Taking flats with my Newt - Nightmare!

[FiveByEagle]

Hey everyone.

Recently picked up a TS Photon8 - got a new focuser installed, cameras, OAG - everything is running nice and smooth!!! First time where there are no digital failures out of the gate and that was so exciting.

However - always a BUT lol. But my flats just do not come out. I use an A3 tracing pad (used on all my telescopes - never let me down yet!!!)and the contrast and vignetting does not match the light frames. The dark corners are not the same, and the flats do not appear to match the pattern on the lights. Unfortunately because of this, I have two full nights of data I can not process or do anything with, which is too bad. My M106 data was SHARP...

Is there something different I need to be doing? Extra diffusion? - I have always just placed the pad onto the opening of the scope and that was all that was needed for perfect quick flats.

 

There was a massive amount of light leaking from the back, so I covered that up with the Astrozap Shower Cap and some gaff tape internally, and that totally eliminated the problem and I have taking good flats - "correct looking" without banding or gross leaks and streaks.

(I was able to take a 30s dark frame and there is only small leakage even in the middle of the day. So we are fairly well sealed up)

My new flats and dark flats show zero signs of light leakage - they just don't match... and I can't seem to get it to work despite the last few weeks of work

 

 

Edited May 21, 2021 by FiveByEagle

[alacant]

27 minutes ago, FiveByEagle said:

an A4 tracing pad

Hi

Are you sure the light source is even right to the edge? A4 isn't sufficiently large to give even light over the aperture of our GSO 8" of which AFAIK the ts is a re-badged version.

A3 works fine though. Until you can test, maybe just use simple t-shirt flat frames. 

HTH

[FiveByEagle]

6 hours ago, alacant said:

Hi

Are you sure the light source is even right to the edge? A4 isn't sufficiently large to give even light over the aperture of our GSO 8" of which AFAIK the ts is a re-badged version.

A3 works fine though. Until you can test, maybe just use simple t-shirt flat frames. 

HTH

That is a typo. I have both A4 and A3 panels - A4 for the refractors and A3 for the SCT and Newt - 

[JP-S]

Sorry to distract but which model of tracing pad do you use? I just got a newt and I'm looking to pick one up but it's very difficult to gauge which ones provide uniform light.

[Clarkey]

I have used an A4 AGPtek one from Amazon that is pretty good and even. It is just about big enough to do the RC8 but the A3 version would probably be better. Fine for up to 150mm.

[david_taurus83]

The thing with a newtonian is you only want light that is reflected from the primary to the secondary to shoot up the drawtube. If you put the light source for flats on top of the tube you risk light bypassing the mirror route and entering the focuser directly which will give you uneven illumination. Can you place the light source a bit further away from the OTA and aim at it? 

Edited May 22, 2021 by david_taurus83
newtonian, not newtonion!

[CraigT82]

Might be a good idea to post up some examples of your lights/flats? 

[Clarkey]

20 hours ago, david_taurus83 said:

If you put the light source for flats on top of the tube you risk light bypassing the mirror route and entering the focuser directly which will give you uneven illumination

That is a very interesting point. I have the kit to image with my newtonian but as yet only used the RC8 and ED80. I was thinking of trying the newtonian as it is F5 whereas the other two are >F6. Looking at this I may need to re-think my flats before proceeding🤔

[FiveByEagle]

On 22/05/2021 at 05:43, david_taurus83 said:

The thing with a newtonian is you only want light that is reflected from the primary to the secondary to shoot up the drawtube. If you put the light source for flats on top of the tube you risk light bypassing the mirror route and entering the focuser directly which will give you uneven illumination. Can you place the light source a bit further away from the OTA and aim at it? 

Oh my goodness - this might be it.

Will try to get it above the scope... ideas?