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f6.5 vs f7, how much of a difference?

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#1 Neinball

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Posted 22 June 2021 - 02:10 AM

From an imaging standpoint, how much of a difference will half a stop in aperture make? I'm looking at two refractors, both have the same focal length, but one is a half stop faster, and I'm wondering how much of a reduction in exposure time that might translate into if any.

#2 premk19

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Posted 22 June 2021 - 02:21 AM

The difference is not half a stop (50%), it's only about 14%. (Ratio of the squares of the two numbers)

Since the focal lengths are the same, the f/6.5 scope should also have a bigger objective size, so it's the better choice as it's both bigger and faster.
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#3 daveco2

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Posted 22 June 2021 - 05:33 AM

One benefit of a faster scope is that you can shoot for shorter time to collect the same signal.  A 14% reduction in shutter time (square of f number ratio as prem points out)  is not very significant. 

 

At the same focal length, the weight of the objective would go up by 14% (prem number again) to which would be added increase in scope tubing, to get total weight increase - possibly of significance depending on mount payload capacity.



#4 vio

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Posted 22 June 2021 - 06:50 AM

First off, half a stop is not from 6.5 to 7, in photographic terms. A full stop is when the amount of light doubles, that would be from 7 to 4.9 (dividing 7 by square root of 2). To be slightly more accurate on how this is expressed, aperture would be 1:7, so one extra stop (2x more light) is multiplying by square root of 2, and it is 1:4.9.

Anyways, the ratio of light reaching the sensor would be 7/6.5 squared (1.16) so your gain is 16%.
This would not be a significant factor for me to decide on. Scope glass, construction, focuser, even weight and length, would likely be more important for me. Depending on your setup, some could be more critical than others.


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Edited by vio, 22 June 2021 - 06:52 AM.

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#5 Neinball

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Posted 22 June 2021 - 05:52 PM

One benefit of a faster scope is that you can shoot for shorter time to collect the same signal. A 14% reduction in shutter time (square of f number ratio as prem points out) is not very significant.

At the same focal length, the weight of the objective would go up by 14% (prem number again) to which would be added increase in scope tubing, to get total weight increase - possibly of significance depending on mount payload capacity.


Weight is a non issue for me, I've got both an EQ6 and a G11, thing I was wondering about is if that small difference in aperture would drop my exposure times down enough to justify the extra cost of the f6.5 over the f7.

#6 Neinball

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Posted 22 June 2021 - 05:58 PM

First off, half a stop is not from 6.5 to 7, in photographic terms. A full stop is when the amount of light doubles, that would be from 7 to 4.9 (dividing 7 by square root of 2). To be slightly more accurate on how this is expressed, aperture would be 1:7, so one extra stop (2x more light) is multiplying by square root of 2, and it is 1:4.9.

Anyways, the ratio of light reaching the sensor would be 7/6.5 squared (1.16) so your gain is 16%.
This would not be a significant factor for me to decide on. Scope glass, construction, focuser, even weight and length, would likely be more important for me. Depending on your setup, some could be more critical than others.


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Yeah, trying to weigh the pros and cons of two options at the moment, I've got the chance to buy a used TS 107 right now, or I could hold out and hope their new FPL55 102mm comes back in stock, the 107 has a good reputation and FPL53 glass, but the FPL55s come with a guaranteed strehl and supposedly better figure, not to mention several hundred cheaper. Trying to figure out if that extra bit of aperture is worth the extra $300 over the 102.

Edited by Neinball, 22 June 2021 - 06:00 PM.


#7 vio

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Posted 22 June 2021 - 10:59 PM

Glass difference, better specs, I’d pay extra for those it I believed it was worth the price for me, in results, convenience, craftsmanship, reputation, or just the satisfaction of getting the scope I wanted.

Aperture difference, not as much. The extra money would find good use towards improved guiding, focusing, software tools like APP or PI, online classes on astro image processing and so on. Not suggesting you have one of these in need of improvement, I’m just showing what I’d put ahead of aperture.
In the end, your priorities may be very different, not to mention you may have all these covered better than I have.
Good luck with your decision!


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