To Edge or not to Edge?

[CSM]

I have a Celestron NexStar 6SE on a wedge with a focal reducer, nicely set up in my Pulsar observatory.  I have used it successfully for astrophotography, both planetary with a webcam and latterly with a small CCD camera.  I want to upgrade to one of the AVX scopes, either a 8" Edge or the 9.25 which isn't Edge.  With the 9.25 I can use the focal reducer but with the 8" I have to buy another and obviously this has a smaller aperture but is allegedly better for astrophotography.  But does the extra aperture overcome that advantage?

[swag72]

My immediate thought is what use you are planning for the scope? Are you thinking about using it for DSO imaging or just planetary? And if the former, what camera are you looking at using it with?

[Grotemobile]

I have gone from this

http://www.firstlightoptics.com/optical-tube-assemblies/celestron-c11-xlt-optical-tube-assembly.html

to this.

http://www.firstlightoptics.com/cpc-deluxe-hd/celestron-cpc-deluxe-800-hd.html

This is one of the best scopes I have had.

I thought, have I done the right thing going

from a 11", scope to a 8" scope.Sold the C11.

I do imaging,& this scope is spot on.

Steve

[CSM]

Hi Sarah/Steve - what I do in astronomy is developing as I go along.  I started out just visual with no thought of astrophotography but have more or less changed over to this now.  Will want to do both planetary and DSO but how far this will go is in the lap of the gods.  Currently thinking that as the 9.25 was developed for astrophotography that this and the extra aperture would give it the 'edge' for planetary but the Edge would be better for DSO and it does have the F/2 capability and the recommendation of Damian Peach.

[swag72]

I definitely think that the C9.25 would be better for planetary than the 8" - But if you are thinking of getting into DSO's then these scopes are not ideal, in my opinion. Firstly for DSO work you are going to be imaging at long focal lengths which places one hell of an ask on your mount. If you are not using a reducer for example, then imaging at f10 is far from ideal and you will need long exposures to get anywhere.

The EDGE has a flatter field than the non EDGE version. I guess whether you will benefit from that will very much depend on your camera chip size. I can pretty much get away with my non EDGE version and my camera chip, but if you have a DSLR then I reckon that will be another ball game.

Yes you can image at F2 - All I can say is that will require some serious collimation and perfection to get that right.

Good luck with whatever you decide. This is just my ramblings after all and others will probably disagree

[CSM]

Thanks Sara.  I already have a focal reducer which would work with the 9.25 and am not seriously considering the F/2 option with the Edge.  The extra .7 focal reducer for the Edge 8" means that there is not much in terms of cost.  Don't have a DSLR at the moment but who knows.  I realise that there are other, probably better options in terms of scopes for astrophotography, but having got used to the Celestron SCT's I am comfortable in going for these bigger and better versions.

[ollypenrice]

I agree with Sara's main post. At these long focal lengths you will need to work hard to get workable tracking. People do manage it but it will not be at all easy, I don't think.

On a small chip the Edge advantage is small.

In choosing a camera be sure that you can get a reasonable sampling rate. This can be done by binning small pixels provided you don't go for one shot colour, which cannot be binned. This calculator is good: http://www.12dstring.me.uk/fov.htm

And as Sara said, you are choosing a difficult route into DS imaging. It isn't the route many of us would recommend.

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2266922474&k=Sc3kgzc

[CSM]

Thanks Olly.  Yes, I appreciate that I am being a bit awkward but we shall see.  At present from your comments I am leanng towards the AVX 9.25.  Once I have the mount then in future, if circumstances and needs change, then maybe a different scope.

[swag72]

Just one further thing to add, many DSO's are really quite large. The C9.25 will give you a rather narrow field of view. Good for small galaxies and planetary nebulas, but emission nebulas tend to be massive in comparison.

[RobH]

I had an Edge 11, but the advantage of the large flat field was lost on me because I used a small chip. The big disadvantage with the edge scopes is that they are tricky to focal reduce unless you buy the ridiculously expensive reducers made specifically for them.

I'm going to be buying an 11 inch SCT again soon, but will go for the standard version with a 0.63x reducer. I want to have a crack at planetary and lunar imaging, which it'll be great for, you can attach a hyper star and image at F2 ( both the standard and edge versions have this ability), and it'll be a great visual scope, which my 12 inch RC isn't. Plus, I can double up on small galaxies etc, with the SCT using my small chip Atik 16 HR, and the RC using my 460eX binned 2 x 2.

[CSM]

Thanks Rob and Sara for further comments.  I am aware of the size of some DSO's but will be quite happy to get the smaller ones.  One reason why I was glad I went for an SCT was due to the back problems that I had a couple of years ago so that using the reflector I had was very uncomfortable.  Rob - you've more or less convinced me that using the 9.25 with the .63 reducer will be sufficient, at least for some time to come.  And it should also be good for visual, especially with my binoviewers.

[steppenwolf]

With the caveats already mentioned above, I'd agree that the 9.5 is the better choice and it is worth bearing in mind that the flat field advantage of the Edge is lost if you use the Hyperstar and image at f2.

Sent from my iPhone from somewhere dark .....

[CSM]

Thanks Steve.  Interesting comment re the Hyperstar.

[pyrasanth]

With the caveats already mentioned above, I'd agree that the 9.5 is the better choice and it is worth bearing in mind that the flat field advantage of the Edge is lost if you use the Hyperstar and image at f2.

Sent from my iPhone from somewhere dark .....

I have a C11 Edge but I don't agree with this comment in entirety. The 9.5 inch Celestron is probably the best imaging platform for it's size in the SCT world. It's optics have been praised and & continues to produce superb planetary pictures- so no problem with that & I have no doubt that the edge version continues this tradition of fine optics.

 I've researched the Hyperstar extensively as it's a significant financial purchase when you also have to select a good CCD camera to couple with it. The Hyperstar lens is essentially a contact lens which corrects the aberrations of the primary mirror when the secondary is removed. It gives a superb flat field & makes the optics five times faster. I really believe that the Hyperstar gives more than it takes away and since the edge optics  no longer feature in this setup means that there "loss" is no detriment as your using the instrument completely differently. The added bonus is that you now have an imaging platform that is 25 times faster than the native optics. This could be  a small price to pay even if there is a detected curvature of field which is not apparent in any of the images I've looked albeit not knowing if cropping is involved.

I don't see why anybody would not go for the Edge version of the SCT other than financial reasons.

[RobH]

I don't see why anybody would not go for the Edge version of the SCT other than financial reasons.

The reason I got rid of mine is that it's a complete pain to focal reduce them while keeping a flat field due to the flattening lenses in the scope. You can do it, with the AP CCD67 for example, but for the best results you need to get the reducer close in to the flattening lens, which means it's then tricky, if not impossible, to use an aftermarket focuser with the scope, so you're stuck using the standard focuser which moves the primary, and isn't really up to the job if you want to use remote focusing, robofocus for example.

Of course, after spending nearly £4k on the scope, you can then go for the reducer that's designed for the scope, but that'll set you back another £700 or so.....far too expensive.

With the non edge version, as long as you don't plan on using a large chip, you can get a good flat field and a 0.63 x reduction using the standard SCT reducer.

[steppenwolf]

The Edge is indeed an excellent optic - my observation was simply that with a Hyperstar in place,the corrective optics at the rear of the 'scope are taken out of play therefore, if the intention is to use the Hyperstar exclusively, there is no advantage in the extra cost of the Edge.

Sent from my iPhone from somewhere dark .....