Would DSLR questions be welcome here?

colin bailey:
Hi I was intending to get a second hand one 300 - 400 pounds I already have a celestron 9.25  so don't wat expense of another scope

I thought we could multiquote here, I guess I'm wrong.

Colin, I've only just started to astroimage, but I've learned quite a few things.  First, you can never have too many telescopes.  Secondly, a good camera lens CAN work well, but may not be better than a new or used scope.  Since you've only been here for three days, I don't really know WHERE you are from.  To image M31 on to a crop sensor size, my Sony A58 or A77, I have a used Tamron 200-400 f/5.6 which would be pretty good, but for not much more money than I spent on that, I purchased a use William Optics ZenthStar 71.  Both fall in the same 400mm focal length category, and surprisingly, both of them have close in aperture size.  There is a LOT wrong with this image, dust on the sensor, out of focus, poor post processing, etc.  But, you can start to make out the edge of the galaxy to show you what that WO 71 telescope can do.

http://www.astrobin.com/204437/?nc=user

I'm going to take a guess, and if I'm wrong, I apologize, but I'll say the U.K. for your location.  For actually about the same cost as an used Canon 70-200, you could pick up this telescope, instead.

https://www.telescopehouse.com/telescopes/telescopes-by-type/refracting-telescopes/bresser-messier-ar-102s-600-hex-focus-optical-tube-assembly.html

You'll need a 'T-Ring' to connect to the camera.  This is the basic interface between the camera and everything else.

https://www.telescopehouse.com/astro-imaging/digital-slr-adaptors/canon-eos-t-ring.html

This camera adapter screws on the rings of the T-Ring,  adding a 'nose' to the camera, and then installs in the focuser of the telescope.

Since the Bresser has a 2" focuser, you can also use an Explore Scientific 2" Camera adapter.

https://www.telescopehouse.com/astro-imaging/camera-adaptors/explore-scientific-adapter-2-on-t2.html

By the way, Bresser and Explore Scientific are owned by the same parent company.

These three things will get you a nice 600mm telescope, which if you use Stellarium, you can actually model the telescope and a crop sensor camera (or a full sensor camea!)  You will find that this camera/telescope combination will put the galaxy edge to edge.

This is basically the same telescope, except branded Explore Scientific.

http://www.highpointscientific.com/telescopes/refracting-telescopes/explore-scientific-102-mm-air-spaced-doublet-refractor-telescope-ota-dar102065-01

It is important to understand that these are 'achromatic', or 'achromat' telescopes.  That means that the three colors of the spectrum a camera utilitzes, red, green and blue, don't all line up at exactly the same point when they are being refracted by the optics of the telescope.  This amounts to a very small amount of 'fringing'.  Technically, this is called Chromatic Aberration, and usually will give you a small fringe around very bright objects.  To combat that, designers developed the 'apochromatic', or triplet lens system.  But, on either the Bresser or the Explore Scientific, for M31 and other Deep Space Objects, it's usually not an issue.  The moon and planets, yeah, you'll notice some.

If you are going to use the Canon 6D, which believe me, after using the Sony A58 (which is a great camera, just a poor Astro imaging camera), you will need to have something called Backyard EOS.

https://www.otelescope.com/index.php?/home/&sku=OTL-BYE-P

You'll also need to get some additional stuff, like an autoguider, guide scope, etc.  You'll need all of that to shoot M31 or any other deep space object, regardless if you use a straight up camera lens or a DSLR on the back of a telescope.  That 6D, with the T-Ring, Adapter, Guide Scope, autoguider, etc, also makes for great planetary shots using the Celestron 9.25.

The last thing I can say is I hope that you DO NOT have a Celestron 9.25 Evolution system, since that uses an AltiAzimuth, single sided mount system, and is horrible for astroimaging.  If you want to do astro imaging, you'll need to purchase a German Equatorial Mount.  Let me clarify that to say that with the Evolution system, you can do solar system/planetary/lunar/ and solar work, where your exposure times are very short.  This is due to something called 'field rotation.'  A German Equatorial Mount, aka a GEM, is aligned so that one axis points directly to the North Celestial Pole.  As the Earth rotates below, the mount automatically compensates for that rotation, and tracks the object across the sky.  Then, all of your exposures will be aligned correctly.  However, with an AltiAzimuth design, the mount will track your object, but each frame will show a different position and angle of your object.  You have to rotate the image in post processing to align it.  The problem is that, depending on where you are, which way you are pointing and at what altitude above the horizon you are pointing at, you can start to the see the effects of field rotation inside one minute.  Since many astro images are made up of exposure of 1, 2, 3, 5, or even 10 minutes, you're going to have stars that have trails on them, effectively destroying any deep space photography.

Here is a great article on Field Rotation.  http://calgary.rasc.ca/field_rotation.htm

Finally, if you have a GEM, something like this:  https://www.firstlightoptics.com/advanced-vx-goto/celestron-c925-sct-vx-goto.html , you'll simply need a polar alignment scope to align the mount with the NCP, and then you'll be able to track up to about five minutes per shot before you get 'drift.'  An Autoguider package would be suggested, but as I said earlier, it doesn't matter if you're doing this with a Canon 27-200 lens, or a Bresser 102mm/600 telescope, you need that for all of this.

GOOD LUCK to you!  Welcome to the daunting, expensive world of astro imaging!