Taking Dark Frames with Tri-band NB filters

[Tak120]

I'm a long time amateur but a relatively new imager.  I have a Tak120 on a EQ6R-Pro mount with a full frame QHY367PRO-C OSC along with several other accessories including an IDAS NB-1 tri-band filter that's in a single filter drawer in the image train in front of the camera sensor and behind my Tak TOA-35 reducer/flattener.   I've been building a library of calibration frames for various temperatures, exposure times, gain/offset, etc. 

I have a basic question.  Do I need to take a separate set of dark frames when I shoot with the IDAS filter?  Since the sensor is blocked with the scope cap and you're shooting darks, I don't see why it would be necessary to take separate darks if you add a filter but I may not be understanding something here.  I've read conflicting advice about the need to take a separate set of darks with a filter/OSC combo as well as a mono/filter combo.  Some people have said that you need to take new darks for every filter and others have said you just need to take one set of darks for all mono filters.  I'm just about to pull the trigger on a QHY600MM-PH so my question is relevant to that scenario too.

I tried searching but couldn't find a thread on this although I'm sure this issue has been discussed.  My apologies if it has.

Thanks.

Paul
State of Washington, USA

[Seelive]

Darks are just capturing the response of the camera sensor (and the associated electronics). Since their is no light reaching the sensor, any objects in the light path are irrelevant. Obviously flats are a different matter.

[david_taurus83]

Hi Paul. No, you don't need to take darks for each filter. As mentioned above, there is no light falling on the sensor so its irrelevant what filter is in place. Perhaps you have read about 'flat darks' (or dark flats as they are commonly mis-named). These are the recommended calibration frames for flats if your using a CMOS sensor. If you were to take flats for every filter then you may need to take a dark frame to match the flat exposure. If your flat exposure was different every time then it would need for a new set of flat darks for every set of flats.

[rickwayne]

One more reason to use bias frames, if your camera takes them stably. Ekos calculates an exposure to give me 25K ADU +/-  1000, so every session it probably is a slightly different exposure.

Of course, for my particular setup, dark current is pretty  negligible for everything but my narrowband flats. Fractions  of a second  for LRGB ones.

Edited January 30, 2021 by rickwayne

[Tak120]

Thanks for the replies, everyone! That confirms what I thought.  I need to match the subs with any filters used during the imaging session to the corresponding dark frames with the same , temperature, etc.   Got it.  That's what I'm doing now.    Star Forming - you said that it was one more reason to use bias frames.  Could you amplify a little on that comment?

[ollypenrice]

Since this is a CMOS camera, bias will not work in the same way as they do with CCD cameras. In CCD you can use a master bias as a dark-for-flats, for instance and it will work fine. When making CMOS flats you must also make darks-for-flats (AKA flat darks) by shooting them with no light and at all the same settings as the flats. No bid deal, though, with OSC.

Olly

[Tak120]

Thanks and understood.  I took my first set of dark flats last night with N.I.N.A. which BTW is really slick and easy.  So, I now have a comprehensive set of darks, dark flats, flats and bias calibration frames.  With dark flats for my CMOS cam, do I also need to use bias frames in pre-processing?  I've heard conflicting views on that topic.