My older-reduced HorseHead Nebula shot

[Daveskywill]

Hi: In my earlier 2 shots of the Veil-those were taken in the past using my Celestron RASA and my Nikon D800...but that same older setup is used in this shot too of the Horsehead Nebula this time.

So I was wondering since I plan to use this same RASA scope in my image train in my future shooting setup for deep-sky...and I'm wondering, why in this shot, I've got:

1. 2 purple halo's (equalateral triangular)
2. A serious Unflat fielding problem...that even has 5 different shifts between light to dark to, etc...
3. And a false of bit of rainbow to the star to the south in the image...see?

If I can train-out all the anomalies in this Celestron RASA I'll be happy!

Thanks.
David

[Daveskywill]

I did subtract a 'Flat' but when I took flats: I used a clean white sheet folded a few times and put it over the camera...and as you know it is a Celestron RASA with the camera on the front of the scope...the sheet made a 'cone-section' between the back of the camera and the sides of the scope...was that an alright setup? Or was that responsible for the problem seen?

[Daveskywill]

Should I use a 'flat lighting source'? Like those 2'X2' flat lighting panels? My friend Terry says I'll need to be able to dim those ones, because the shutter speed can only do so much...Like what technique for flats should I use for this RASA scope? Thanks. David

[rcheshire]

David. The light is divided by the flat. But I know what you mean. You want the flatest even light source available to take flats. Rest the A4 sheets so that they are flat on top of the scopo. That should eliminate one variable.

EDIT: You could make cardboard frame and add layers until you get the flat illumination correct.

[cometcatcher]

I just use the sky for flats. With no clouds of course.

[Atmos]

I recently put my D810 on the RH200 F/3 and noticed something quite similar. Some of what you have could be caused by bad flats but other parts, like what I got, are caused by something more irritating.

The sensor in a Nikon (at least their FX sensors) is housed in a deep rectangle that is about the same size of the sensor. For me at F/3 the light angle is quite steep, for you at F/2.2(?) it is even steeper. On-axis light isn't affected but the further you move off axis the more light is blocked by the interior walls of the camera.