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Can I use Barlow for imaging?

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#1 hongxu chen

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Posted 22 November 2020 - 09:57 AM

Hi all.

I have a fast OTA and the FOV of my camera is very large. So, the nebula is always very small in the image. And the details are very hard to see. Can I use a Barlow to “zoom in”? Will the Barlow or Powermate shrink the telescope's image circle? Or cause the edge of the field curve?
Btw, is the Televue powermate worth to buy? They announced that it is not a Barlow. Then what is the difference?


Edited by hongxu chen, 23 November 2020 - 09:30 AM.


#2 Traveler

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Posted 22 November 2020 - 10:47 AM

When using a barlow/powemate,your focal length will getiing longer and thus your exposure time will be longer as well...

 

Also keep in mind: when doing astrophotography with longer focal length, your guiding has to be better

 

Barlows/powermates are used for planets astrophotography....



#3 sg6

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Posted 22 November 2020 - 10:48 AM

You can, but in general the final result deteriorates a bit. Also your exposures need to be longer or more of them.

 

Difference is that a barlow is basically an achromatic negative lens and so makes the image bigger by diverging it.

 

I assume that a powermate operates more as 2 achromatic lens where the first is positioned such that its focal plane is coincident with the prime image and so the output is collimated. This feeds into the second and that would have a focal length of 2x the first. Then the formed image is again 2x in size. Optically the scope it is used on should not matter either.

 

I assume the Powermate design is better, also the expansion is fixed at 2x (or whatever). Will be longer, and I expect by a reasonable amount.

 

Better warn you this is guess work as I have never pulled one apart, nor looked for the optical configuration but that is what I half expect.

 

One ignored aspect of a barlow is most are sort of generic and not for a specific scope. So don't expect match optics.



#4 Alex McConahay

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Posted 22 November 2020 - 11:16 AM

Yes, you can use a Barlow or Powermate. You get a bigger image with either. You need to make adjustments for backfocus with either. 

 

I would say the average Powermate is of better optical quality than the average Barlow. But that is mostly because the Barlow is somewhat ubiquitous, and sometimes the manufacturers do not try quite as hard as Nagler's outfit does on the Powermate. A high quality Barlow is relatively rare (although available for a price) . All Powermates are high quality. 

 

There are optical differences between the two that make the exit rays of the  Powermate identical (except in magnification) to the entrance rays. That is not true in the Barlow, where the further away you place the sensor the larger the magnification. For more, simply google "Powermate v. Barlow." 

 

Alex



#5 hongxu chen

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Posted 23 November 2020 - 09:30 AM

Yes, you can use a Barlow or Powermate. You get a bigger image with either. You need to make adjustments for backfocus with either. 

 

I would say the average Powermate is of better optical quality than the average Barlow. But that is mostly because the Barlow is somewhat ubiquitous, and sometimes the manufacturers do not try quite as hard as Nagler's outfit does on the Powermate. A high quality Barlow is relatively rare (although available for a price) . All Powermates are high quality. 

 

There are optical differences between the two that make the exit rays of the  Powermate identical (except in magnification) to the entrance rays. That is not true in the Barlow, where the further away you place the sensor the larger the magnification. For more, simply google "Powermate v. Barlow." 

 

Alex

Alex,

 

Thank you for your reply. Will the Barlow or Powermate shrink the telescope's image circle? Or cause the edge of the field curve?



#6 Alex McConahay

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Posted 23 November 2020 - 11:05 AM

I do not know the answers to those questions. I have used barlows and powermates only to magnify the central point of the field. I do that rarely with visual (preferring shorter eyepeices) and imaging (for planets, etc.). In neither application do I need to worry about the outer edges of the field of view. My impression is that they can fill the image circle, but I am just not prepared to say. 

 

Alexce




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