TheSkySearchers

I have a dielectric mirror diagonal. and a prism erect image diagonal
and there seems to be a weird tradeoff between them

dielectric seems sharpest and best contrast. no noticable scatter or anything. as sharp as straight-through focuser

however the prism has noticably lower CA in my orion ST80 telescope. LOWER false color than the mirror diagonal. rather than increased.
the red green and blue wavelengths definitely seem closer to each other at high magnification with the prism diagonal. instead of the mirror one.
but the downside is there seems to be a miniscule decrease in small details at high magnification like a very small blur. airy disks seem a little "swollen" and larger if that makes sense.
as well as glare/flares on bright objects ghost double images that "float all around" the view at low power like on venus. jupiter and the moon. its very odd. strange reflections. they do not happen with the dielectric mirror diagonal

On normal stars they look practically indistinguishable at low power to high power. theres too much light pollution for any "spike" to be visible with the prism diagonal. whether thats on venus jupiter or bright stars. I just dont see any spike at any power really.

On a far away porch streetlight there was a slight hint of a spike with the prism but nothing really noteworthy

Any thoughts? the prism is worth about 15$ if you were to buy it new.

Over to you Bill Paolini!

In case Bill doesn't see this question, check out his prism comparisons here to hopefully help answer your question: https://www.cloudynights.com/articles/c ... ison-r2877

- Dean

ive read this article before but it's not quite the specific thing im asking about. my brand of prism isnt on the list either. its something shipped with the svbony 70mm scope that I dont have anymore.

also have the celstron dielectric diagonal mirror (Swapped the mirror out of the original celestron diagonal body into a different one. with MUCH MUCH improved open aperture and much better supression of internal reflections.)

I suspect that the CA that is produced by your prism diagonal coincidentally cancels out some of the CA of the ST80: more by luck than design. If you used that diagonal with an apo refractor you would probably find it introduced unwelcome CA.

Have a look at the discussion on CN here: https://www.cloudynights.com/topic/7970 ... ront-lens/ - in particular posts #11 - #20 As JG in #19 (j.gardavsky: also a member of this forum) points out, Wofgang Rohr made some tests for CA with prism diagonals, but I suspect that no-one else has done much: it is probably cheaper to get an apo scope than to try to develop some way of compensating for CA in an f5 achro like the ST80.

All the best,

Dean

Is it possibly to obtain an ED doublet 80mm F5 lens??? to swap into my ST80 instead of buying a whole complete new telescope? Would that be cheaper?
then I wouldn't have much CA to deal with and with a field flattener I'd have a crisp flat field image with no vignetting or significant aberrations.

realflow100 wrote: ↑Mon May 16, 2022 10:04 am Is it possibly to obtain an ED doublet 80mm F5 lens??? to swap into my ST80 instead of buying a whole complete new telescope? Would that be cheaper?
then I wouldn't have much CA to deal with and with a field flattener I'd have a crisp flat field image with no vignetting or significant aberrations.

I don't think so. Others have thought about it though, have a look at the CN discussion here:
https://www.cloudynights.com/topic/6588 ... -for-st80/

You could try this for the closest possibility:
https://www.teleskop-spezialisten.de/sh ... anguage=en

- or go the whole hog for this!
https://www.apm-telescopes.net/en/skywa ... -refractor

ok wow thats waaaaaaaaaaay too expensive and out of my price range.
I payed 100$ for my whole scope. thats more than 4 times more for less CA and nothing extra.

The Dielectric mirrors were made for the Army with their spotting scopes in north Africa wars as the sand gets in so they could just wipe it off , it is certainly more robust but no better than a good quality normal one , no modern top of the range scope uses Prisms only for correct view
I have never had any top quality from SV body and the 2 I did have were bad , but that is a few years ago so perhaps others can confirm

realflow100 wrote: ↑Mon May 16, 2022 12:50 pm ok wow thats waaaaaaaaaaay too expensive and out of my price range.
I payed 100$ for my whole scope. thats more than 4 times more for less CA and nothing extra.

That is the joy of the ST80. Surprisingly good optics for a very low price: but removing CA comes at a cost, especially for fast optics like f5.

Yeah I'm quite surprised it has such low aberrations BESIDES the CA
no coma spherical astigmatism ect. less thjan 1/10 wave even very likely. and I really did a lot of comparing too.
thought collimating is a pain with how its built. having to loosen and tighten the lens cell holder over and over rattling and shaking it around at different angles. such a pain! but once airy disks finally look right its got razor crisp sharp usable images even up to the maximum magnification. 160x even!
Seeing is more of a problem than the optics most of the time where I live the seeing is always horrendous. sometimes so bad its like Im always aiming above an air conditioner radiator or something. theres no clear areas that dont have hot tile rooftops or road in some direction or another.