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Old 10-12-2012, 10:56 PM
Langaandy (Andrew)
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Focal ratio

Hi guys,

Just a question on f ratios.

What is better for DSO and astrophotograhphy in general. It's lower isn't it. Eg f/4 is better than f/5. And does this affect viewing at all.

Also my camera is connecting prime focus

Cheers
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Old 11-12-2012, 12:04 PM
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Geoff45 (Geoff)
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Huge can of worms you're opening here. Ask three people this question and you'll get six different answers. See these links for interesting discussions:
http://www.stanmooreastro.com/SamplingFratioMyth.htm
http://www.stark-labs.com/help/blog/...ioAperture.php
The second one is the easiest to understand.

Also here is a long thread on Cloudy Nights on this topic
http://www.cloudynights.com/ubbthrea...2/Main/3085977

If you google "focal ratio myth" you'll get a lot of hits.

Geoff

Last edited by Geoff45; 11-12-2012 at 12:24 PM.
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Old 11-12-2012, 12:24 PM
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Merlin66 (Ken)
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Faster f ratios favour nebula, slower f ratios favour stars.....
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Old 11-12-2012, 01:21 PM
Langaandy (Andrew)
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okay so after a lot of reading, Cheers for the sources.

ive come to the conclusion that an 8" f/4 vs 8" f/5 for the same price is 6 of one half a dozen of the other.

i guess it comes down to the camera you are using as well.

Thanks guys
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Old 11-12-2012, 01:29 PM
LewisM
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I use a Vixen Fl102S refractor for my astrophotography. I operate it at f/9 (a little more, but may as well say f/9). It handles anything I point it at well, in fact VERY well, and is only "hampered" by a lack of aperture (I don't perceive it as much of a hampering really!).

I also have a dedicated f/4 astrograph R200SS Newtonian (8"). The exposures are shorter for a comparable result (not as aesthetically pleasing IMHO because of the darned diffraction spikes).

I have never put a focal reducer on the Fl102S (though do want one), but at present, I can live with it being f/9. I have taken better pictures with it than all the other scopes I have/had.

The only time I use the R200SS is when there is cloud about, as I can fit in more exposures in a shorter time span.
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Old 11-12-2012, 02:35 PM
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I look at it with respect to the field of view. Lower F Ratios provide a wider FOV which is good for large nebulae, whereas higher ratios are good for a closer view, for example galaxies.

Guiding requirements vary in both as well, with the lower ratios more forgiving on errors than longer focal length.
Hence why some people have RC's at F/10 as well as Newt's at F/4 or 5.
I personally prefer lower F Ratios.
You also have to deal with Coma at faster ratios.

I guess its upto the nature of objects you want to image, your guiding setup, pixel scale, etc.
Similar for viewing. Longer focal lengths are better for planetary viewing, and shorter ones for wider views. exact same applies to Eyepieces. 9mm's are good for planets and 20mm or above is suited for DSO's.
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Old 11-12-2012, 03:30 PM
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Quote:
Originally Posted by Langaandy View Post

i guess it comes down to the camera you are using as well.

Thanks guys
Yes, if you double the pixel size (or bin 2 x 2) the effect is similar to halving the focal ratio.
Geoff
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