is it better to correct doublet for center to edge sharpness. or for some other aberration?

Discuss your refractor type scopes here.
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realflow100 United States of America
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is it better to correct doublet for center to edge sharpness. or for some other aberration?

#1

Post by realflow100 »


is it better to correct doublet for center to edge sharpness. or for some other aberration? like coma. field flatness. or chromatic aberrations?
Like adjusting the spacing between the elements?

I have a super cheap (Probably not even true doublet) 70mm refractor telescope with 360mm focal length approximately.
The spacing with the original spacer causes severe blurring to the edges of the image. but if I remove the original plastic ring spacer and use 4 tiny rectangles of duct tape on 4 corners. 2 layers tall. much thinner than the original spacer
its significantly sharper from center to edge. but I'm not sure if theres any other negative effects.

its a very very cheap refractor telescope. like under 40 bucks i think. I had to do a lot of modifications to make the image brighter in it. plastic ring thing in the focus tube. as well as focus tube being absurdly too long. causing vignetting. as well as a metal ring in the tube of the telescope also causing vignetting and stopping the lens down most likely to F11 or something.
but with all those rings taken out it gives a much brighter image at night.
The diagonal it came with was terrible though. had to replace it with a cheap but still infinitely better one. giving even more image quality.
Svbony SV503 70mm ED F6 420mm FL refractor telescope (New)
Canon EOS 100D/SL1
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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: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#2

Post by JayTee »


RF, I moved your post over to refractors so it gets better visibility plus your topic is about a refractor.

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∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
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Re: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#3

Post by notFritzArgelander »


The most important aberration to correct is spherical aberration which is most likely what you have done. If there are two lenses in the objective then it’s a doublet, there are a few different designs for doublet achromatic refractors. Since it’s already so slow CA isn’t likely an issue. If you don’t mind the work, though, another optimization would involve rotating one lens with respect to the other. That might reduce aberrations further. @Lady Fraktor has written about how to do that.
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: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#4

Post by realflow100 »


I see plenty of CA with it. especially in the daytime. but sharpness seems somewhat ok. Not great but not terrible. at least with lower magnifications.
my plossls give me the sharpest view from center to edge (nearly indistinguishable from perfect) but I still see chromatic aberrations. even at the lowest magnification possible with a massive absurd overkill 40mm plossl.
but they dont seem to be too much of an issue. especially at night time the chromatic aberrations arent bad enough to degrade the view in any meaningful way. I find i have more trouble just from the lack of stars. light pollution.
I thought F5.1 was closer to fast than slow. (thats what its focal ratio should be)

Ive tried rotating the lenses separately from each other. but seems to have no positive effect. The astigmatism (slight cylinder shape to one of the lenses.) just rotates with it. and rotating the other lens does not cancel it out at any time. so it seems only one of the lenses has slight astigmatism/cylinder but the other is near-perfect.
best I can do is line it up 90 degrees to my own eyes astigmatism. so that its least noticable. (it is very hard to tell. I have to use very high magnification with an eyepiece DIRECTLY in the focuser with a very long extension tube to reach focus. like a 6mm eyepiece)

I think my MOST significant problem is the slop in the focuser between the thing it slides in and out of. Craaaaaazy slop. focusing causes the image to shift up and down a lot. Most noticable in vertical direction.
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Re: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#5

Post by notFritzArgelander »


Sounds like you have done all you can. CA is often dependent on spacing btw so if you are overall pleased with the new spacing then let it go at that.
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: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#6

Post by John Baars »


Increasing the spatial distance shifts focus towards blue. Decreasing it shifts focus towards red. ( this is what you did) SA is altered with shifting too. I am not sure in which direction. Lady Fraktor used to have a scheme for it.
CA can't be altered this way. In an existing lens the only way to suppress CA is by vignetting it. ( an old optician's tric) That way you use only ( the best) middle part of the lens. SA is smaller too.
By removing all parts that worked like a diaphragm you increased CA, and simultaneously made the image brighter and less sharp.
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Re: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#7

Post by Lady Fraktor »


Good information from nFA and John but if the focuser drawtube is not solid it can produce aberrations by itself especially in short tube refractors.
Once you get the drawtube to stop moving around and collimated then you can see if changing lens parameters will help.
I will post a under/ over corrected image later and some visual aberrations.
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
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Re: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#8

Post by realflow100 »


Ive tried shimming it with some thin plastic strips but its too tight and stiff then.
I think the part that just holds the draw tube centered is too short in length. so theres no way to prevent it from wobbling in some direction.
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: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#9

