Harrington, John Wiley & Sons, Inc,, 2007, pages 6-9.

Smallest Resolvable Angle = [wavelength nanometers]/ [Diameter in nanometers]
Smallest Resolvable Angle (arc-seconds) = [114/ Diameter in nanometers]
for 70 mm = 1.6 arc-seconds
Observers Handbook 2021, Royal Astronomical Society of Canada, page 49.

The simple equation for Dawes' limit is: a = 11.6 / D
Where D is the aperture in centimeters, a is the measure in arc-seconds of the smallest separation between double stars of near-equal brightness that can be positively identified as double by elongation of the image.[10]
[10] [This is a lengthy footnote baed on another reference, A Manual of Advanced Celestial Photography by Wallace and Provin, Cambridge, 1988. In part: "... the ideal diffraction disc on the Airy model increases from 0.6 seconds of arc at 4000 A to to 1.2 seconds at 7000 A in an unobstructed systems of 6 inches (150 mm) aperture."]
Care of Astronomical Telescopes and Accessories by M. Barlow Pepin, Patrick Moore’s Practical Astronomy Series, Springer-Verlag, London, 2005, page 25.
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Michael E. Marotta
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: "A good 70-mm refractor..."

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Post by notFritzArgelander »


:lol: The sense of humor in veering among extremes is amusing.
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: "A good 70-mm refractor..."

#10

Post by notFritzArgelander »


In any case the Dawes criterion for separation double stars is not the only criterion. Where a is the angular separation in arc-seconds and D is the aperture in millimeters it is:

a(Dawes) = 116/D

The Dawes criterion is not derived from the Airy disk though. It is an empirical relation. The criterion that is slightly more pessimistic and based on Airy disk diffraction patterns is the Rayleigh criterion. It obeys a formula similar to Dawes's but with a different constant for the arc-second millimeter bit.

Rayleigh is

a(Rayleigh) = 142/D.

It has been suggested that for scopes that are centrally obstructed and the resulting redistribution of light into the diffraction pattern that replacing D by D-CO where CO is the linear size of the central obstruction might be a decent approximation. :shrug:

That might be too pessimistic. For a well collimated centrally obstructed scope the point spread function gains power at higher frequencies and so one might get away with something rather better.

Rayleigh is the source of the 1/v wave RMS wavefront error for a good scope.
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: "A good 70-mm refractor..."

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Post by mikemarotta »


40-Millimeter Shootout
First night
30-31 October 2021


Celestron AstroMaster 70 AZ Refractor F=900 f/13. $189.95 from Mile High Astro. Comes with two oculars, 20mm and 10mm.
Meade StarPro AZ 70mm Refractor F=700 f/10. $159.99 from Mile High Astro. Comes with four eyepieces, 26mm, 9mm, and 6.3mm oculars and a 2X Barlow lens.
[National Geographic 70mm Refractor $109.99 from Explore Scientific – not arrived yet.]

Keep the telescopes. Throw away the mounts.
  • The right ascension gear does not work at all on the Meade. No matter how much I turn the handle, the telescope does not move.
  • The action on the Celestron pan-tilt handle is so tight that it lifts the tripod and telescope off the ground without moving the telescope up-down or -left-right.
(I will contact Mile High Asto on Monday to see what they know and recommend.]

So, I mounted each in turn to an Explore Scientific Twilight, which I use for my Explore 102-mm refractor. From there, I tested the optics. Both were all right for what they were. My original intention was to test these and then give them away. Now I want to keep them both. In truth, I will not keep these though they are temping as “grab-and-go” instruments.

31 October
02:00 hours Central US Time.
Celestron 20mm and 10mm oculars: Orion Nebula is OK in each. Trapezium is easy to distinguish.
20mm on Sigma Orionis group. 20 mm shows the close companion to sigma Ori. 10mm shows it better at the Airy limit.

02:25 Found eta Cassiopeiai with 10mm. At the Airy limit.

02:45 Meade 25mm on the Pleiades, very nice.
02:48 25 mm. Orion nebula easy focus, wide view. Trapezium easy. Sigma Orionis good. Shows companion.
02:53 9mm. M42. Trapezium. Better contrast to the cloud than in the Celestron, perhaps because it is now higher in the sky.
02:57 9 mm on Sigma Orionis.
03:02 6.3 mm on sigma Ori shows the double red pair. (This was impressive. I recorded this group earlier in the year but I saw the faint red paid for the first time on 29 October using my ES 102-mm.)
03:05 26 mm on M 41 open cluster.

03:10 Celestron with 20mm on M41.
03:18 Celestron 20 mm on eta Cassiopeiai. 10 mm shows better with Airy disc.

0340 – Dew point. Brought everything in.
Sigma Orionis Perez 1.jpg
It is interesting to consider the different design decisions that went into the creation and production of these beginner scopes. I will write more about the engineering aesthetics later.

