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Luke

Refractors, aperture size for imaging?

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Hi!

Does aperture size matter much for imaging?

If you had a 66mm refractor and and 120mm refractor and they both had good optics, would there be much difference between them for imaging?

Cheers!

Luke

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Luke, I've been using 80mm fracs and they have both been working great. Ok, one of them has much better optics than the other. The main difference is likely to be the focal length providing massively different fields of view, therefore the 66 is going to be great for wide starfields and large numbers of fuzzies, albeit very small, whereas the 120 is going to get you in closer to the fuzzy...

I know the resolution is controlled by the aperture, but I'm not sure at the scales you're talking about it's going to make too much difference, although the 120 giving you the closer view is going to get better resolution.

I'm sure there's bound to be something, or several, I've missed :icon_rolleyes: but someone will be able to answer whatever it is soon.

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The main difference is likely to be the focal length providing massively different fields of view, therefore the 66 is going to be great for wide starfields and large numbers of fuzzies, albeit very small, whereas the 120 is going to get you in closer to the fuzzy...

Ah thanks John, that's making sense to me!

Cheers,

Luke

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Luke,

I think Celescope is using two refractors as his main imaging set up. I think they are the Equinox 120 and the Equinox 80. Its always good to look at the gear some of the imagers are using.

John

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The F ratio of the scope will dictate the length of time you will need to expose a target. The lower the F ratio the shorter amount of time you will need to set the exposure to get the same level of brightness. The F ratio is a square law so for example an F4 scope will require half the amount of exposure of an F5 scope.

If you had two scopes one 8 inch apeture F5 focal ratio and the other 4 inch apeture F5 focal ratio. They would both capture the same brightness of image but the 8 inch scope would be able to resolve more detail.

Regards

Kevin

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I have seen some awesome AP shots from the little 66.

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If you had two scopes one 8 inch apeture F5 focal ratio and the other 4 inch apeture F5 focal ratio. They would both capture the same brightness of image but the 8 inch scope would be able to resolve more detail.
The 8 inch will capture 4 times as many photons and have twice the signal-to-noise ratio per spatial element on the sky. Or to put it another way, at the same resolution the 8inch will do much better than the 4 inch.

NigelM

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I think the example given above really depends on whether they are both same 'type' of scope (i.e. reflector or refractor). If it's an 8" reflector vs a 4" refractor the difference may not be so dramatic due to such factors as optical path and mirrors vs lenses (the latter usually providing better contrast). However, I'm sure someone here will correct me if I'm wrong?

Note: I'm sure I read somewhere recently that a 4" refractor is the equivalent of an 8" reflector (could possibly have been Sir PM but I'm not sure).

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If you visit my Flickr site (www.flickr.com/photos/lesgranges) and open the nebulae set you can compare the widefield (rather pink) Horsehead/flame image in the ZS66 with the close-up of the horsehead taken in a Meade 127 refractor. You see the difference in image scale that way. You can't say one is better than the other, it's just the difference, if you like, between a wide angle lens and a telephoto. (The widefield image was taken on a poor night so I had to lose most of the green, hence the dodgy colour! Don't blame the 66.)

I don't know the Skywatcher 120 doublet as an imaging scope. I suspect it will be good. I'm delighted with our Meade 127 triplet though. It's anther one to consider.

Olly

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Thanks very much for the advice!

Olly, the image comparisons make it nice and clear, thank you. Your M101 shot is awesome btw! I think I would pass out if I ever managed to take an image that nice!

Celescopes images are great to check out, thanks John. We had been thinking about the Equinox already, so it's good to see what it is capable of in expert hands.

Cheers,

Luke

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Luke, you'll get there. There is no magic to it, just a lot to remember. Good focus and lots and lots of exposuure time and then double it!

Olly

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Thanks for the encouragement, Olly!

Cheers,

Luke

Luke, you'll get there. There is no magic to it, just a lot to remember. Good focus and lots and lots of exposuure time and then double it!

Olly

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