Post by Lady Fraktor »


What is the make of your telescope?
I would like to see the focuser so I may give some advice.
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
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Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Takahashi prism, TAL, Vixen flip mirror
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Re: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#10

Post by realflow100 »


https://www.amazon.com/gp/product/B071XX1WX9
One of these.
Its only really worth maybe 28$ realistically. the asking price is too much for what low quality you get out of it. Too much lens element spacing. cheap crummy eyepieces (Even the eyepieces have bad chromatic aberrations. and distorted small field of view)
bad blurry edges
poorly placed and sized "rings" in the focuser and telescope tube causing excessive loss of light and vignetting issues Focuser tube excessively long causing even more additional vignetting and loss of light.
Sloppy focuser mechanism as well. and focusing is extremely difficult due to theres a lot of friction you have to overcome when trying to turn the focus knob. making it jerk instead of slowly focus smoothly.
Surfaces too shiny. even the lens hood is just a cheap shiny black plastic.
Poor quality lens elements as well. One of them is even slightly cylinder-shaped with astigmatism.
Extremely bad cheap star diagonal. its just a thin slab cut-off of a mirror. jammed in with a piece of foam. and its extremely imprecise and bent. causing blurring and warping to the image.
Svbony SV503 70mm ED F6 420mm FL refractor telescope (New)
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canon 50mm STM F1.8
svbony 8-24mm zoom eyepiece
svbony goldline 66 degree 9mm and 6mm + 40mm plossl + 2x barlow.
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Re: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#11

Post by Lady Fraktor »


Well my cure for any 66° eyepiece is to hit them repeatedly with a heavy mallet until they work properly ;)

I am not sure how you are determining the lens spacing is not right, is it over or under corrected?
If the baffles are vignetting the view you can remove them, spray the inside of the tube afterwards with either Krylon Ultra Flat Black or black chalkboard paint.

The focuser drawtube is like a see-saw with the focuser knobs at the pivot.
Your eyepiece is at one end and so you need to place 2 shims above the focuser tube inside so the tube cannot drop down at the eyepiece end.
Place these shims/ blocks at 10 and 2 of the clock.
If there is a plate on the bottom of the focuser which holds the knobs and focuser shaft in place, undo each f the screws about a eighth of a turn until it moves reasonably well.
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
EQ: TAL MT-1, Vixen SXP, AXJ, AXD
Az/Alt: AYO Digi II/ Argo Navis, Stellarvue M2C/ Argo Navis
Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Takahashi prism, TAL, Vixen flip mirror
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Re: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#12

Post by realflow100 »


Ive adjusted all the screws and whatnot. but I think its a lost cause at this point.

The lens spacing is not right because after shortening the distance. the edges of the field of view become significantly sharper and clearer compared to the center of the field of view. especially when using my camera with it.
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: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#13

Post by realflow100 »


On a very slow ratio telescope with a doublet lens inside could the lenses be spaced to minimize chromatic aberration to as little as possible. but still have acceptably sharp image field from center to edge? like what would be the best way to space them?
do chromatic aberrations and field sharpness become a non-issue past a certain focal ratio?

or do you want to go a different way and try to get the best balance of lens spacing for chromatic berrations as well as field sharpness? (if that is possible or even an issue?) on a moderately long focal length?

Somehow I got the spacing just right (at some point) where I had fairly sharp enough image from center to edge with my dslr. while also simultaneously decently low chromatic aberrations. I forgot how much spacing I had though. it was a couple tiny squares of duct tape cut up and stacked on each other.
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: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#14

Post by Lady Fraktor »


The air gap is determined by the optical design of the doublet so changing the distance will only degrade the image.
I think it is more likely that you need to correctly space your camera to the focus plane instead of moving the telescope lens.
If you are stacking pieces of ducting tape as spacers then you have definitely spaced the lens to much, a fraunhofer doublet only needs spacing in the thousands of a inch.
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
EQ: TAL MT-1, Vixen SXP, AXJ, AXD
Az/Alt: AYO Digi II/ Argo Navis, Stellarvue M2C/ Argo Navis
Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Takahashi prism, TAL, Vixen flip mirror
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Re: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#15

Post by notFritzArgelander »


Lady Fraktor wrote: Sat Jul 31, 2021 4:11 pm The air gap is determined by the optical design of the doublet so changing the distance will only degrade the image.
I think it is more likely that you need to correctly space your camera to the focus plane instead of moving the telescope lens.
If you are stacking pieces of ducting tape as spacers then you have definitely spaced the lens to much, a fraunhofer doublet only needs spacing in the thousands of a inch.
And almost all achromats are Fraunhofer.
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: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#16

