Orion Optics CT10 Backfocus and other issues

[mbalkham]

New CT10 scope has finally arrived in NZ and I've attached my Baader Diamond Steeltrack 2" Newt focuser waited for a clear night and headed out. Hopefully the 3 month wait would be worth it!

Nope can't achieve focus. I can't get the camera (Canon 6d with Baader protective T ring & MkIII MPCC Coma Corrector) close enough to achieve focus. I've pushed the primary as far forwards as it will go and still seams to be a good few mm off.

Does anyone know the focus distance for this scope? I have asked Orion Optics but not yet managed to get that number from them. The standard focuser they ship it with is 65mm-105mm and my steeltrack is 62-102mm so I would imagine it should work.

Also I cant attach the Orion Awesome Autoguider (80mm short tube refractor) as the tube rings wont attach to the plate. So looks like I need some new tube rings that will fix via M6 bolts.

Lastly I also need some extra counterbalance weights as I'm pushing the NEQ6 out onto the extension bar. Not a biggie just another thing I hadn't given proper thought to! 

Any ideas on backfocus solutions? I don't want to spend that much money on an OTA and have to start chopping up the tube/moving the primary. And shipping back to OO in the UK would be a real pain.

[andrew s]

Have you tried an eye piece? This should give you and idea where the focal plane is. (If it's a simple eye piece then the field stop should be at the focal plane.)  Then you will have some idea of the task you face.

As for extra weight I use roofing lead in various ways to get balance!

Regards Andrew

[brantuk]

According to this page FL = 1200mm: http://www.orionoptics.co.uk/CT/ct10-10l.html

Iirc dslr's have a back focus of 55mm with a reducer - but I could be wrong so double check that one first. Hth

 

[mbalkham]

So using some basic (potentially flawed!) maths (with dims to be measured when I'm back in front of the tube!)

Tube length 1050mm

less distance from end to primary (say 50mm)

less distance from end to secondary (say 75mm)

plus distance from secondary to base of focuser (say 150mm)

equals 1075mm

Focal Length =1200mm

less number above = 125mm from tube to focal point?

Less backfocus (55mm) should = where I need sensor? = 70mm from tube?

[andrew s]

8 minutes ago, mbalkham said:

Less backfocus (55mm) should = where I need sensor? = 70mm from tube?

It's is easy to be just a few mm out so make a measurement. If you don't have an eyepiece put some frosted paper/thin plastic over the end of the focuser and focus on the moon.

Regards Andrew

 

[mbalkham]

Thanks Andrew I'll give it a go when I get home tonight.

[mbalkham]

How about a glass path compensator? Anyone used one of these? would it work in my optical chain which includes coma corrector & filters?

[andrew s]

25 minutes ago, mbalkham said:

How about a glass path compensator? Anyone used one of these? would it work in my optical chain which includes coma corrector & filters?

I think this would be a bad idea. I suspect that to have a large enough effect it would introduce significant aberrations. I would look at a very low profile focuser or modifying the tube but see what OO say.  

Regards Andrew

[mbalkham]

Not sure i can take much more out the optical chain

[mbalkham]

I managed focus with the 10mm EP but not with the 25mm. So I make back focus 62mm plus 10mm ~ 72mm (assuming I am measuring and doing the maths correctly).

25mm EP does not achieve focus with 60mm ~ 75mm back focus

Camera is 44mm (flange distance on Canon 6d) plus 25mm ~69mm but I can't get focus

 

[mbalkham]

Incidentally it's a cloudy night so I am focusing on a streetlight a fair distance from the house. Prob need a little more back focus on a real target.

[mbalkham]

Just realised I'm measuring to the wrong part of the focuser really so prob need to take another 10mm off the numbers above. The Baader manual suggests I can get 10mm out of removing the 2" clamp if I can live with an M48 thread but this would prevent me easily changing orientation of the camera or between visual and imaging having to loosen 6 grub screws to adjust the M48 adapter.

[Peter Drew]

In your OP you say that you have moved the primary as far up the tube as it will go, what's stopping it from going further?. If it's because you have run out of length of the adjusting screws then longer replacements could help. One caveat though, OO tend to supply minimum size secondaries in the interests of minimal obstruction, so be careful not to introduce vignetting.    

[andrew s]

Two things to try. 

