Flats in r,g,b very different

[lucchett]

Hi,
My flats are quite different in the three filters.
I am not talking about spots or dust traces, but the illumination gradient.
I take my flats with an anitak panel.
Filters are 50x50mm.
Any ideas why?
Thanks a lot,
Andrea

[gvanhau]

Hello Andrea

This coul have a lot of explanations.
For instance, if you have a gap between the panel and the scope, it could be reflected light of sorrounding objects.
I you dont have a gap between scope and pannel, then it could be reflected light of sources inside of the scope, or even a light leak; Some pannels generate different intencities at different wave lengts, this mean that your exposure times likely will be different for each color, so, reflections or a light leak may influence differently your flat.

Regards
Geert

[lucchett]

Thank you Geert.
I Took the flats during the night, in my observatory.
Also, I tried rotating the panel.

I was thinking at flat exposure.
I noticed that the bad ones are taken at 5 sec, the good ones at 6 secs.
The ccd is big, it is a 16803, so I wonder if the shutter can be the problem.
I analyzed the rejection maps of the bad master flat and there are lots of pixels.
I suspect is the shutter movement.
What do you think?
Thank you,
Andrea

[gvanhau]

Hello

5 secs is a lot of time, I doubt that there could be a problem with the shutter.
Do you take FlatDarks?  If you dont, maybe this is the issue.
Can you show the flat calibration settings?

Regards
Geert

[lucchett]

yes, I take dark flats.
the rejection is due to the fact that the single flats are a bit different (visually).

I am attaching my calibration settings.

thank you again,
Andrea

[gvanhau]

That seems ok to me unless you are calibrating darks with bias. If so you should also add bias to flat calibration.

Inst there any light source in your obs. like a PC screen that could influence the flats? Some times the fan holes at the back of a telescope allow light into the tube....

Regards
Geert

[lucchett]

Took sometime to verify a few things and:

- flat time was indeed too short and set it to 6 sec minimum.
- still, flats are different in r,g,b,l
- to exclude light leaking, I will try to wrap the camera with aluminum foil, but I work with a quality refractor and tend to exclude leaking.

- flat with ha filter is even more strange: it is like if a smooth light cross connects opposite angles. I will send a picture for this.

Thanks a lot, andrea

[msmythers]

Andrea


Just a question, are your using STF to inspect your flats? If so are you using the STF settings from one flat with the others or are your using auto individually with each flat of like exposure time? I would think would want to inspect with the same stretch with each flat of the same exposure time to see the variation between flats. I would like to know from others if this would be the correct way of  visual inspection.


Mike 

[lucchett]

Hi Mike,
You made a good point, but in my case is the overall illumination pattern that differs from filter to filter.
For example, for r I get some vignetting, for b there is a darker edge on the left, for ha I get a strange pattern.
Thank you,
Andrea

[lucchett]

this is the ha flat

[gvanhau]

Very strange symetrical pattern.

For me it seems a reflection, a light leak or both.
It could be a seal defect in the camera itself or in any other equipment in the ligth path.

You could put a light cap on the scope and take some exposures in same conditions as flats, an see if there is coming any light into the camera.

Geert

[lucchett]

Thank you Geert.

I did a test, wrapping the camera with aluminium foil (as suggested by R. Crisp to take day flats).
The pattern is still there, so I'd exclude light leaking.

I think internal reflections are one possibility.
I did change my adapters and exchanged the original focuser for a FLI ATLAS.
I will try to improve the adapters, but that is not going to be quick.
I will test with the light cap, but I am bit skeptical because it is a refractor and I wrapped with aluminium from rear flange of OTA up to the camera.


Still I don't understand why the pattern is different for the various filters:

R: almost fully illuminated
G,B: one side darker near the edge
Ha: the strange cross pattern

I am going to ask also Optec if they had any issue like this, but I don't think is tied to the panel. (and don't know how to test it)


thank you again,
Andrea

[gvanhau]

Suppose you discard the telescope and the camera, what else is in the light path? (Filter wheel, Focuser, OAG,....)
It seems that you will have to test and replace one by one until you find the problematic one.

Still I don't understand why the pattern is different for the various filters:

the main reason is that the coatings, paints and any material inside the optical path have different absorption and reflection coeficients for each wavelenght. Also the filters may have different responses to unwanted wavelegths (UV, IR)

Geert