Post by realflow100 »


Lady Fraktor wrote: Sat Jul 31, 2021 4:11 pm I think it is more likely that you need to correctly space your camera to the focus plane instead of moving the telescope lens.
i extremely doubt my camera would be spaced wrong or the image would just be completely out of focus. it is definitely in-focus for sure.
it came with a clear plastic ring spacer thing which was like a millimeter or 2 in size. Probably 2mm
it was slightly oversized as well. Way too tight fit. just getting it to stay in and not get wedged between the threads and stuck at an angle was a nightmare. so i got rid of it and used tiny pieces of duct tape.
How thick is a usual piece of duct tape? Would there be a recommended amount of layers of duct tape to use (Also accounting for the fact duct tape has some give to it.)
it says 360mm focal length. and is a 70mm lens.

If I want to use it with eyepieces I definitely need a better diagonal for it.

Also the entire length of the focuser tube is a fraction smaller than the width of an eyepiece 1.25" except for the VERY end where the diagonal/eyepiece would slot in. so I cant stack too many filters for example.
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: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#17

Post by realflow100 »


I re-did the spacing again and also this time used some strips of packaging tape to keep the elements from rotating while screwing on the cap. keeping 4 tiny little cut rectangles off a super thin plastic-paper thing I got off a bandaid wrapper. they are pretty thin and I dont see any rainbow-looking rings in the center of the objective. so the elements aren't touching.
though it was pretty hard to get them to be perfectly spaced out right at the edge of the lenses. hopefully not too much of an issue with 4 of them spaced around instead of 3 like i did before.

I tried with my camera again and all the corners are still severely blurred :( Dont really think its over correction or under correction. because there is no point where the edges become even close to sharp. no matter what spacing i try.
hair-thin plastic-paper thing from a bandage wrapper. a single layer of duct tape. a single layer of packaging tape. or multiple layers. its all the same. I use tiny rectangles cut to the same size. it just gets worse the more spacing there is. but theres no point where it gets good at the center and edge at once :(
no spacing at all seems to be the closest i think. theres just a small concentric ring rainbow in the center of the lens with no spacing of the elements.

Center looks fairly sharp or at least decent. but edges look like a dump. Sad. all this effort and the edges still look poor.
Heres a screenshot.
Focus might be a fraction off. but thats nothing compared to the disgusting blur-mess of the edges! YUCK

With an eyepiece (From 6mm to 40mm) the image quality is pretty much perfect. negligible amount of chromatic aberrations. details razor-crisp sharp
Need to wear my glasses to even tell when using the 40mm eyepiece though. due to the enormous exit pupil magnifying my own eyes astigmatism.
with my glasses and 40mm eyepiece the image quality is near-perfect. I cant really detect any major faults. its pretty much crisp from center to edge evenly.
And with a 6mm eyepiece (with or without my glasses) it still looks acceptable. though quite a dark image. with a low-contrast look. however fine details look distinct and nearly at the limits of my vision. Actually i think the atmosphere is more of an issue than the telescopes optical quality in terms of sharpness. Crazy heatwave rippling on everything.
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Last edited by realflow100 on Sun Aug 01, 2021 11:54 am, edited 2 times in total.
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: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#18

Post by realflow100 »


heres another test photo. Sorry its a bit dark and noisy. took it very early-morning
this one seems a little better for some reason (Focused on a object half a mile to a mile away just about)
Also was a bit cloudy
I took about 10 shots and picked the sharpest looking one with a 5 second timer delay. one by one (not continous)

Dangit my image is getting resized down for some reason!!! Why is the uploader doing that?? heres a discord upload with the full resolution and file.
For some reason the edge blurring is significantly less here.

Image
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: is it better to correct doublet for center to edge sharpness. or for some other aberration?

#19

Post by realflow100 »


heres a better test image showing whats going on with the focus. Nothing is really "in focus" even when I get as much as possible in focus.
the edges are blurry and smeared. and the center is just "kinda a little sharp but not really"
The focus plane is all over the place and blurrier at the edges even if I focus specifically for the edges they still arent really "sharp"
I'm starting to think its impossible because the geometry of the lenses is slightly off. so that I can't achieve sharp focus from center to edge no matter what the spacing.
One or more lens surfaces might be slightly too concave or convex.
Image
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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