Find the good backfocus with my 200/800 SkyWatcher

ZWO ASI1600MM COOL Equipment Reflectors Coma
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Valentin JUNGBLUTH avatar
Valentin JUNGBLUTH avatar
Hello guys,

I have a Newtonian SkyWatcher 200 Quattro P (200/800) with the SkyWatcher Coma Corrector GPU 4-lenses and ZWO ASI 1600 MMC Pro.
Filters are ZWO LRGB SHO 31mm with 1.9mm of thickness.
However, I'm getting some troubles in order to find the best backfocus with my setup.
I read at different places that the good backfocus with this coma corrector model is : 53.66mm with 800 mm of focal lenght + 1/3 of filter's thickness.

First question :
Among astrophotographers on this forum, some people have the same configuration ? If the answer is yes, could you tell me the exact backfocus you have on your setup ?

Theoritical backfocus for 800 mm focal lenght :
53.66mm (BF for 800mm) + 0.63mm (1/3 thickness of filter) = 54.293 mm

Is it true up to now with my calculation ? 

Second question : my optical train with 54.30mm
Ring 14mm (9mm + 5mm) + (spacer 0.8mm) + Ring 11mm + Ring M48/M42 2mm + Filters wheel 20mm + ZWO ASI 6.5mm

Do you think there is a better coma corrector with higher tolerance than mine ?

Thank you very much
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andrea tasselli avatar
andrea tasselli avatar
I have a similar, but not identical, setup: SPX 300 f/4 with same CC and ASI 294MC (same backspace) with a filter holder with filter around 2mm thick. My back focus spacing is 55.6 mm +/- 0.1 mm and I can't find it being an issue. TS-GPU and SW GPU (or SharpStar) are basically identical optically and at that price point hard to beat. Honestly, I can't see variations in back focus of the order of 0.5mm causing any issue as far as optical performance is concerned. In fact, I've tested it to death and still can't tell whether there is any difference at all (just remove a filter and you'll see).
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Earle Waghorne avatar
Earle Waghorne avatar
Hi Valentin,

I have the same system, a Quattro 8cf which I use with a ZWO 1600 or 294. The Skywatcher f4 coma corrector is quoted at 55 mm backfocus and I've found that this works. It is usual to add the 1/3 times the filter thickness.

Rather than stressing about the exact distance, I would suggest getting it close, say go for 56 mm and try it out. If the stars at the edge are long and aligned towards the center you need to increase the distance and if they are long but aligned around the image you need to shorten it. In fact this system is fairly forgiving, you probably can't see changes at .5 mm. I also have an old GSO corrector, with a 75 mm backfocus and this quotes a tolerance of +- 5 mm.

Hope this helps,

CS, Earle
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Leonardo Landi avatar
Leonardo Landi avatar
Same setup too and found that 55 mm works pretty well. In my opinion on that scope collimation matters much more than backfocus… and it's quite a mess. Idiot question: are you sure that your problems are caused by BF error?
Valentin JUNGBLUTH avatar
Valentin JUNGBLUTH avatar
andrea tasselli:
I have a similar, but not identical, setup: SPX 300 f/4 with same CC and ASI 294MC (same backspace) with a filter holder with filter around 2mm thick. My back focus spacing is 55.6 mm +/- 0.1 mm and I can't find it being an issue. TS-GPU and SW GPU (or SharpStar) are basically identical optically and at that price point hard to beat. Honestly, I can't see variations in back focus of the order of 0.5mm causing any issue as far as optical performance is concerned. In fact, I've tested it to death and still can't tell whether there is any difference at all (just remove a filter and you'll see).

Thank you very much @andrea tasselli with your answer. So in your calculation, you took the 55mm backfocus and you added the filter thickness in order to get 55.66mm ? I have to try this to see what happen exactly with the same backfocus.
Unfortunately, they are too much clouds since 3 months on my site.
Earle Waghorne:
Hi Valentin,

I have the same system, a Quattro 8cf which I use with a ZWO 1600 or 294. The Skywatcher f4 coma corrector is quoted at 55 mm backfocus and I've found that this works. It is usual to add the 1/3 times the filter thickness.

Rather than stressing about the exact distance, I would suggest getting it close, say go for 56 mm and try it out. If the stars at the edge are long and aligned towards the center you need to increase the distance and if they are long but aligned around the image you need to shorten it. In fact this system is fairly forgiving, you probably can't see changes at .5 mm. I also have an old GSO corrector, with a 75 mm backfocus and this quotes a tolerance of +- 5 mm.

Hope this helps,

CS, Earle

Hi @Earle Waghorne, thank you very much to take time in order to answer me. I can read at different places 55mm is given for 1000mm focal lenght. So you have about 55.5 / 55.6 mm by adding filter thickness in your configuration ?
I have to do some tries in order to see the impact with 55.5 / 56mm of BF.

Thank you anyway !

Leonardo Landi:
Same setup too and found that 55 mm works pretty well. In my opinion on that scope collimation matters much more than backfocus... and it's quite a mess. Idiot question: are you sure that your problems are caused by BF error?

Hi @Leonardo Landi, you have 55mm + about 0.6mm added due to filter thickness ? I would like to be sure that my BF is good before to look if I get tilt or something else.
To get return from other people which have the same setup is important in order to compare.
ManuManu avatar
ManuManu avatar
Hello Valentin,
I have a Newtonian 800/200 with the same GPU and IMX571C sensor. With this configuration (APC-C format) the coma correction is OK but there is no margin.
After several hours my experimental backfocus value is 54mm ….. So your calculation is right but perhaps add a 0.5mm spacer in your setup. Better should be adding a M48 ring of 26mm to avoid cascade of rings and tilting!
CS
Emmanuel
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