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
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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.
orion skyscanner 100mm F4 reflector telescope. Canon EOS 500D/rebel-T1i. ioptron Smartstar E8500R alt-az goto mount and tripod. no EQ mode limited controller options
Canon EOS 100D/SL1 ! New camera!
Tamron 18-200mm F3.5-F6.3 II VC lens
canon 50mm STM F1.8
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JayTee
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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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notFritzArgelander
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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: 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 III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
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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.
orion skyscanner 100mm F4 reflector telescope. Canon EOS 500D/rebel-T1i. ioptron Smartstar E8500R alt-az goto mount and tripod. no EQ mode limited controller options
Canon EOS 100D/SL1 ! New camera!
Tamron 18-200mm F3.5-F6.3 II VC lens
canon 50mm STM F1.8
svbony ultra wide 66 degree eyepiece kit
15mm 9mm and 6mm + 40mm plossl
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notFritzArgelander
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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: 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 III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
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John Baars
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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.
Telescopes in Schiedam in frequency of use : * SW 150mm Achromat F/5, *SW Evostar 120ED F/7.5, *grabngo: SW 102 Maksutov F/13,
*Vixen 102ED F/9, *OMC140 Maksutov F/14.3, on Vixen GPDX.

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Most often used binoculars: *AusJena 10X50 Jenoptem, *Swarovski Habicht 7X42, *Celestron Skymaster 15X70,
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Lady Fraktor
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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.
Telescopes: 1 more than I need but 2 less than I want
Mounts: 5 Equatorial, 3 Az/ Alt
Diagonal: 3 mirror, 2 Amici, 1 Herschel
Eyepieces: Possibly more than 1 person requires
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realflow100
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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.
orion skyscanner 100mm F4 reflector telescope. Canon EOS 500D/rebel-T1i. ioptron Smartstar E8500R alt-az goto mount and tripod. no EQ mode limited controller options
Canon EOS 100D/SL1 ! New camera!
Tamron 18-200mm F3.5-F6.3 II VC lens
canon 50mm STM F1.8
svbony ultra wide 66 degree eyepiece kit
15mm 9mm and 6mm + 40mm plossl
svbony UHC 1.25 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.
Telescopes: 1 more than I need but 2 less than I want
Mounts: 5 Equatorial, 3 Az/ Alt
Diagonal: 3 mirror, 2 Amici, 1 Herschel
Eyepieces: Possibly more than 1 person requires
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realflow100
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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.
orion skyscanner 100mm F4 reflector telescope. Canon EOS 500D/rebel-T1i. ioptron Smartstar E8500R alt-az goto mount and tripod. no EQ mode limited controller options
Canon EOS 100D/SL1 ! New camera!
Tamron 18-200mm F3.5-F6.3 II VC lens
canon 50mm STM F1.8
svbony ultra wide 66 degree eyepiece kit
15mm 9mm and 6mm + 40mm plossl
svbony UHC 1.25 filter
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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.
Telescopes: 1 more than I need but 2 less than I want
Mounts: 5 Equatorial, 3 Az/ Alt
Diagonal: 3 mirror, 2 Amici, 1 Herschel
Eyepieces: Possibly more than 1 person requires
Image
realflow100
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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.
orion skyscanner 100mm F4 reflector telescope. Canon EOS 500D/rebel-T1i. ioptron Smartstar E8500R alt-az goto mount and tripod. no EQ mode limited controller options
Canon EOS 100D/SL1 ! New camera!
Tamron 18-200mm F3.5-F6.3 II VC lens
canon 50mm STM F1.8
svbony ultra wide 66 degree eyepiece kit
15mm 9mm and 6mm + 40mm plossl
svbony UHC 1.25 filter
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