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which scope for deep sky imaging ?


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Hello Rob,

There's no 'one size fits all' as far as scopes are concerned.

If you want a reasonably wide field of view, such as you'd want when imaging the Orion nebula for example, and good widefield shots of M81/82, M51 etc, then an 80-100mm apo refractor would be a good bet, and there are a lot of decent ones around and they won't break the bank.

For a higher resolution, you will need a bigger refractor, or something like a newtonian, or an SCT of reasonable light grasp.

Refractors get rapidly more expensive, and heavier, the greater the aperture though.

It very much depends on what you want to image, as different types of DSO need different scopes.

The thing that will determine your particular choice is the load carrying capacity of your mount, which in the case of the CG5, is a maximum of 35lbs I believe.

For accurately guided astrophotography, the total weight of the gear you put on it...scope, cameras etc, should not be very much more than half of this, so an absolute max of 20lbs, and that may be pushing it a bit.

Give Steve or James at First Light Optics a call and have a chat with them. They'll be able to give you good advice and steer you in the right direction.

Cheers

Rob

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cheers Rob,

"good name" i was thinking of galaxies to image but am getting confussed about which scope to look at as some say reflectors others a large newt, i have a 6" sct for planets but have not got round to trying it on any thing else yet i.e. nebs or galaxies or do you think i`d need a bigger aperature for an sct ? or are sct`s no good for this type of image

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Hi Rob ( Yes, a fine name!)

A newt is a 'Newtonian reflector', so it is actually a reflector.

You can image galaxies with an SCT of course, but you'll need to lower the focal ratio as at its native F10, your 6 inch has a focal length of 1500mm, which means your guiding will need to be good as your exposures wiill all be long ones due to the faintness of the image at this focal length with only a 6 inch scope, and your field of view will be quite small.

You can however put a focal reducer on, Celestron, Meade and others do one which screws onto the rear cell of your scope, and this will drop it down to F6.3, which will give it a focal length of 945mm and is much more useable.

You will need to get an adapter to attach your camera to the focal reducer too.

Again, FLO will be able to help.

To have a look at how objects will look with your scope/camera combination, download this excellent calculator and input your camera details.

Your scope will be on the drop down menu.

The New CCD Astronomy Home Page

An 8 inch newtonian should also be fine on your mount, and this will give decent light grasp plus a sensible focal length, but you might want to try a focal reducer on your SCT first.

Cheers

Rob

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sorry ment to say refractor not reflecter above, still getting used to all the terminolgy,

as for camera i have a philips sp900nc webcam i`m using at the moment for planets, seems to be a very cheep way to get started, now i`m thinking of a dmk 21 mono with a filter wheel, what do you think ? still trying to get my head round all the imaging aspects as i thought that you needed a longer focal lenght to get more detail out of objects, or is this only for planets ? an 8" newt sounds good and are fairly cheep.

all help needed please.

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Hello Rob,

The way it works in theory is that the greater the aperture, the more resolution.

But....the best resolution you can get is limited by your seeing, and in this country, most of the time, the general opinion is that a 10 inch scope is about the max.

Planets are small, so you need long focal length in order to get a good image scale, but they are also bright, so you don't need long exposures to image them, so an F10 SCT is fine, and most lunar and planetary imagers will go to much higher focal ratios than this by using barlow type lenses.

The more you increase focal ratio, the dimmer the image.

This doesn't matter with planets, because they are bright, but DSO 's are a different thing altogether.

DSO's are many times fainter than planets, and so need long exposures. A DSO needing an exposure time of 5 minutes at F5, will need 20 minutes at F10, as a doubling of focal ratio requires 4 times the exposure.

Your image scale ( the size of the image at your camera), is a direct function of focal length.

An instrument of 2000mm focal length will produce a larger image scale, and hence a smaller field of view, than one of the same aperture but only 800mm focal length.

An 8 inch scope with a focal length of 1000mm, will produce the same image scale as one with a focal length of 1000mm and an aperture of 4 inches, but will be much brighter as it is capturing more light.

As you can see, all of these factors need to be taken into account when looking for the right scope.

Refractors tend to produce better contrast than reflectors or SCT's, because they don't have a secondary mirror, which obstructs the light path and reduces contrast due to diffraction effects.

Reflectors are cheaper per inch of aperture, and apochromatic refractors above 5 inches in aperture start to cost a phenomenal amount very quickly for each aperture increase.

Regarding cameras.

If you want to image DSO's, a DMK is not the camera for you.

These are designed primarily for lunar and planetary imaging, and have a very high frame rate in order to take advantages of short periods of very stable seeing.

For DSO's, you need either a dedicated CCD, or a DSLR.

A DSLR, matched with something like and ED80 type refractor, will give you a good field of view and be a great way to start DSO imaging.

It will be light enough not to strain your mount, you won't need to be quite so accurate with your guiding to begin with, and you can justify the purchese to the wife more easily :D

A CCD will be more sensitive, and they come in mono or colour versions, but are more expensive.

Your SPC webcam will be fine as a guidecam.

Hope this is some help and not too confusing....it can be a steep learning curve at first!

Cheers

Rob

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I have a DSLR and a mid size CCD.

The telescopes I use are a 8" Newt and 72mm refractor.

Both of these scopes are available relatively cheaply, and I find they give me enough range to cover most targets.

As Rob suggested download CCDCalc, and play around with various camera/scope combinations, and you can find which set up will offer you the greatest amount of your choice targets.

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A free and easy to use piece of software is Deep Sky Stacker.

You will be stacking many images together.

Until you start guiding you can expect a maximum of about 2 minutes per single exposure.

When you start guiding, that will shoot up.

Of course the longer the single exposure, the more faint details get picked up.

But by stacking many single exposures, you can bring out even more details, and reduce the noise that will plague a single or short stacked image!

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thanks, my brother is into his cameras, will blag a go on his camera, shall have to invite him for sunday lunch !

i guess i`ll need an adapter also to attach it, have also read that some cameras have been adapted (not sure what this means exactly) but is it needed for todays high quality cameras ?

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would you think that my Skywatcher 130p could be used for deep shy imaging ?

i know there might be a proplem with the focuser travel but it is a fast scope, F5, and has a short focul length being 650mm. would also use this on my Celestron cg5 gt mount.

what do you think ?

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