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Focal Ratio: What difference does it make?


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While reading (and participating) in this thread I started to wonder just how much difference focal ratio made to performance. From my recollection it was very little, but I realise that I have never done a comparison on the same night, and while a side by side will be impossible I will be doing observations in quick succession.

The scopes used will be the ST80 (F/5), ED80 (F/7.5) and Vixen 80M (F/11.375) and the required comparisons will be gathered over the couple of months, clouds permitting.

For targets I'm thinking things like the Moon, Jupiter, M45 and M42, some nice open clusters and perhaps some double stars. With each telescope I'll use the same eyepiece, which will be any out of this list:

TeleVue: 24mm Panoptic, 14/12mm Delos, 32/25/20/15/10.5/8mm Plossl, 3-6mm Nagler Zoom

VIxen: 50*/30/18/7/2.5*mm LVs

*It's unlikely that I'll use these, as they are on the extremes of either side, but I could give them a go in the relevant scopes.

I'm anticipating I'll find little to no difference, but I'm looking forward to it anyway :). Obviously it is not entirely a fair test because it is two achromats and one doublet, but I hope I can look past that (CA will probably be ignored for testing purposes).

Edited by Naemeth
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It's not just the focal ratio which matters, it's the design and optical quality. The ED80 will bw the best by some way due to it's CA corrected optics, the ST80 will be the worst due to f5 being demanding for an achromatic objective. The f11.75 will show some CA and may be acceptable on moderate targets.

CA is important as in large amounts it will affect contrast and clarity as well as providing a poor observing experience.

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It would be good to see a comparison at ~2mm exit pupil using your televue plossls. Then you're really taking it down to just focal ratio if you do that comparison.

I suppose 11mm with st80; 15mm with ed80; 25mm with 80M. Not exact but close enough

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It's not just the focal ratio which matters, it's the design and optical quality. The ED80 will bw the best by some way due to it's CA corrected optics, the ST80 will be the worst due to f5 being demanding for an achromatic objective. The f11.75 will show some CA and may be acceptable on moderate targets.

CA is important as in large amounts it will affect contrast and clarity as well as providing a poor observing experience.

It is indeed, which is why in part I'm worried this won't be a fair test at all.. 

I'll be sure to include CA, and definitely a disclaimer about quality of optics :).

It would be good to see a comparison at ~2mm exit pupil using your televue plossls. Then you're really taking it down to just focal ratio if you do that comparison.

I suppose 11mm with st80; 15mm with ed80; 25mm with 80M. Not exact but close enough

Ah yes, I'll do that too. Thanks Jim :)

Edited by Naemeth
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Focal ration basically means that any image taken at that focal ratio will/should have the same exposure parameters as another at that focal ratio.

In effect the image is the same brightness/intensity on the sensor.

On different focal lengths the image will be bigger or smaller.

On the assorted optical designs other aberrations may be more or less apparent.

One aspect that I suppose goes with focal ratio is the curvature of the image, so fast systems can be expected to have a higher degree of image curvature.

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I'm anticipating I'll find little to no difference, but I'm looking forward to it anyway :). Obviously it is not entirely a fair test because it is two achromats and one doublet, but I hope I can look past that (CA will probably be ignored for testing purposes).

Yep, that's why I suggested the same exit pupil, all 80mm scopes so at 2mm exit pupil 40x mag. In the 2 achros like Mr Spock said you'll see differences in CA which will kill sharpness and differences in eyepiece corrections and curvature like Ronin said.

Longer FR should be more robust against bad seeing conditions so maybe another test for the ED80 and 80M. Both should have good CA correction but if you can sort out a 0.8mm exit pupil with same range eyepieces on Jupiter on an average night it could be interesting to see if you can tell there's a difference.

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Focal ration basically means that any image taken at that focal ratio will/should have the same exposure parameters as another at that focal ratio.

Focal ration = the number of scopes that your partner will tolerate at one time  :grin:

Sorry ronin  :smiley:

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Focal ration = the number of scopes that your partner will tolerate at one time  :grin:

Sorry ronin  :smiley:

No-one seems to believe the line 'but they all do different things' in response to, 'why do you have 6 (soon to be 7) telescopes?' :rolleyes:.

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Focal ration = the number of scopes that your partner will tolerate at one time  :grin:

Sorry ronin  :smiley:

:laugh: I think I have a bit of room there. Only yesterday telling the missus I had bought my pentax 10.5mm, but she was complaining  she wanted a bit more FOV last week on the double cluster and some others. I stressed I had bought the pentax instead of the 28mm Maxvision while it turned up. Now she has offered to get he 28mm for Xmas if I really want one ...  tempting :0)

Great to have partners that understand this hobby, so far rationing has not entered the equeation ... yet.  :D 

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Has she got any sisters?

RL

Ha, not anymore, as soon as I started astronomy they all got sold on astro buy and sell.  Sold out like Televue plossls as if they going for 40 pounds a shot, you know, they appear for a few hours, then gone  :0)

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No-one seems to believe the line 'but they all do different things' in response to, 'why do you have 6 (soon to be 7) telescopes?' :rolleyes:.

My father in law can't quite understand why I won't give/sell him my Celestron 130SLT when I have 4 other scopes (including the 10" LX90, which everyone who sees it thinks must be the scope to put all the others out of commission, even though it's probably my least used these days). I tell him "they do different things, mate" but I know he doesn't understand. Funny that people think the 10" must show little green men smiling and waving back at you!

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