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Flat frames when multiple bands over the course of the evening

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#1 AutopilotEngaged

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Posted 14 January 2018 - 12:02 AM

So I'm imaging M81/82 tonight. Super bright target which means I will not be spending 8 hours on each channel. I started with some lum frames and before I switched to my Ha filter. I got to thinking that I will need to refocus which then made me consider the fact that I hadn't collected flats for the Lum data....Are you guys taking your flats in the field before changing to the next channel? Do I need a lightbox? It seems pretty inefficient to gather a single channel over night just to get your morning light flats for that channel, then move onto the next the next night. 



#2 Kaos

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Posted 14 January 2018 - 04:06 AM

As long as you do not move the camera you should be able to take flats at the end. My typical project runs 3-4 nights and I never take flats until I am finished taking my light frames. I refocus in between filter changes as I have SGP set to do that. Until recently, (one of the bearings cracked in my ficuser due to the extreme cold), my focus held fine during the 2-4 hours I spent per filter. But, I generally refocus as necessary if I notice the frames starting to look like the focus has changed. I am getting ready to upgrade to a focus motor that has a temperature input and I will probably set SGP to refocus on certain temperature changes. My filters are pretty close to being parfocal, so I can wait until I am done.

 

I use the Aurora Flatfield Panel and flats are a breeze with that panel. Once you run the flat calibration wizard in SGP, you put the panel on the end of your scope, set up a sequence to run your flats and you are done in a few minutes. 

 

Kaos


Edited by Kaos, 14 January 2018 - 04:07 AM.


#3 mike8888

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Posted 14 January 2018 - 10:46 PM

+1 to what Kaos stated - spot on.



#4 AutopilotEngaged

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Posted 15 January 2018 - 09:07 AM

Interesting. So you're refinding fine focus through each filter then? The flat wizard aids you with that? 



#5 RDBeck

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Posted 15 January 2018 - 01:51 PM

A follow on question, how close does the focuser position need to be for the flats to calibrate reasonably?  I think I know the drawtube position within a millimeter of correct focus.

 

One of my friends only takes flats occasionally because he never breaks down the scope, FW, camera assembly (at least not until he got a second scope).  I was thinking since it will cloudy for several days after I get my ASI 1600MM Pro that I could work on flats and flat darks (or dark flats, can't remember the "correct" order) at the temperatures I expect to operate.



#6 Kaos

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Posted 15 January 2018 - 02:35 PM

Interesting. So you're refinding fine focus through each filter then? The flat wizard aids you with that? 

I have a motorized focuser (currently JMI Motofcous with PC control). I am about to upgrade to the PrimaluceLabs Sesto Senso robotic focuser with temperature input. I try to keep my optics clean and my equipment is located in an observatory so it stays as is from night to night. Since my focuser bearing cracked, I am waiting for the replacement focuser to arrive from Orion. I use Astrodon LRGB filters and they are pretty parfocul each reaching focus within a couple of hundred reported steps in SGP.

 

SGP controls my focuser.. I generally start out finding rough focus (usually pretty close) manually operating the focuser through SGP and sometimes I run the autofocus routine. I set the sequence to refocus at the start of the sequence and with each filter change. Since I have an electronic filter wheel, when a filter change occurs, SGP will rerun the autofocus. You can also set it to run autofocus with given temperature changes (e.g. like refocus of every 2* of temperature change). This helps to maintain a really tight focus throughout the night. The JMI does not have a temperature sensor. I generally monitor the refocus while it is running and the results. If the results look less than optimal (both visually and by inspecting the SGP focus graph) I will rerun the autofocus. Ideally, if you set the filters up in SGP at the right focal steps, SGP will move the focuser to that step position when the filter changes and run autofocus using default step as a a start. If your filters are parfocal, the default steps will generally be close to each other (mine are within a couple of hundred steps). As long as the default steps are entered correctly to achieve pretty decent focus by default, there should be no issue when you take your flats. ( I periodically update my defaults in my equipment profile to account for any draw tube slippage).

 

The flats wizard in SGP will find the appropriate exposure time for each filter to achieve a predetermined target ADU value. For example, I target 21,000 ADU +/- 500 for each filter with my ASI 1600MM Cool. The wizard will save the default exposure time to your filter profile when completed (and it will do 1x1, 2x2, 3x3, and 4x4 bins all during the same wizard session if you tell it to). I only do 1x1 for my Lum and narrowband filters and do 1x1 and 2x2 for the RGB. Once my imaging session is complete, I load my flat sequence into SGP and simply tell it where to save the flats. I place the Aurora Flat Field Panel on the end of my OTA and run the sequence. It generally takes 5-10  mins to take flats for 4-5 filters. The focuser will change the to the default value for each filter if you have the defaults set up in your equipment profile. If not, it will use the last focus position. 

 

Kaos


Edited by Kaos, 15 January 2018 - 02:38 PM.


#7 Kaos

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Posted 15 January 2018 - 02:46 PM

A follow on question, how close does the focuser position need to be for the flats to calibrate reasonably?  I think I know the drawtube position within a millimeter of correct focus.

 

One of my friends only takes flats occasionally because he never breaks down the scope, FW, camera assembly (at least not until he got a second scope).  I was thinking since it will cloudy for several days after I get my ASI 1600MM Pro that I could work on flats and flat darks (or dark flats, can't remember the "correct" order) at the temperatures I expect to operate.

See my previous post. I generally only take bias, darks and flats. I take the bias and darks for each exposure time I am likely to use and different sets from the Highest dynamic range and Unity gain profiles (being these are the only two gain settings I use). I generally take flats with each session as I for one reason or another move my camera (e.g. like rotate the camera to achieve the desired target orientation in the frame) so prior flats are pretty useless. 

 

I have not had an issue with flat correction using the default values I set up in SGP. They are usually within a few hundred steps of the actual focal point used during the session (and that equates to a very small draw tube movement on my scope).

 

Kaos


Edited by Kaos, 15 January 2018 - 02:51 PM.


#8 AutopilotEngaged

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Posted 16 January 2018 - 11:31 AM

Excellent responses. I am not running any autofocus or autofilter wheels yet. One thing at a time right? :)




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