OAG Adapter Advice

[dpwood]

I have a Celstron C9.25 a NexGuide Autoguider, and I've recently bought a TSOAG9T2 OAG, my problem is I don't know how to connect the OAG to the telescope!

I have an EOS adapter that lets me connect my DSLRs (5d2 and a 550d) to the camera side of the OAG. Unfortunately the way they connect means the guider has to sit under the camera and can't be rotated. It would be nice if the whole camera and OAG could be rotated instead so I at least have a hope of finding some guide stars. A push fit might be better for this, rather than something that screws on?

The threading options have me totally confused. The OAG has "a short 2" nosepiece with M48 thread"... the telescope has as... 1.25" eye piece holder? If I remove the visual back there is what I believe to be a 2" threaded hole but the OAG won't fit over it and it'd be bumping into the focusing knob anyway.

Is the idea you remove the visual back and attach the OAG to it via one of these?

http://www.teleskop-express.de/shop/product_info.php/info/p368_Verlaengerungshuelse---2--Filtergewinde---30mm-Laenge.html

-All the gear, no idea

[Oddsocks]

Not the item linked in your post, that is just a 2" extender and won't connect directly to the visual back of your telescope.

Several users of SCT telescopes use the Baader Click Lock system, this screws to the existing visual back thread of the telescope and provides a 2" eyepiece/nosepiece/diagonal universal clamping attachment.

You then slot the 2" nosepiece adaptor of your off axis guider into the Click Lock with the camera attached to the "T" mount thread on the other side of the OAG. The click lock adaptor will bring the OAG away from the back of the telescope by 47mm and allow you to rotate the OAG and the camera.

http://www.teleskop-express.de/shop/product_info.php/info/p2427_Baader-2--Click-Lock-eyepiece-and-Adapter-holder-for-2inch-SC-thread.html

If you need still more space between the back of the telescope and the OAG then you can use the extension piece you originally linked to, threaded into the filter thread of the OAG nosepiece and this would bring the OAG another 30mm away from the rear of the telescope (77mm total).

There are many other possible combinations of visual back adaptor rings as well, some less expensive, some more.

William.

[Oddsocks]

Here is a less expensive alternative to the Click Lock, a standard SCT visual back thread to 2" adaptor, there is no height data for this on the retailers web page or at the suppliers web page but it will be approximately the same as the Click Lock, you can ask F.L.O. to measure it for you before purchase if necessary:

http://www.firstlightoptics.com/adaptors/baader-sct-to-2-inch-adapter.html

One other thing to consider, if you need to quickly swap between the camera-OAG set-up and the Celestron 1-1/4" diagonal and eyepiece then you will find it easier to obtain a 2" diagonal with 2" to 1-1/4" eyepiece adaptor then you won't need to keep unscrewing-screwing the various visual back adaptors, not essential but will make things easier.

http://www.firstlightoptics.com/diagonals/skywatcher-di-electric-star-diagonal.html

William.

[dpwood]

I think it being called a "nosepiece" is mis-leading, bad translation perhaps. It suggests there is a protuding piece from the OAG but there isn't. It's a recessed M48 female thread:

http://teleskopy.pl/images/TSOAG9T2.jpg

(Picture seems to have prism installed backwards, that's looking at the telescope side)

The Baader Click Lock looks like you slide a barrel into it so I don't think it'll work?

I have a 1.25" visual back adapter on the scope, and a t-thread to 1.25" adapter that I currently use to attach my DSLRs to the scope. So I'm thinking I could buy an M48 to t-thread adapter, into which I can feed my t-thread to 1.25" adapter, and then plug that into the back of the scope? I already get some heavy vignetting on my full frame 5D when I try attaching it using a 1.25" adapter though so I'm wondering if that's such a great idea.

I've realised I have a second problem. The guider mounts straight onto the T-Thread of the OAG which is fine I hope, the problem is I have no way of attaching an eyepiece to the OAG in order to find the guide stars. I need some sort of female t-thread to 1.25" eye piece holder?

Do people normally end up with piles of adapters?

