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# Magnification question.

4 replies to this topic

### #1 mikeDnight

mikeDnight

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Posted 18 April 2024 - 02:06 AM

Usually I observe the Moon by attaching a 2X Ultima SV Barlow to the nose piece of my Maxbright viewer. This gives me great views while still being able to use long focal length eyepieces for comfort and eye relief. Last week however, I decided to buy my first GPC which has a amplification of 1.7X. I like the GPC as it gives me great low power views of the moon when I use my 35mm Ultima's and 25mm Parks Gold. But I hate that it's so fiddly, as it screws into my diagonal prism, which means I've to screw it in or screw it out depending on what I want.
Last night I fit the 1.7X GPC to my diagonal, but rather than unscrew it so I could up the power by using 2X Barlow, I simply added the Barlow as well. So the GPC was in my diagonal while I'd attached a 1.25" nose to my binoviewer, and the 2X Barlow sandwiched in-between. It was a wonderful sight and a very nice experience using the 35mm's & 25mm's with their large eye lenses and long comfortable eye relief at such high powers. So here's my question. What is the best way to calculate the magnification with this combination?

### #2 Astronomigo

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Posted 18 April 2024 - 04:49 AM

Start by measuring your FOV.  For example:

https://www.cloudyni...-fov/?p=6871340

Then, knowing the field stop of your eyepiece in mm, you can get your working focal length. The FOV is given by the field stop divided by the working focal length. That result is in radians, and to convert to degrees, you would multiply by 360/2pi. So we have 1 radian is 57.3 degrees.

Now, the working focal length is given by the field stop times 57.3 degrees divided by the FOV (in degrees). As you probably already know, your magnification is given by the working focal length divided by the eyepiece focal length.

Now just plug and chug

Edited by Astronomigo, 18 April 2024 - 04:53 AM.

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### #3 Redbetter

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Posted 18 April 2024 - 05:38 AM

Drift time across the center of the field with a known star for determining true field of view.  Assuming the eyepiece has a known field stop, you can then calculate the effective telescopic focal length.  You do need to apply the cosine correction for the declination (on date) of the star from the celestial equator.

I do several runs for a star on/near the meridian to make sure I am getting the start and finish accurately, and to make sure the star is passing through the center of the field.  Generally, the longest value will be correct, because the most likely errors are not being quite centered, starting the timer too late, or stopping too early--all of which bias to shorter elapsed times.  It is rare that I wait too long to stop the timer, and if I do I am aware of it and throw that reading out.

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### #4 betacygni

betacygni

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Posted 18 April 2024 - 06:41 AM

Drift time across the center of the field with a known star for determining true field of view. Assuming the eyepiece has a known field stop, you can then calculate the effective telescopic focal length. You do need to apply the cosine correction for the declination (on date) of the star from the celestial equator.

I do several runs for a star on/near the meridian to make sure I am getting the start and finish accurately, and to make sure the star is passing through the center of the field. Generally, the longest value will be correct, because the most likely errors are not being quite centered, starting the timer too late, or stopping too early--all of which bias to shorter elapsed times. It is rare that I wait too long to stop the timer, and if I do I am aware of it and throw that reading out.

I go a step easier, time the drift in mono mode, then with same eyepiece try again this time with binoviewers and given Barlow/glasspath, divide the mono number by the binoviewer/amplifier number is your amplification. For example if it takes 60 seconds in mono, then 30 seconds with binoviewer/glasspath setup, you have a 2x amplification value. Nice thing about this method is you don’t have to know anything about the eyepiece FOV or do any fancy math. Doesn’t matter which star you use or where in sky, as long as it’s the same for both timings.

Edited by betacygni, 18 April 2024 - 06:46 AM.

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### #5 mikeDnight

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Posted 18 April 2024 - 04:06 PM

Thanks guys. Youve all been very helpful!

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