So not to long ago I went out and got my self a shiny new GSO GS-600 Newt along with one of the Bintel Coma correctors. Finally the weather cleared up and I was excited to give it a whirl.
After setting up, I started my alignment process. It was during this phase I noticed that I couldn't achieve focus with the DLSR at all, I tried with some eye pieces and all was well. I took the Coma corrector out and had just my 2" to 1" adapter and my Camera with the T-ring + adapter. I was still out of focus, though it did help a tiny bit, it seemed as if I needed more in-focus but the tube couldn't go any further in.
So my question is, what's the solution? Sure I could buy a low profile adapter but then that would mean the Coma corrector will be pointless as I wouldn't be able to use it Or I could use a barlow to extend the prime focus point but then imaging won't be as good. I'm somewhat confused as the scope was marketed as an imaging optimized scope, unless it's me doing something wrong.
The standard Canon DSLR sensor setback distance is 55mm. That is the distance from the base of the T-Adaptor flange on the camera to the sensor surface. Your scope must be able to come to focus at the camera sensor. I assume you are using a standard EOS T-Adaptor bayonet fitting and something like the Bintel T-Adaptor, like these: http://www.bintel.com.au/Astrophotog...oductview.aspx
If setup using these, simply put the 2" adaptor into your focuser, with the flange right against the focuser tube top flange.
Try this first without a Coma Corrector!
Now as to Coma Correctors, ones designed for visual use may not be suitable for camera use. They have specific setback requirements. I have found the best coma corrector for use with Canons standard 55mm setback is the Baader MPCC (Multipurpose Coma Corrector). As it comes out of the box it has the precise setback you need for a Canon. I personally could not be bothered fooling around with other coma correctors that require spacers to get it right.
So come back here after you have tried to attain focus without a coma corrector, that is step one. If you cannot attain focus with just the scope adaptor, and you think you need more in-focus range to do so, then the solution is usually to move the primary mirror up the tube slightly (often this can be done within the range of the adjustment screws supplied). You can get as much as 5mm this way, maybe more, but you don't want to adjust it out so much that a mirror ring screw comes loose, you still need to be able to lock it. You can get longer primary screws, stronger and longer springs, etc if you need more range. If a scope is being sold as an imaging optimised scope, then the focal length should already be setup to be able to attain focus IMHO. However, don't muck around with moving the mirror until it is clear that is the last option.
Finally, I don't understand your comment about a 2" to 1" adaptor, because that should have no place on the imaging train setup for a DSLR. You want to be 2" all the way into the camera.
If setup using these, simply put the 2" adaptor into your focuser, with the flange right against the focuser tube top flange.
Try this first without a Coma Corrector!
So come back here after you have tried to attain focus without a coma corrector, that is step one. If you cannot attain focus with just the scope adaptor, and you think you need more in-focus range to do so, then the solution is usually to move the primary mirror up the tube slightly (often this can be done within the range of the adjustment screws supplied). You can get as much as 5mm this way, maybe more, but you don't want to adjust it out so much that a mirror ring screw comes loose, you still need to be able to lock it. You can get longer primary screws, stronger and longer springs, etc if you need more range. If a scope is being sold as an imaging optimised scope, then the focal length should already be setup to be able to attain focus IMHO. However, don't muck around with moving the mirror until it is clear that is the last option.
Finally, I don't understand your comment about a 2" to 1" adaptor, because that should have no place on the imaging train setup for a DSLR. You want to be 2" all the way into the camera.
Sorry, by that I mean that standard apdator you get when purchasing the telescope, that goes into the 2" focuser but allows 1.25" eye pieces and the such.
For the T-Adapter, it's a Celestron 1.25 Universal. I'll pick the one you linked and see how I go.
If you use a 1.25" adaptor your cutting out alot of the light getting to the sensor and effectively cropping the image (vignetting). A dslr requires a 2" adaptor.
I am sick to death of banging my head against a brick wall regarding this
problem. Many new buyers will at some time decide to take some pictures
with their new scope, only to find out that they can't. WHY DO RETAILERS
NOT DO THE RIGHT THING AND INFORM BUYERS OF THIS ISSUE.
Skywatcher [and Saxon, same thing] Newts do allow prime focus imaging,
but most other brands don't, unless they are astrographic models.
The standard fitting on SW Newts, which doesn't require the purchase of any adaptors, other than the appropriate T-ring, works at 1.25." All bar one of the prime focus images I have posted over the years have been taken with this set up, and vignetting has not been a problem. The T-ring is flush with the outer end of the focuser.
I have found that very often the mirror needs to be moved up the tube a
lot more than 5mm Glen, necessitating much longer adjusting screws.
I feel so sorry for people in this situation, it is so frustrating.
Rant over.
raymo
He is ok Raymo, it is a GSO imaging optimised scope, it will achieve prime focus. He just needs the right adaptor bits. But i agree with you that some retailers don't mention that their scopes are unsuitable for photography, other than afocal.
Fella's, Ive had this problem with my 10" hand crafted Newt. It was a real headspin but I managed to get around it. It is dual purpose for visual and photograpghy(not an astrograph which will possibly happen if you move the primary mirror). I solved it by two actions. 1st- I unbolted the rack&pinion focuser and inserted it from the inside of the OTA which effectively brought it closer to the mirror by 15mm.
2nd- I ground down the outer flange on both my 2" and 1.25" adaptors which enabled them to travel further into the focuser tube allowing up to 7mm closer to the mirror(this was time consuming using a dremel a grinder and a metal file). In total 22mm closer to the prime mirror therefore enabling prime focus with my canon 60D. I know it may seem extreme but It worked for me albeit a butchered set of adaptors and a rough and ready look to the scope. But hey there not meant to look pretty sittin in the lounge room anyway...
If the scope was advertised as 'optimised for imaging' then you should be able to reach focus with the adjustments on board. Try the primary mirror collimation screws: turning them all the same way by the same amount will move the mirror up/down the tube: you want to move it up as far as it goes while still being able to adjust the collimation and lock it in place with the locking screws. If this does not do it, you have the right to return the scope (it's not fit for purpose), or better ask the shop to find a solution (which should come at no extra cost to you, unless there is something unusual about your camera ...).
If you got the scope at Bintel I'm sure they'd be happy to do the right thing and sort you out. (Based on my experience with their now closed Melbourne branch.)
If the scope was advertised as 'optimised for imaging' then you should be able to reach focus with the adjustments on board. Try the primary mirror collimation screws: turning them all the same way by the same amount will move the mirror up/down the tube: you want to move it up as far as it goes while still being able to adjust the collimation and lock it in place with the locking screws. If this does not do it, you have the right to return the scope (it's not fit for purpose), or better ask the shop to find a solution (which should come at no extra cost to you, unless there is something unusual about your camera ...).
If you got the scope at Bintel I'm sure they'd be happy to do the right thing and sort you out. (Based on my experience with their now closed Melbourne branch.)
I've got a 2" T-Adapter on the way, I'll give that a try and then if not I'll do exactly this.
For those who have to do the DIY thing, the following link has the "register" distance for a variety of camera bodies. For the most part, there is no difference between film and digital bodies for the same manufacturer. http://www.graphics.cornell.edu/~wes...-register.html
Thank you everyone for your replies, the 2" Adapter from Bintel solved my problems. I just kind of assumed my Celestron Univeral 1.25 would do the trick. But all is well now!