Calibration frames & Filter Wheel Question

[Vinnyvent84]

Hey all, forgive me if this is a silly question…. I’m trying to get started soon with taking a shot at AP. My limited knowledge is that when it comes to darks you can make a library of those generally refreshing every 3-6 months or as needed. And the remainder of calibration frames must be done on a per session bases. 
 

In addition from what I understand - you cannot touch the imaging train in anyway in relation from when you take the calibration frames to doing the actual imaging run. So for example if I was taking flats - I would not be able to alter or touch the imaging train in any way as that may move around dust and render the flats useless? If this is the case how do things work in relation to a filter wheel? 
 

I have read people taking alternating subframes between different filters when doing mono for example. Does the rotating of the filter wheel not constitute “touching” the imaging train during a session? I would think that just the movement of the wheel could introduce new dust or relocate existing dust particles if your bouncing between filters on the wheel from frame to frame? I’m I understanding this incorrectly?

[Hughsie]

Hi Vinny.

You are correct regarding darks. These can be captured in advance and are not affected by changes to the image train. What you do need to remember is that your darks need to be captured using the same exposure time, Gain, Offset and Temperature as your light frames. I have a 'Dark' Library for my ASI1600mm and ASI294mc all at a temperature of -10c. For each camera I have exposure times for 60s, 120s, 180s and 300s. The Gain and Offsets for the 1600mm are based on whether I am doing narrowband or broadband imaging. For the 294mc I tend to use a Gain of 121 and offset of 30. I refresh mine every 12 months.

It's best practice to take flats after every session to ensure the best calibration outcome. You can use flats from previous sessions only if your image train has not changed and assuming you keep your optics free from dust. As you mention, changing the image train between capturing your lights and taking your flats will result in poor calibration so don't touch anything. 

In regard to the filter wheel, as you take your flats for each filter the wheel will rotate round and your filters will be in the same orientation as they were when you captured your light frames so this doesn't impact the image train. The only way this will not be the case is if you alter your image train by moving the filter wheel or unscrew the filters and put them back in again. As to the rotation of the wheel moving dust, you would have to be pretty unlucky for this to happen, however, it does no harm to periodically inspect your optics to ensure no dust or other particles have snuck their way in.

Hope this helps.

John

Edited April 23 by Hughsie

[Vinnyvent84]

1 hour ago, Hughsie said:

Hi Vinny.

You are correct regarding darks. These can be captured in advance and are not affected by changes to the image train. What you do need to remember is that your darks need to be captured using the same exposure time, Gain, Offset and Temperature as your light frames. I have a 'Dark' Library for my ASI1600mm and ASI294mc all at a temperature of -10c. For each camera I have exposure times for 60s, 120s, 180s and 300s. The Gain and Offsets for the 1600mm are based on whether I am doing narrowband or broadband imaging. For the 294mc I tend to use a Gain of 121 and offset of 30. I refresh mine every 12 months.

It's best practice to take flats after every session to ensure the best calibration outcome. You can use flats from previous sessions only if your image train has not changed and assuming you keep your optics free from dust. As you mention, changing the image train between capturing your lights and taking your flats will result in poor calibration so don't touch anything. 

In regard to the filter wheel, as you take your flats for each filter the wheel will rotate round and your filters will be in the same orientation as they were when you captured your light frames so this doesn't impact the image train. The only way this will not be the case is if you alter your image train by moving the filter wheel or unscrew the filters and put them back in again. As to the rotation of the wheel moving dust, you would have to be pretty unlucky for this to happen, however, it does no harm to periodically inspect your optics to ensure no dust or other particles have snuck their way in.

Hope this helps.

John

John, thank you so very much. This is extremely helpful. I have ALOT to learn and my gear gets here on Monday. I have a ASI533MC and am pairing it with a Redcat 71 WIFD ASIair Plus and AM5. Your information most definitely helps. 
 

Im currently trying to research how long my sub exposures need to be considering I’m in a Bortle 8 and using OSC. This, along with many other things - but you now scratched one concern off my list so thank you again!  

[Hughsie]

1 hour ago, Vinnyvent84 said:

John, thank you so very much. This is extremely helpful. I have ALOT to learn and my gear gets here on Monday. I have a ASI533MC and am pairing it with a Redcat 71 WIFD ASIair Plus and AM5. Your information most definitely helps. 
 

