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Will comet move off FOV when imaging too long?

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#1 mannys

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Posted 27 May 2024 - 05:04 PM

Hi CNers!

 

I'm planning to image comet C/2023 A3 (Tsuchinshan-ATLAS). I will be using sidereal tracking, which means tracking the stars instead of the comet head. My question is: if I image too long a time period, say 3 hours, wouldn't the comet head eventually move off the FOV since it's moving faster than the stars being tracked? Specially if I nudge the scope to move the head off the FOV center and nearer to the edge of the FOV, in order to show more of the tail? So does this mean I am limited as to how long I can image, like instead of 3 hours I can only do 1 hour? 

 

Some info: I'll be using a 12.5" scope, and the FOV is .8°x.53°. The comet's hourly motion is 01'20".

 

Another question: should there be a delay between frames? If so, why?

 

Thanks a whole bunch!


Edited by mannys, 27 May 2024 - 05:16 PM.


#2 t-ara-fan

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Posted 27 May 2024 - 05:29 PM

FOV is 48' wide.  In 3 hours it will move 4'.    So it will not move out of the frame of view during the entire night.  The comet is moving but it isn't a FAST comet. Yet.

 

I don't delay between frames.  Shoot back to back images.  You can always process just every nth image if that gives better star rejection.  I shoot short subs i.e. one minute so the comet doesn't trail during the exposure.  In a minute it moves 1.3 arc-seconds, so if that is your image scale any longer will blur. 


Edited by t-ara-fan, 27 May 2024 - 05:31 PM.

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#3 acrh2

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Posted 27 May 2024 - 05:32 PM

If you know how fast the comet moves relative to the stars, you can plan accordingly - how short your exposure should be so that the comet moves only a single pixel across your sensor frame, and whether or not you will have to recenter the mount to the target comet over the length of the imaging session.

 

You can get the speed information from a planetarium app like Stellarium or CdC. The most common scenario is that the exposure length would be on the order of minutes, and you would not have to recenter over several hours. But the exact numbers depend on the particular target and your imaging system. 


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#4 danny1976

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Posted 27 May 2024 - 06:10 PM

I would not recenter when imaging. This can get you into trouble when aligning the images on the comet. Dithering is not a problem.

 

Just take a few exposures and watch how the comet moves. Then position the scope so the tail is in the fov and start imaging. If you have a total of 3h you’ll have a great picture. I never got that far.


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#5 Luna-tic

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Posted 27 May 2024 - 06:37 PM

This subject interests me, I'd like to try for the comet this fall also. Can you shoot it like you would do planetary? How about guiding on the comet itself instead of the background field, is that possible?

 

I'll probably use my Edge 8" with a .7 reducer, ASI533MC pro and OAG with ASI120MM. If that is too much FL, I can do a C6 with or without a .63 reducer, or a 3" 478mm APO. Something will fit.

 

I managed a few single image exposures on Neowise a few years ago with a crop sensor DSLR in the Edge8, they came out pretty good unguided, but with the expected star streaking. Best I can remember, I was shooting 25 sec  exposures at ISO 6400.



#6 t-ara-fan

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Posted 29 May 2024 - 02:36 PM

This subject interests me, I'd like to try for the comet this fall also. Can you shoot it like you would do planetary? How about guiding on the comet itself instead of the background field, is that possible?

 

I'll probably use my Edge 8" with a .7 reducer, ASI533MC pro and OAG with ASI120MM. If that is too much FL, I can do a C6 with or without a .63 reducer, or a 3" 478mm APO. Something will fit.

 

 

Why wait?  C/2023 A3 already has a tail. Now is a great time to work your way up the learning curve.

 

Right now: your Edge-8, 0.7x,  and '533MC would be a good combo.  Guide on stars, 60 second subs.  Hopefully in the fall you will need to use the wider field of the 3" APO to get the entire comet in the field of view. 
 

Here is my recent shot @ 1,000mm FL.  https://www.cloudyni...ail/?p=13476750

 

I want "before and after" pics so I got some "before pics" this past weekend. 




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