Firstly, you need to check where the field stops are on your eyepieces. Look in the non eye end of the eyepiece and you should see, assuming it is a simple design, a ring that defines the field. If you put a pencil (blunt end) in so that it contacts the ring and look through the eyepiece you should see it in focus. This is where the focus will be rather than the ledge where the eyepiece meet the focuser.

I would also try to focus the camera without the coma corrector. I am not sure where the MPCC needs to be wrt to the focal point to get its optimal performance. Maybe some one who uses one knows.

Regards Andrew s

[mbalkham]

The bolts look like m8 with 35mm one moving the primary and 30mm locking it. I recon i need 10-15mm extra but am wondering if that will introduce a heap more flex/vibration into the primary?

[Peter Drew]

It shouldn't do and it's the easiest, cheapest fix.   

[Space Hopper]

FWIW with my own OO 10" (VX10L) i can comfortably get my own 6D into focus with prime focus imaging.

My primary is on the bottom 'level' as well.

 

With eyepieces i was unable to reach focus as well.

It amazes me that OO put these scopes out without the ability to focus eyepieces minus some sort of extension tube.

The focuser on my own scope was a piece of junk.

I upgraded to a Moonlite which was a breath of fresh air. But even with a non standard 60mm drawtube, the focuser is almost fully racked out with my 2" eyepiece.

 

[mbalkham]

I've heard back from Orion Optics. There emails are much less helpful than before I paid them the money and we're down to one line answers which aren't much help.  I have got out of them that the standard tube to focal point is 116mm on the CT10. Why they don't have this discussion upfront is beyond me they knew I was intending to image with this scope and it's a bespoke one (made to order and supplied without focuser) so you would have thought they would have prompted a question in the buying process.

Anyway that being the case I have little chance of achieving focus (steel track 9mm+62mm plus T ring 15mm plus Canon flange 44mm = 130mm).  So by this calc I am ~14mm out, changing to from 35mm to 50mm m8 bolts on the primary should in theory get me into the right ball park (we'll see tonight)

I can get my 10mm to focus and the Baader Hyperion Zoom mkiii fine but just not the Canon 6D or the 25mm EP.

[Uranium235]

Ok, here is a way to buy yourself about 9-10mm of in travel....

 

1) Buy an ultra-short M48 EOS adaptor like this:

http://www.teleskop-express.de/shop/product_info.php/info/p6527_TS-Optic-ultrashort-Adapter-from-M48-to-Canon-EOS-EF-Bayonet---only-1-mm-optical-l.html

2) Put the MPCC into M48 mode (remove the t-thread and stop collar).

3) Use a short M48 extension ring(s) to achieve the correct spacing of 57.5mm (thats the spacing in M48 mode).

This will have the effect of sinking the corrector further into the drawtube. Im not sure the steeltrack M48 adaptor will work since I dont think it has a thread scopeside (ie: nowhere to mount your corrector... which is a bit daft IMO).

[andrew s]

10 hours ago, mbalkham said:

Why they don't have this discussion upfront is beyond me they knew I was intending to image with this scope and it's a bespoke one (made to order and supplied without focuser) so you would have thought they would have prompted a question in the buying process.

I appreciate your frustration but (and I realise this is too late for you) I would urge anyone specifying a bespoke telescope to be explicit about what they require especially back focus. I recently got OO to make me a 400mm ODK optimised for spectroscopy. I explicitly stated the required corrected field, longitudinal aberration and back focus from an atlas focuser. 

The difficultly for the supplier if you are not explicit is that, for example, imaging may mean a backfocus form 13mm to 55mm or more.

In your case I would move the mirror up and if needs be replace the secondary if it proves to small in practice (try it first and see if the vignetting is acceptable). You can use NEWT http://stellafane.org/tm/newt-web/newt-web.html  to asses the options. f you do need a new one PM me as I may have one that is suitable that I am willing donate FOC.

Regard Andrew

[mbalkham]

Cheers Andrew. I understand that people's needs vary but most people aren't buying these things everyday and they are selling them everyday so why wouldn't you ask and do you best to make sure your clients are happy with the product? Not just ship what you always do and hope for the best. Different if I was buying a standard off the shelf product.

Anyway currently I have achieved focus on a street light prob a km away so we have progress. Just need a star to check it properly! I now need some spacers so that the springs work on the longer bolts. Thinking M10 nuts on the M8 bolts unless anyone has a better suggestion.

Sorted the guidescope last night too so once the clouds part I might get first light.

Just extra counterweight, new collimator, new bahtinov mask to sort now!