Flats, darks and leaky focusers

[luka]

I have been struggling with flats on my RC8 (with Moonlite focuser) since getting the scope. This was quite a surprise coming from ED80 where at least the flats were easy. I have tried sky flats, LED panel, diffused LED panel, diffused light sources but the flats were never right. Sometimes over-correcting (similar to this) and mostly not fully correcting the vignetting.

Eventually I have found that the issue was caused by:
- taking darks with the camera still on the scope (to save time and preserve alignment for imaging over multiple nights)
- making the dark library also during daytime (in a darkened room with blinds down)
- focuser leaking light - I definitely did not expect this.

Basically the resulting flat darks (and darks) were not darks and, of course, the flats never worked properly. And thinking more about it, this probably also explains why sometimes the amp glow would not properly calibrate out.

The 2" Moonlite focuser leaks light where the drawtube enters the focuser body and also where the 2" tube enters the drawtube. Google indicates that this is common with most of focusers and that this is normally not of concern for nighttime imaging.

By the way, the easiest way to test for light tightness of the imaging train is to have dust on the sensor. Any light leak will be quite obvious

The moral of the story is that focusers may not be light tight. Take darks in darkness with camera only, i.e. do not try to cut corners.

Hopefully my experience helps someone else with similar issues.

[gregbradley]

You should be able to take darks at night with the scope cap on.

When you think of the number of nights you can't image due to clouds you could accumulate a massive amount of darks.

But this is one area where CCDs are better as CCD cameras have shutters which should block out stray light. I would still only trust them at night though.

CMOS only have electronic shutters which is good from a cost/maintenance viewpoint but not from a flats/dark viewpoint.

Greg.

[JA]

Originally Posted by luka

The 2" Moonlite focuser leaks light where the drawtube enters the focuser body and also where the 2" tube enters the drawtube. Google indicates that this is common with most of focusers and that this is normally not of concern for nighttime imaging.

By the way, the easiest way to test for light tightness of the imaging train is to have dust on the sensor. Any light leak will be quite obvious

The moral of the story is that focusers may not be light tight. Take darks in darkness with camera only, i.e. do not try to cut corners.

Hopefully my experience helps someone else with similar issues.

Hi luka,

If you are concerned about the light leaks in the focuser or anywhere else, but still want the ease of leaving your system setup and want to continue with daytime Darks, you could use a blanket to cut out the extraneous light together with a lens cap.

Perhaps even an o-ring could be placed over the drawtube and slid up against the focuser housing when required to make it light-tight. If that doesn't slide as easily as you'd like then cut a washer from closed cell neoprene foam (wet suit material) or if suitable for the outer diameter of your drawtube, cut a 10-20mm slice from a cylindrical neoprene stubbie holder and use that like the o-ring.

Best
JA

[multiweb (Marc)]

I always did darks at night. When I shoot bias frames on the field I point the scope up plug it and cover it with a fabric to minimize light leaks on the camera. Every bit help.

[AdamJL]

when using a DSLR, I did darks on-site.
Now with a cooled cam, I do them at home, at night when there are no lights on and let it run for however long it needs to. Sometimes I cover it for extra light blocking capability, sometimes I put it in the second fridge in the garage if it's a hot day and can't get it too temp.

haven't had any issues with that yet. Flats? Yep. Flats can be tricky to get right, but darks are fine.