Dark frames in LL - a question and a feature request

[aparker]

Question for anyone who wants to chime in - how many dark exposures should I be doing before swithching over to light frames? My default has been 10, on the assumption that there's some averaging taking place to create a "master" dark, and that this averaging is subject to diminishing returns as more frames are added. But that's totally a guess. For context, I'm almost always doing 10sec subs and find the dark frames essential to control hot pixels with my mono Lodestar X2.

Feature request for Paul: could we get a checkbox to temporarily disable use of the current darks? I often want to switch down to 1-2 sec unstacked exposures for object finding and centering, before going back to 10 sec for stacking, and having the 10 sec dark active really bolluxes up the 1 sec exposures, making almost everything but the brightest stars disappear. Right now the only way I know to stop using the current darks is to switch LL off and back on (to get to no darks) or to restore a saved master dark (which has not worked well for me in the past), or to record new darks over the existing ones. It would be nice to just toggle use / don't use the current set.

Thanks again for everything you've done with LL!

[Paul81]

The master dark is composed of a median stack of the dark frames - I tend to get at least 5, normally 8-10. As you say its diminishing returns. I often capture darks as I am setting my mount up (the scope and camera have been outside at this point for an hour or so cooling) and in that case I end up with around 10-20 but I'd say you should aim for 5-8 for a decent result.

In terms of your feature request - this is what focus / alignment / framing mode is for. Just switch to that mode (it will auto stop the camera exposures), find / centre your object as necessary and then switch back to image acquisition (reset the current image on the exposure tab when you move to a new object).

The focus / alignment / framing mode doesn't apply darks, and processes everything in mono for speed. You can switch modes and back to image acquisition and if you do not reset then you can carry on as you left off.

Hope that helps!

[Martin Meredith]

Hi Alex

If you take insufficient dark frames you're actually adding noise into your image, so the advice is to take as many as possible. For short subs there is no hardship in taking 30+ and recently I've started to increase the number of darks I use, though I haven't checked the results.

Slightly off topic, since LL doesn't currently support these things, but still relevant to the general 'darks' discussion:

(i) While suitable darks handle hot pixels, there is another approach based on bad pixel mapping. Check out the Nebulosity documentation for more on this. I find that hot pixels do re-appear some way into a session and that is a cue for me to update my darks. Bad pixel mapping would, I think, avoid this.

(ii) Another approach to the darks question is based on creating thermal frames, which removes the need to take fresh darks once a suitable initial set has been built up (if I remember correctly). At any rate, the idea is to take plenty of darks for the longest possible exposure you'll ever use, and then interpolate at the desired exposure length. This is covered in Berry and Burnell's excellent Handbook of Astronomical Image Processing. I'm away from the text at the moment so don't have the details at hand, but it goes under the name of advanced calibration. I can look it up in a couple of weeks if there is any interest.

cheers

Martin

[aparker]

The master dark is composed of a median stack of the dark frames - I tend to get at least 5, normally 8-10. As you say its diminishing returns. I often capture darks as I am setting my mount up (the scope and camera have been outside at this point for an hour or so cooling) and in that case I end up with around 10-20 but I'd say you should aim for 5-8 for a decent result.

In terms of your feature request - this is what focus / alignment / framing mode is for. Just switch to that mode (it will auto stop the camera exposures), find / centre your object as necessary and then switch back to image acquisition (reset the current image on the exposure tab when you move to a new object).

The focus / alignment / framing mode doesn't apply darks, and processes everything in mono for speed. You can switch modes and back to image acquisition and if you do not reset then you can carry on as you left off.

Hope that helps!

Paul,

Thanks - that makes perfect sense.  I think I'd gotten stuck on the "focus" part of focus, framing, and alignment, and had not thought to use it other than for the focus assist mode (which works very well, BTW).  So problem solved for me!

Alex

[aparker]

(i) While suitable darks handle hot pixels, there is another approach based on bad pixel mapping. Check out the Nebulosity documentation for more on this. I find that hot pixels do re-appear some way into a session and that is a cue for me to update my darks. Bad pixel mapping would, I think, avoid this.

I recall this from reading about nebulosity.  It is notable for me that I always wind up with 1 or 2 hot pixels that survive the dark subtraction and show up in my live stacks.  Agree that some algorithmic detection and elimination of these (which would probably be dead easy) would be a nice thing to have.