Prime Focus / Barlow

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Prime Focus / Barlow

Postby steve-munsey » Wed Sep 30, 2015 8:47 am

I know how to figure magnification with lenses. FL of scope divided by FL of eyepiece.

In prime focus Astrophotography how do you compute magnification? example 8" f10 2000mm focal length with Camera at prime focus?
what if I add a Barlow between camera and scope?
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Re: Prime Focus / Barlow

Postby edward-fraini » Tue Oct 06, 2015 12:54 pm

I have talked to imagers at TSP about this and got two answers, one is a rule of thumb I think and what he said was to divide your telescope focal length by 50. I have no idea where the 50 comes from. My CCD vendor said to divide the telescope focal length by the diagonal of your imaging chip. You will have to do a little digging to find this value. If I do that for my FSQ and my CCD Chip I get a value of 3X.....and that seems wrong.

So I am interested in some more input of this as well.

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Re: Prime Focus / Barlow

Postby edward-fraini » Tue Oct 06, 2015 5:33 pm

OOOOPS

Had a math error, my calculation now yields 23X which IS believable.

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Re: Prime Focus / Barlow

Postby steve-fung » Thu Oct 08, 2015 7:05 am

Hi Ed. The “magnification=focal length/50” rule comes from the fact that for a 35-mm film/full frame sensor format with 24 x 36 mm image dimension has a “normal lens” focal length that can be approximated by 50 mm (defined by Oskar Barnack, creator of the Leica camera). Your CCD vendor is also correct in that a lens with a focal length about equal to the diagonal size of the film or sensor format (assuming aspect ratio ~ golden ratio of 1.62) is considered to be a normal lens. For a 35-mm film/full frame sensor format, the image diagonal is 43.3 mm, so any lens with 40-50 mm focal length would be a good approximation of a “normal lens”. For 4/3 digital format, you would use a 22 mm lens. For APS-C or DX digital format, you would use a 27-28 mm lens. “Normal” here is defined as a field of view angle of about 53° diagonally that looks natural to a human observer under normal viewing conditions. With “normal focal length” defined as 1x magnification, it is easy to see that using a lens of other focal length would have a magnification of “lens focal length/normal focal length”. A telescope set up as prime focus is essentially a giant camera lens. To calculate magnification with a Barlow lens, just multiply the telescope-camera magnification by the Barlow magnification to get the overall magnification of the telescope-Barlow-camera setup. Hope this helps.
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Re: Prime Focus / Barlow

Postby edward-fraini » Fri Oct 09, 2015 6:30 pm

Ok, I think I understand. if so then I would postulate that the reason the rule of 50 and the diagonal calculation do not yield the same answer is that the chip I have deviates significantly from the 1.62. My chip is close to a 1.3 to 1 ratio.

I do understand the principal so thanks for the knowledge share.

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Re: Prime Focus / Barlow

Postby steve-fung » Sun Oct 11, 2015 1:16 pm

Just to clarify, “normal focal length = 50 mm” for 35-mm full frame format and “normal focal length = image diagonal” are simply operational definitions of “normal focal length”, which creates an image that approximates the human central field-of-view or cone-of-attention (varies 40-60° but on average 53-55°). I found two websites that explain this better.

http://petapixel.com/2012/11/17/the-camera-versus-the-human-eye/
http://www.cambridgeincolour.com/tutorials/cameras-vs-human-eye.htm

I personally prefer “normal focal length = image diagonal” rule, which is more generic, so that for a 35-mm full frame image sensor, which has image dimension of 24 x 36 mm, the image diagonal is 43 mm, so technically its normal focal length is 43 mm, not 50 mm, but no one makes a 43-mm camera lens. Even this is not a precise definition if a sensor’s aspect ratio deviates from 1.6 or if an image is cropped. For example, I use a full frame DSLR for astrophotography but only use the central square of the image sensor (24 x 24 mm or less) because of vignetting. Instead, I think it is more precise to define normal focal length as that resulting in 53-55° (0.93-0.96 radian) field-of-view on your final image as 1x magnification, and go on from there in your calculations.
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