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Shooting Flats With Different Filters

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#1 Juno18

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Posted 05 May 2021 - 07:24 PM

Seems like shooting flats has been involved in several recent topics.

 

I am really looking forward to getting out tomorrow night for some imaging. Haven't had a good night since 4/11. 

 

I would like to shoot several hours on M101 with the L-Enhance filter to collect some red channel data to add to the image that I recently posted here https://www.cloudyni...9252-m101-help/.

 

Then, slew to M13 for some data using my IDAS LPS D-1 lp filter. Possibly repeating the same two targets the following night if I am so lucky to get two nights in a row!

 

My dilemma is how do I shoot flats for each filter? Usually, I shoot flats the next day after imaging. I have never swapped filters during a night of imaging. I use a ZWOfilter drawer.

 

Currently, the IDAS lp filter is in the filter drawer. I would change to the L-Enhance tomorrow and focus before starting the sequence in N.I.N.A. 

 

I really don't want to fumble around at 1 a.m. trying to shoot flats with my ipad after imaging M101 with the L-Enhance, then slewing to M13, focusing, and imaging M13 with the IDAS lp filter.

 

Does anyone have any suggestions how I can "comfortably" shoot flats for each filter? Is focus critical for shooting flats? Can I install the L-Enhance tomorrow and shoot flats/dark flats, image M101, swap filters to the IDAS LPS D-1, image M13, then shoot flats with the IDAS filter the next day?

 

Thanks!



#2 kathyastro

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Posted 05 May 2021 - 07:50 PM

You need separate flats for each filter.  Focus should remain unchanged from the imaging session: infinity focus. 

 

You definitely do not want to change filters between the imaging and the flats!  Whether you unscrew filters or slide a drawer, it would be difficult to ensure that the filter was in exactly the same orientation and position for the flats.  If it is not in exactly the same position, then, instead of eliminating dust spots, the flats will make then look 3-dimensional.

 

Either stick with one filter for the entire night, or shoot flats before changing filters.

 

This is why most imagers use electronic filter wheels.  You can change filters and the electronics will reproducibly position the filter in exactly the same spot every time.


  • NMCN, Juno18 and bignerdguy like this

#3 Juno18

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Posted 05 May 2021 - 07:58 PM

You need separate flats for each filter.  Focus should remain unchanged from the imaging session: infinity focus. 

 

You definitely do not want to change filters between the imaging and the flats!  Whether you unscrew filters or slide a drawer, it would be difficult to ensure that the filter was in exactly the same orientation and position for the flats.  If it is not in exactly the same position, then, instead of eliminating dust spots, the flats will make then look 3-dimensional.

 

Either stick with one filter for the entire night, or shoot flats before changing filters.

 

This is why most imagers use electronic filter wheels.  You can change filters and the electronics will reproducibly position the filter in exactly the same spot every time.

Thanks Kathy! That is what I thought, but was hoping for something unrealistic!

 

I wasn't 100% sure though and you told me just what I needed to hear!

 

 Thank you much!




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