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Newbie question: barlow+FF or just barlow?

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#1 sbharrat

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Posted 18 October 2021 - 09:42 PM

Apologies as this is a repeat of a portion of a prior post... but this is now a more specific question for which I never got an answer. Hoping by being short and specific, I can get an answer this time.

 

I have a DSO imaging setup which comprises an 80mm triplet apo w/ an asi533 camera. The FOV is hence rather large so I want to use at least a 2.5x barlow for some lunar imaging. Right now for DSOs I need to use a field flattener to keep a flat image throughout (even for the small asi533 sensor). If I am using a barlow, do I use BOTH the FF and barlow, or I just remove the FF? If the former, is it telescope->FF->barlow or the reverse? 

 

Thanks in advance...

 



#2 maroubra_boy

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Posted 18 October 2021 - 11:41 PM

Do you really need the field flattened?

Why don't you do some good old fashioned trial and error on this one to figure it it? Something very novel, do one lot of imaging without the FF, & a second lot with it and see what gives. Same night so conditions are as much the same as possible. You can also see if there are not other artefacts introduced in to the images both with and without the FF. This is something only you can determine as you may not have the same gear as me. I found out one particular Barlow of mine is no good with the Moon, something only I would have found out by trial and error.

Alex.
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#3 sbharrat

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Posted 19 October 2021 - 08:14 AM

Do you really need the field flattened?

Why don't you do some good old fashioned trial and error on this one to figure it it? Something very novel, do one lot of imaging without the FF, & a second lot with it and see what gives. Same night so conditions are as much the same as possible. You can also see if there are not other artefacts introduced in to the images both with and without the FF. This is something only you can determine as you may not have the same gear as me. I found out one particular Barlow of mine is no good with the Moon, something only I would have found out by trial and error.

Alex.

Yep, of course. I just assumed that this was a basic question for lunar imaging and I was going to quickly get something like "don't be an idiot. Of course you don't use a barlow and field flattener together" grin.gif



#4 maroubra_boy

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Posted 19 October 2021 - 08:31 AM

By my book the only dumb question is the one that isn't asked.

I'm just trying to inspire a bit mongrel experimentation from you. To just have a go to find out for yourself. A lot of good people post here, but a lot of good people are also mistaken about many things because they read about something that was mistaken to start your they didn't have a full handle on things to begin with. Here you are presented with an opportunity to figure something out for yourself. I'm not suggesting any result or another will happen. Instead I will suggest that all optical components are active and will have an effect, just not always what you expect.

Alex.
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#5 ch-viladrich

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Posted 19 October 2021 - 02:36 PM

Hello,

 

Some field flatteners reduce a little bit the quality at the center of the field in order to improve the quality at the edge of the field. This is not a problem for DSO imaging since the resolution of the image is not very high.

 

However, for high resolution imaging, e.g planetary imaging, you look for the best possible quality on the optical axis (center of the field). So, no corrector or field flattener for planetary imaging ;-)

 

Lunar imaging is a bit in between. The answer depends on the field of view of the camera / Barlow combination (large or small), on the sampling (fine or coarse), and on the size of the diffraction limited field of the telescope without the corrector. You also have to consider that some Barlow flatten the field, while others don't.

 

So, the short answer is ... try with or without it :-)

 

In any case, the Barlow come after the field corrector/flattener.


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#6 sbharrat

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Posted 19 October 2021 - 02:56 PM

Hello,

 

Some field flatteners reduce a little bit the quality at the center of the field in order to improve the quality at the edge of the field. This is not a problem for DSO imaging since the resolution of the image is not very high.

 

However, for high resolution imaging, e.g planetary imaging, you look for the best possible quality on the optical axis (center of the field). So, no corrector or field flattener for planetary imaging ;-)

 

Lunar imaging is a bit in between. The answer depends on the field of view of the camera / Barlow combination (large or small), on the sampling (fine or coarse), and on the size of the diffraction limited field of the telescope without the corrector. You also have to consider that some Barlow flatten the field, while others don't.

 

So, the short answer is ... try with or without it :-)

 

In any case, the Barlow come after the field corrector/flattener.

Thank you! Exactly what I didn't know...




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