Both are fine for the entry-level astronomer, if we define what “entry level” means. In the current economy, for an adult working fulltime my recommendation is to save your money and buy a better telescope for your introduction to the hobby. You never dive into the pond until you know how deep the water is. If you want to know if astronomy is a hobby for you, and if you can easily afford a $350 mistake, other choices would be better. It is a fact that if you deeply enjoy the hobby of astronomy, then your first telescope will not be your last. Moreover, even as a hobby, astronomy is open to professional-level engagement including collaboration in peer-reviewed publications. For myself, even though I edit for the History of Astronomy Division of the American Astronomical Society, I just like going out in the backyard and looking at the stars. I leave the photographing of galaxies and nebulas or the discovery of new asteroids, comets, and planets, or the spectroscopic analysis of stars to other amateurs.

The other beginners are children. You do not know if yours will take to this hobby or abandon it for something else. If the youngster is serious, they will get a lot of use out of this level of instrument, regardless of make or model. For the cost, it can be a good investment in your child’s education, given that the parent is also engaged. It is a sad truth that in astronomy, we do not see the level of passion that causes parents to rush onto to the playing field to argue with the field umpire. But if you want a telescope to be used for three to five years, then you have to be present for the proceedings.

The testing will take time as the weather is good now but rain is forecast for the coming week. I want to be thorough and careful over time in order to learn well the strengths and weaknesses of these entry-level astronomical instruments. One night of viewing is not enough. I quickly found it hard to manage two instruments in parallel. It goes better if I work in series. And I prefer a spiral approach, learning new material, integrating it to what I know, then returning to put more information about subject into its proper context in order to establish a foundation or framework for the next level of learning. So, here in The Sky Searchers, this will be a continuing project and open discussion ahead of any more formal publications. I trust TSS. I participate in half a dozen discussion boards. This one works best for me largely because I have learned to trust the opinions offered here.
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Michael E. Marotta
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: "A good 70-mm refractor..."

#12

Post by mikemarotta »


It will be clear again tonight and I just got notice that my new National Geographic 70-mm refractor is being delivered. I took the opportunity to buy what seems to be among the last in a current batch at Explore Scientific. Two other models in the series are sold out and the price on this was bumped up $10 over advertised, actually -- not a big deal, but an indication... Anyway, more on that later. As for last night:

31 October 2021 - Epsilon Lyrae.

Whie my wife covered the front door for Halloween, I went out back for an hour while Epsilon Lyrae was above the trees, to compare the Meade StarPro and the Celestron AstroMaster.

The Meade StarPro comes with a range of oculars - 26 mm, 12 mm, and 6.3 mm, and a 2X Barlow lens.
1940 hours. Finding e Lyrae easily with the 26mm, I stepped all the way down to the 6.3mm with 2X Barlow for an effective magnification of 222 before the view was acceptable though not unequivocal. At that magnification, it was dim, yellowish, and granular, past the threshold for good resolution. I noted that I could identify each star and its companion only because I knew what to expect. The pair on the right (corrected view) was separated but not on the left, only a bulge of light on the star disc.

1945 hours. Switched to Nagler 7mm with 2X Barlow from my Celestron Lens & Filter kit. No joy.

Celestron Astromaster 70-mm comes with a 10mm and a 20mm ocular and I did not bother with those time around having been there and done that with these when they first arrived.
1948 Installed Celestron Lens&Fliter kit Ploessl 6mm and kit 2X Barlow and successfully split the companions E Lyrae E1A and E Lyrae E2A.
E Lyrae E1 E1A E2 E2A.png
E Lyrae E1 E1A E2 E2A.png (3.65 KiB) Viewed 7248 times
(about like this, but not as large)
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Michael E. Marotta
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: "A good 70-mm refractor..."

#13

Post by AstroBee »


I'm pretty sure they meant to say Bahtinov mask, not Maksutov mask...

Image
Greg M.~ "Ad Astra per Aspera"
Scopes: Celestron EdgeHD14", Explore Scientific ED152CF & ED127 APO's, StellarVue SV70T, Classic Orange-Tube C-8, Lunt 80mm Ha double-stack solar scope.
Mounts: Astro-Physics Mach One, iOptron CEM70EC Mount, iOptron ZEQ25 Mount.
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Filters: 36mm Chroma LRGB & 3nm Ha, OIII, SII, L-Pro, L-eXtreme
Eyepieces: 27mm TeleVue Panoptic, 4mm TeleVue Radian, Explore Scientific 82° 30mm, 6.7mm , Baader 13mm Hyperion, Explore Scientific 70° 10mm, 15mm, 20mm, Meade 8.8mm UWA
Software: N.I.N.A., SharpCapPro, PixInsight, PhotoShop CC, Phd2, Stellarium
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Re: "A good 70-mm refractor..."

#14

Post by mikemarotta »


31-October-2021
Celestron AstroMaster 70-mm refractor with oculars from the standard Celestron Lens&Filter kit.

