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Focal Length Question

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#1 Dave Lee

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Posted 22 June 2013 - 05:42 PM

Just a question here. Keep in mind that this is totally hypothetical.

But if you could have some "optically perfect scope" for use in astrophotography (pick your own focal ratio and assume that you have a mount that can handle whatever that thing is), what focal length would you choose. Again assume that 'you are stuck' with this choice (no reducers, barlows, other scopes, etc).

Obviously a choice of 2500mm will render M13 quite well, but not so good on M31. But that great 500mm lens (on M31) will be far less satisfying on M13. And so on.

I don't have much experience here, but 800 to 1000 mm seems to be a decent compromise here. At least for the range of objects convenient from the northern hemisphere.

Just curious as to what others think about this.

dave

#2 terry59

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Posted 22 June 2013 - 06:48 PM


I think image scale is an important factor so since you said only one scope, I'd look at multiple cameras (unless that is also not acceptable). I would probably pick a camera with a Kodak 8300 chip and one with a small Sony sensor and pair them with a Takahashi TSA-102s.

#3 CounterWeight

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Posted 22 June 2013 - 06:54 PM

As Jared mentioned in another post, the CCD choice starts to figure into this as well. Like many, I don't feel there is a one size fits all "well". Smaller aperture ~f.6 widefield using a field flattener and something like the 8300M or a DSLR for the imaging would be a great place to start though. As far as galaxies another story sort of...

I think at the end of the day - budget trumps all, but if was stuck with just one to learn on and get 'keepers' I'd go the widefield fast focal ratio and an imager with small pixels. I have another post up 'motion in the ocean' that shows somewhat the deal with sort of thing.

#4 Dave Lee

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Posted 22 June 2013 - 06:58 PM

Fair enough. And I guess you can (kind of) change fl by changing sensors.

But my choice (given the silly parameters I outlined) would be an f3 refractor (with no CA or other aberations) of fl 800 mm :-)

dave

#5 CounterWeight

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Posted 22 June 2013 - 07:02 PM

Yes, or maybe a Riccardi-Honders 200mm f/3 like the one reviewed in S&T a month or two ago. (it is my new 'dream scope')

but that is some serious $cha-ching$!

There too are the Tak FSQ's that get pretty fast with their reducers, also $$$

#6 Wouter D'hoye

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Posted 23 June 2013 - 03:42 AM

Hi,

Ideal focal length is determined by a lot of factors. First of all the focal lenght dethemines your field of view provided you only have one camera. So based on the kind of objects you want to image uou may need to make a choice already. (Broad nebulae or smallish galacies,...

If field of view is not an issue then there comes resolution. Resolution is determined by three things: focal length, pixel size and most of all seeing conditons.

My camera has 7.4ยต pixels at 1000mm fl this results in a theorethical resolution of 1.5 arcsec. if you then consider that average seeing conditions at home are 2-3 arcsec I'm pretty close to the ideal resolution for my camera. While still keeping enough field of view for general deep sky imaging.

Kind regards,

Wouter.

#7 StarDust1

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Posted 23 June 2013 - 11:17 AM

800 to 1000 mm is a nice focal ratio, depending on the camera, and type of interest. If you like galaxy's then maybe 1500 or so, is a nice focal ratio. These focal ratios are also less demanding on the mount then 2000 plus ratios.


I really like refractors for wide field. For faint fuzzy this newton seems to do really nice job:

http://tinyurl.com/m56m37v






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