Im currently trying to research how long my sub exposures need to be considering I’m in a Bortle 8 and using OSC. This, along with many other things - but you now scratched one concern off my list so thank you again!  

A great resource for research is Astrobin. If you haven’t heard of it it's a community of photographers who post their images/have forum discussions. It is free to join. You can search by location, say New York, and view images posted by individuals near you. In the main they will list the equipment used and exposure times. You can also search by equipment such as your camera and view images and see what camera settings have been used.
 

It’s a great way to see first hand what combination of equipment/exposure times will produce and to look for potential targets. Worth a look if you haven’t already.

[carastro]

The bias can be re-used as well.  I must confess I am a bit lazy and rarely clean my optics but then the filterwheel and camera are kept together and sealed even when not on the rig.   I do take flats every time though................

Except for - I have a camera and filterwheel attached to a Samyang lens which I don't separate, and I have re-used flats upto 18 months later and they have been fine.   But you are correct in saying nothing should be moved. 

My separate Camera and filter wheel that I do remove from the telescope now and then, I take new flats every time the camera is moved.

HTH

Edited April 23 by carastro

[900SL]

10 hours ago, Vinnyvent84 said:

John, thank you so very much. This is extremely helpful. I have ALOT to learn and my gear gets here on Monday. I have a ASI533MC and am pairing it with a Redcat 71 WIFD ASIair Plus and AM5. Your information most definitely helps. 
 

Im currently trying to research how long my sub exposures need to be considering I’m in a Bortle 8 and using OSC. This, along with many other things - but you now scratched one concern off my list so thank you again!  

For exposure with a 533MC Pro, aim for 3000 ADU average at 100 gain which you can find in the histogram data. This will ensure you are not clipping, and not overexposing. 

[Vinnyvent84]

20 hours ago, Hughsie said:

A great resource for research is Astrobin. If you haven’t heard of it it's a community of photographers who post their images/have forum discussions. It is free to join. You can search by location, say New York, and view images posted by individuals near you. In the main they will list the equipment used and exposure times. You can also search by equipment such as your camera and view images and see what camera settings have been used.
 

It’s a great way to see first hand what combination of equipment/exposure times will produce and to look for potential targets. Worth a look if you haven’t already.

Awesome thank you so much that’s great info I’m going to do some searches this evening! 

[Vinnyvent84]

11 hours ago, 900SL said:

For exposure with a 533MC Pro, aim for 3000 ADU average at 100 gain which you can find in the histogram data. This will ensure you are not clipping, and not overexposing. 

Thanks for the info! So I understand the gain; and u understand the clipping part but forgive me for sounding dumb I’m totally clueless in regard to “aim for 3000 ADU average”. What exactly is that and how would I determine that I’m averaging that 3000 number? 

[ollypenrice]

I believe it's best to think through what it is that your calibration frames actually do. Darks record camera noise and no light reaches the chip so you can do them in any situation in which no light reaches the camera.

Flats record the illumination arriving at the chip, which should be perfectly even but won't be. Why won't it be? Mostly because of vignetting, which is not affected by which filter you use. Also by dust somewhere in the light path. The closer to the chip the dust is, the worse the 'dust bunny' it will create. This dust might be on a particular filter but is more likely to be closer to the chip, perhaps on the chip window. This means that you might need flats per filter but might not. With observatory based rigs I used luminance flats for all filters and had hardly any problems with this. If I did - once every few years - I'd shoot a filter flat or clean the rig.

If using an uncooled camera you will probably do best without darks entirely and with a large dither between subs.

With a CCD camera the only dark-for-flat you need is a master bias. With a CMOS camera you need a matching dark-for-flat.

My real point is that there is no need to make a mystery of calibration files, nor is there any need to treat them as a religious ritual to be followed to the letter every time. If you think them through and look at how they work for you, you'll be fine. With my CCD rigs, flats per filter would have been a total waste of time and luminance flats lasted 6 months to a year.

Olly

[900SL]

2 hours ago, Vinnyvent84 said:

Thanks for the info! So I understand the gain; and u understand the clipping part but forgive me for sounding dumb I’m totally clueless in regard to “aim for 3000 ADU average”. What exactly is that and how would I determine that I’m averaging that 3000 number? 

Start reading here

https://theskysearchers.com/viewtopic.php?t=13708

[900SL]

And maybe watch this