2310 Hours Eta Cassiopeiai
17 mm identified and 13 mm better split.
2319 Hours Pleiades
32 mm nice view.
2330 Hours
Jupiter. 32 mm. Hazy. White. Bands very faint.
Jupiter. 17 mm. Hazy. White. Bands very faint.
Jupiter. 13 mm. Bands better but still foggy. Hard to get the moons sharp.
The telescope comes with a correcting prism. So, I got two different prisms to try.
2341 – Explore Scientific prism from 102-mm refractor. No change.
2345—National Geographic (old telescope) prism somewhat better but still not good.

Meade StarPro
2354 Hours
Jupiter. 32 mm. better but still very white.
Jupiter. 17 mm. check. Bigger. No change.
Jupiter. 8 mm. Bands somewhat more distinct.
01 November 2021
0009 Hours.
Jupiter. Explore Scientific 25-mm “Super Plössl” View is better but still bad.
---------------------------------------
Michael E. Marotta
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: "A good 70-mm refractor..."

#15

Post by mikemarotta »


AstroBee wrote: Mon Nov 01, 2021 5:36 pm I'm pretty sure they meant to say Bahtinov mask, not Maksutov mask...
Of course! That was my mistake. I put the text over their placard for accessories to go with this. I will fix the image.
Thanks!
Mike M.
Williams Optics Gran Turismo 71.jpg
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Michael E. Marotta
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: "A good 70-mm refractor..."

#16

Post by mikemarotta »


Monday, 01 November 2021
National Geographic 70-mm Refractor (new) from Explore Scientific.
Comes with Bresser 26mm and 9.7mm Ploessl oculars and Bresser 2X Barlow.

1908 hours: Set up and aligned finder with Venus. Added Celestron neutral density filter (NDF “Moon filter”) to cut the glare of the planet and see phase. Also noted that this is not a correcting diagonal, just a mirror. West was to the right and the bright side of Venus was to left (east).
(The Celestron 70-mm and the Meade 70-mm come with correcting prisms.)

Mount action is tight. It is easier to loosen the clutches and move the telescope by hand. Altitude (declination) control twist works better than the azimuth (right ascension) thumbwheel which is too tight.

26-mm Bresser Ploessl. Venus.
9-mm Bresser Ploessl. Chromatic aberration is pronounced, top to bottom of Venus blue-yellow-red. Difficult to focus.

1947 hours: epsilon Lyrae. 9.7 – I can almost see the companions.
1950 hours: epsilon Lyrae. tried 8mm Celestron Ploessl. Closer, no cigar.
1953 hours: epsilon Lyrae. Tried 9.7 Bresser with Bresser Barlow. Past the resolution limit. Just blobs of color.
2002 hours: epsilon Lyrae. Tried Nagler 7mm – more promise. Tantalizing but no, not yet.
2009 hours: epsilon Lyrae. Tried Celestron 14 mm – and can almost see the companions when the telescope shakes. This is a phenomenon I have read about: wiggling the telescope reveals details because the eye responds to motion.
2012 hours: epsilon Lyrae. Tried Celestron 8mm with Celestron 2X Barlow. Colored blobs. No joy.

2021 hours: Eta Cassiopeiai. 9.7mm Bresser with 2X Bresser Barlow. Identifiable only because I know what I was looking at. The image is a colored blob (vertical: blue-yellow-red) with another white dot to the right.

2029 hours: Saturn. Bresser 26 mm. Ring is discernable.
2033 hours: Saturn. Bresser 9.7 mm. Better gap between ring and planet. But planet has blue-yellow-red vertical chromatic aberration and a ghost image above.

2039 hours: Saturn. My old National Geographic with Bresser 26-mm. Saturn is clean and sharp. Yellow. The ring is crisp. Titan is a likely dot.
2045 hours: Saturn. New Natl G: With Explore 26-mm Super Plössl. No help.
2045 hours: Saturn. Old Natl G: With Explore 26-mm Super Plössl. No problem.

Side by Side: My old Natl G with the new Natl G. Both with Explore 26-mm Super Plössl.
Target: Albireo. Old stars are more round. New stars are more sparkly.

Cloudy now for a few days.
Next tests on all three (new) 70-mm entry level refractors will be on the waxing crescent Moon this weekend Friday to Sunday 5-7 November.
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Michael E. Marotta
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: "A good 70-mm refractor..."

#17

Post by mikemarotta »


We may be extending the original work of William Rutter Dawes beyond its useful limits.

"Hence it follows that the tests of separating power furnished by close double stars are by no means to be relied on as determining the character of a telescope; and further experience has fully confirmed me in this opinion." -- W. R. Dawes, 1867.
See the fuller post under "General Optics" here:
https://www.theskysearchers.com/viewtop ... 29#p177529

That being as it may, it remains that the better view is better and if your instrument will resolve binary stars or the features on Mars, then that is to the good.
---------------------------------------
Michael E. Marotta
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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