-Dave

[Oddsocks]

To reduce vignetting you need to use 2" adaptors throughout, reading the OAG specification the solution is I think:

1: At the telescope fit the SCT to 2" visual back adaptor, either the Click Lock or the standard compression clamp model:

http://www.teleskop-...-SC-thread.html

http://www.firstligh...ch-adapter.html

2: At the internal "nosepiece" thread, 2" / M48x0.75mm, on the OAG fit the 2" extension tube, this will slide into the Click Lock/SCT standard 2" adaptor:

http://www.teleskop-express.de/shop/product_info.php/info/p368_Verlaengerungshuelse---2--Filtergewinde---30mm-Laenge.html

3: At the off axis T2 port of the OAG fit the T2 to 1-1/4" compression clamp adaptor to take your guiding eyepiece:

http://www.teleskop-express.de/shop/product_info.php/info/p209_Visual-Back-T2-to-1-25----with-additional-T2-male-thread---short.html

4: The camera T2 adaptor screws to the protruding T2 thread on the OAG

I think the off axis prism is reversable so the pictures may show the prism facing the wrong way.

There is a picture of the above set up on the TS website, shown half way down the page used with a refractor and the Click Lock adaptor, so the nosepiece extension and most of the Click Lock adaptor is hidden inside the body of the refractor's focuser but you can see the rest of the guiding cross-hair illuminated eyepiece, OAG and camera.

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p641_Off-Axis-Guider---nur-9mm-Baulaenge---T2-Anschluss---fuer-CCD.html

Either the Click Lock or the standard SCT to 2" adaptor will give you around 50mm of clearance between the OAG and the back of the telescope.

To go back to normal visual then the Click Lock or standard SCT to 2" adaptor has to be unscrewed and replaced by the Celestron SCT to 1"1/4 diagonal'adaptor, or you can invest in a 2" diagonal (normally supplied with a removable 2" to 1-1/4" eyepiece adaptor) noted in the post above:

http://www.firstligh...r-diagonal.html

You will end up with a whole ironmongers store of metal adaptors after participating in this hobby after a few years!

William.

[dpwood]

Thanks, William, that's exactly what I needed. I'm filling another expensive shopping basket.

A lot of talk seems to be given to getting the separation distances right so that the guider and main camera can both be brought into focus. There's very little adjustment room for focusing when you attach the autoguider directly to the T2 thread, quite easy to slam the prism stalk into the sensor in fact. I guess this is where T2 extension tubes are necessary but I'm not sure how to calculate it. Optionally, the autoguider comes with an adaptor to turn its female T2 thread into a 1.25" barrel, so I guess I could use that and plug it into the new adapter that I'll be using for the eyepeice. Would make swapping between the two much easier.

-Dave

[Jammy]

I've been looking for the last week for a posting that explains how I fix an OAG to my C9.25 and finally I've found one

This forum is fantastic!  Thank you

[Jammy]

Just a thought... Dave do you intend to use a focal reducer with your setup?  I've been told that guiding without a focal reducer is very difficult on the C9.25 because of the long focal length.

I've not tried either option yet but thought I'd point this out.

Let me know how you get on with all your adapters, I'm going to be ordering an OAG soon.

Regards

[dpwood]

I have another scope, with a shorter focal length, so I'll play with that some more before I look into focal reducers. I'm sure people guide with bigger scopes?

I have already run into a bigger problem though, I can't get my guider to focus!

I went with the baader 2" click lock system, and everything else suggested in William's post. The problem is that once I have my DSLR in focus, the focal point for the guider seems to be intollerably close to the end of the prism stalk. I had to disassemble my crossaim reticle eyepiece, removing the barrel and free-holding it, just to get close enough to achieve a fuzzy focus! I don't understand how anyone can use an eye-piece to find a guide star with such a setup?

I also dissambled the NexGuide camera so I could attach it directly to the OAG T-thread, this put the stalk within touching distance of the sensor but it did achieve some sort of focus. As I wasn't able to use an eye-piece I had to rely on blind luck. Fortunately Pleiades was available with plenty of bright stars and I managed to guide for 10 minutes but the result was extremely disappointing. Possibly dec backlash, I can't expect perfection on my first attempt.

I spent a frustrating 2 hours outside last night and didn't get a single thing to appear in my guide camera.

My setup is not workable, and I don't know how to fix it.

