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i should use proper backfocus distance for planetary imaging right?

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9 replies to this topic

#1 adamphillips

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Posted 19 October 2019 - 08:36 PM

this is a very dumb question, but I have an edge 11hd. ive done some planetary with it before. it seems like it could do better.

would using celestrons adaptor and getting the perfect backfocus distance help?

 

I have an ASI178mc. I could order a couple adapters to get it right.

I think the reason I didn't pay attention before is because people use all kinds of barlows on planetary so I assumed they didn't have stuff perfect, but maybe they do.



#2 jimr2

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Posted 19 October 2019 - 10:47 PM

Correct backfocus distance...? It can't hurt....

 

-jim-



#3 Dan Crowson

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Posted 19 October 2019 - 11:04 PM

This is probably a better fit in the solar system imaging forum. Let me know if you'd like me to move it.

Dan


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#4 t_image

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Posted 19 October 2019 - 11:09 PM

this is a very dumb question, but I have an edge 11hd. ive done some planetary with it before. it seems like it could do better.

would using celestrons adaptor and getting the perfect backfocus distance help?

 

I have an ASI178mc. I could order a couple adapters to get it right.

I think the reason I didn't pay attention before is because people use all kinds of barlows on planetary so I assumed they didn't have stuff perfect, but maybe they do.

Sorry, I can't relate to your dilemma.

?Don't you have some function where you have the live view and can aim at something simple like the Moon?

It should be easy enough to simulate any back focus distance by hand holding the camera in the optical train and seeing if your focus improves or not [as you move it towards or away from the scope].

such is a simple test method for determining back focus distance without having the proper fittings on hand............


Edited by t_image, 19 October 2019 - 11:09 PM.


#5 Stelios

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Posted 20 October 2019 - 12:53 AM

Without a flattener in the imaging train, as long as you can bring the camera to focus, you're done. You probably *should* be using a barlow. SCT's have a very large focusing area--but if you use a flattener or reducer, you are restricted to that the focal point must be a specific distance *from the reducer*. This is not the case with planetary imaging.

 

Your main challenge should be focusing. You can focus on the object (planet) but it may be easier to focus on a nearby bright star using a Bahtinov mask, then sync your mount, then slew back to the planet. When you sync on a nearby object (how you do it depends on mount) the slew should place you close enough to view the object in the camera. If not, you can always fall back on a flip-mirror diagonal. 



#6 adamphillips

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Posted 20 October 2019 - 10:34 AM

This is probably a better fit in the solar system imaging forum. Let me know if you'd like me to move


Dan


ya Dan, could you move it?

#7 adamphillips

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Posted 20 October 2019 - 10:40 AM

Without a flattener in the imaging train, as long as you can bring the camera to focus, you're done. You probably *should* be using a barlow. SCT's have a very large focusing area--but if you use a flattener or reducer, you are restricted to that the focal point must be a specific distance *from the reducer*. This is not the case with planetary imaging.

Your main challenge should be focusing. You can focus on the object (planet) but it may be easier to focus on a nearby bright star using a Bahtinov mask, then sync your mount, then slew back to the planet. When you sync on a nearby object (how you do it depends on mount) the slew should place you close enough to view the object in the camera. If not, you can always fall back on a flip-mirror diagonal.

thanks for that. I wonder if there is anybody with a counter argument, one thing I do remember is the celestron white paper did mention a specific back focus for the edge 11. they even made a camera adapter just for that purpose.
maybe it is because the edge has that built in flattener. but I never used that distance when I was starting with my dslr and it seemed to work fine

Edited by adamphillips, 20 October 2019 - 10:50 AM.


#8 AhBok

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Posted 20 October 2019 - 11:53 AM

You should use the proper backfocus with the Edge for best coma correction and flat field. I’m not sure how important that will be practically for planetary imaging, but it should be considered a “best practice” with the Edge.

#9 Tom Glenn

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Posted 20 October 2019 - 12:02 PM

Basically, it won't matter.  Planetary imaging is done on-axis, and the entire image is captured within a few mm of the central axis.  You won't notice any consequences to the image quality.  If possible, it can't hurt to get close to the proper back focus, but in planetary imaging (and unlike in other types of imaging) there are usually many elements in the imaging train (barlows and ADCs) which make achieving the precise distance impossible.  


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#10 RedLionNJ

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Posted 21 October 2019 - 10:52 AM

+1 on what Tom G said, above.

 

A non-sharp planetary image is going to be 99.9% due to factors other than a precise back-focus.  Eliminate everything else (including seeing, poor focus, lack of ADC, etc.) - then revisit backfocus.




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