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Filter size needed to get no vignetting at f4 using a qhy 163

Bella123
11 Nov, 2019 00:10
Hi, I am thinking about getting an 8 inch imageing Newtonian at f 3.9  I have a qhy 163m and I know the chip sits back further in these than the zwo 1600 mm. My question is what size filter will cover my chip without vignetting, will a 31 mm work or do I need a 36 mm? Any help will be appreciated. Thanks, Shawn.
tim@the-hutchison-family.net
11 Nov, 2019 01:24
I have a ZWO EFW in front of my 1600 and I use 31mm mounted filters without issue.  Slight vignette, but nothing flats don't cure.
dvdearden
11 Nov, 2019 02:29
My experience is the same as Tim's: slight vignetting, easily fixed using flats. I used to have a much bigger problem, but it turned out to be the off-axis guider prism from my Orion Thin Off-axis Guider. I rotated the guider 90° and the problem pretty much went away. ASI 1600MM-cool with ZWO EFW with 31 mm unmounted filters (also ZWO) on an 8" f/4 Newtonian astrograph.
Edited 11 Nov, 2019 02:55
Bella123
11 Nov, 2019 02:37
Thank you both for your comments ,  just worried because the sensor is further back in the qhy camera than the asi, and I've had problems with serious vignetting at f6 with 1.25 inch filters. But I will look into it further. Thank you, Shawn
tomtom2245
11 Nov, 2019 03:22
I use 31mm with my 1600 in an EFW and I've shot with am f/2 lens with the previously mentioned very slight vignetting. Nothing that flats can't handle and nothing that would make me want larger filters
DerPit
11 Nov, 2019 08:18
I don't know the backfocus of the camera, it seems to be somewhere between 10 and 18mm, is that correct?  added to thet the distance in the filter wheel, probably some additional 4-5mm

At F/4 you would need additional 14/4=3.5mm or even 23/4=5.8mm in addition to the sensor diagonal (at 21.6mm).
So in worst case (27.4mm) that should still reasonably work for unmounted 31mm filters (assuming 2-3mm for their support, around 28mm free aperture).
36mm would be the safer bet.
Chris-PA
12 Nov, 2019 18:48
One reason why I chose the ZWO over the QHY for the 1600/163  is precisely this: 6.5 mm vs. (I think) 17.5 mm of backspacing. 1.25" filters work fine at f/4 for the 1600, but I don't think it'd work for the 163. I'd call whomever you bought the camera from (Highpoint, OPT Corp., etc.) and ask them. They should be able to tell you exactly what size filters you need.
TareqPhoto
13 Nov, 2019 05:37
I am using 1.25" filter with my QHY163M, i will do more test, but i don't have serious nice results to post anywhere but only Facebook because i am just testing, and i am also working on all options possible so i can have 1.25" used and i fulfill the backfocus of my imaging system with corrector/reducer,…etc.

Already ordered Chroma 1.25" filter and waiting it to arrive to give it a try, i am using ZWO filter wheel just temporarily because it is mainly for planetary imaging with ZWO planetary mono cameras, and i use Canon lens, i like to test and see.
Bella123
13 Nov, 2019 10:35
Hi everyone I already ordered a new 36 mm filter wheel and lrgb, and narrowband filters , it was time for a step up anyway
 
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