Can a corrector be wrong ? - Forums

Topic starter

27 days ago · Edited: 27 days ago

Hello All,

Do you know if it is possible to have a wrong corrector ?
I own a TS-Optics 76EDPH - 6-element Flatfield Apo (so with the flatner/reducer shipped with the tube)

It is given to have 342 mm focal length but after ASIAir platesolving -> it is corected to 345mm

Since few weeks, I own a 6200mm and, on my first picture, It seems that i have a wrong back focus symptom in the angles (see here)

could it be linked to the reducer ? ( I suppose this due to the adjusted focal length I mentioned before)
And if yes, how can it be corrected (I guess I can't add spacers since it looks like too long back focus)

Maybe I'm totaly wrong, but if you have any idea, I would take it

Thanks a lot

andrea tasselli

Frequent contributor

27 days ago · Edited: 27 days ago

Lens are spec'd to within 1-1.5% of the nominal focal length so nothing wrong with it and I doubt that's the issue there. If it has a reducer/corrector I can hardly remember any giving perfect stars across the baord in a FF sensor with these tiny pixels. Looking at the picture there is a bit of comatic flaring which might be related to tilt but not a lot. And yes, the corrector can be in fact either poorly made (rare) or poorly assembled (wrong distances). You can try to increase the spacing between corrector and focuser to see whether there is any improvement, say adding 1 mm spacer/shim and see how/if the image gets better. If it does keep adding until it improves no more.

P.S.; I also see some field curvature so obviously the field isn't very flat either.

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PierreG

Topic starter

25 days ago

Thanks for your answer, very clear !
could you show me an example in the picture for me to detect the coma flaring and the field curvature ? if it is not bothering you of course

in order for me to exerce my eyes !

thanks a lot again

andrea tasselli

Frequent contributor

24 days ago

Thanks for your answer, very clear !
could you show me an example in the picture for me to detect the coma flaring and the field curvature ? if it is not bothering you of course
in order for me to exerce my eyes !

thanks a lot again

*You need to make the file available at full resolution (Google drive, Filebin, etc)

Poor Bunny

13 days ago

From what I understand, the TS 76EDPH with the 6-element corrector can be quite sensitive to back focus. Since your plate solving shows 345mm instead of 342mm, maybe you're just slightly off the optimal spacing? I agree with Andrea that trying shims/spacers in 1mm steps might help fine-tune it.

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ScottF

Frequent contributor

13 days ago

The 55mm baakfocus is just a starting point for bigger sensors IMO, especially once a reducer is tossed in. I've found out the hard way that it's something that needs to be tinkered with if you want the best out of your scope. My Pleiades 68 scope is very challenging to nail down, but once there, it's fine.

Noah Scott

11 days ago · Edited: 10 days ago

I'll definitely try adding some 1mm spacers incrementally and see how it affects the corners. It sounds like this kind of slope fine-tuning is just part of the process with scopes like the TS 76EDPH.

Well Written

Krzysztof Gaudy

10 days ago

I had this telescope, and in my honest opinion, you simply can't expect perfect stars all the way to the corners with this telescope and a full-frame camera. Every corrector covering such a large field of view with tiny CMSO pixels will introduce some imperfections in the stars, especially in the corners.

Spacing might be an issue, but to determine in which direction to adjust the corrector, I strongly recommend using NINA and Hocus Focus. They show the spacing very clearly. Before that, you should also check for tilt - correct that first, then adjust the backspacing.

This is from my current setup, tested yesterday. At the top, you'll see the Backfocus Error, which shows whether you're within the Critical Focus Zone (CFZ) or if adjustments are needed. The Critical Focus Zone (CFZ), also known as Critical Focus Depth, is the range of focus positions where a telescope produces a sharp image, and you won't notice any difference in star shape within this range. Since your telescope is very fast, the CFZ will be quite narrow.

I'm sure the tilt will be significant if you haven’t corrected it yet. Run tests in different parts of the sky - you might get it perfect when pointing west, but then see different results when pointing east. In such cases, your setup is likely too heavy for the telescope's focuser, or there may be some flexures. You can either adjust it or live with it

If you’re not using NINA, install ASTAP first and check the tilt on your current images - either Luminance or Blue filter. ASTAP -> Go to Tools → Image Inspection → Tilt.

CS, Chris
https://astrophotomarket.com

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