[Jammy]

This is a bit of a wild guess, but if you move your dslr further back from the OAG (with extenders) does that shift the position of the focal point further from the OAG?  In turn will this also move the focal point from the prism?

I'm far from an expert in these things, perhaps you need to ask the good people on here with a new posting.

[Oddsocks]

Hello Dave,

You just need to adapt the focal length of the OAG assembly to suit and match your imaging camera back focal distance.

This is done by adding a simple T2 extension ring between the camera T2 adaptor and the T2 flange at the camera side of the OAG.

Whatever the length of T2 extension you add at the camera port will push the focal point for the guider/eyepiece away from the stalk by the same amount.

Standard extensions come in lengths of 7.5mm, 15mm, 20mm and 40mm, you just need to choose the closest length(s) that will allow you to swap between the eyepiece and the guider.

Sometimes you may need to get a little creative with how you use the eyepiece by not fitting it all the way in to the clamp and making use of a slip over distance (par focal) ring on the barrel so that you can quickly swap between guider and eyepiece, you set up the OAG for perfect and safe focus with the guider and then fiddle with the eyepiece distance using a slip over distance ring on the barrel that you can make from a piece of scrap plastic water pipe or electrical conduit, cut to length, split along one side and then clipped over the eyepiece barrel.

From your description above it sounds as though you probably need both 20mm and 15mm extensions on the camera port to give you an extra 35mm extension on the guider port or 20mm and 7.5mm extensions on the camera port will give you 27.5mm on the guider port, but you need to take a little time and determine the best length to suit your guiding eyepiece and Nexiguide.

Here are four links to the various lengths of T2 extensions:

7.5mm:  http://www.365astronomy.com/t2-75-mm-extension-tube-75mm-optical-length-p-476.html

15mm:   http://www.365astronomy.com/t2-15-mm-extension-tube-15mm-optical-length-p-474.html

20mm:   http://www.365astronomy.com/t2-extension-tube-20mm-optical-length-p-2584.html

40mm:   http://www.365astronomy.com/t2-40-mm-extension-tube-40mm-t2-extender-black-p-3398.html

William.

[dpwood]

I had come to a similar conclusion. The flange focal distance of my cameras is apparently 44mm, so I'l have to do some measuring to determine how much I need to add to bring the guider side into focus. Unfortunately I don't see this being a simple affair with my OAG. I'll post this as a warning, if nothing else:

The OAG uses 3 compression screws to secure an adapter of your choice. Having Canon cameras I went with the EOS adapter, which connects directly to the camera body:

http://www.teleskop-express.de/shop/product_info.php/info/p6262_Adapter-ring-for-EOS-bayonet-at-camera-end-for-the-TS-OAG-9.html

This seemed logical, but it doesn't allow the camera and OAG to be spaced any further apart, which makes it pretty useless given my current focusing problems. They also make adapters for M42, M48 and T2. Here's the T2:

https://www.teleskop-express.de/shop/product_info.php/info/p1649_Adapter-ring-for-T2-thread-at-camera-end-for-the-TS-OAG-9.html

So, by replacing my current EOS adapter with this T2 adapter, adding T2 extensions as William has described, and adding an EOS-T2 adapter at the camera end (which I already have). I might be in business? Given how much talk is given to having slim OAGs, I had really expected my problem to be the focal distance being too far in the other direction and having to extend the guider side, not the camera side! The guide camera even came with a parfocal ring for the eyepiece, how optimistic!

To come back to Jamie's question about focal reducers. 2350mm is a lot and I admit I was bitten by the MUST HAVE MORE bug. I have honestly had better results with a f/2.8 200mm lens than I have with my C9.25" so far, though that's no doubt my lack of skill and understanding. I was trying to capture some of M42 last night and even with a full frame 5DII I was failing to frame the shot and find a suitable guide star. I would like to reduce the focal length some day, but then I'll probably need another dozen adapters and a bank loan!

[dpwood]

I think I have it sussed now:

1. I pulled the T2-1.25" adapter on the guider side, down the stalk, as close to the OAG as I could (it's slightly obstructed on the 5DII creating a 2.5mm gap at the base)
2. Attached the eyepiece and brought it into focus.
3. Moved the DSLR out away from the scope until it achieved focus. This was by moving it 12.5mm.
4. Repeated above but using the guide camera. This time I needed to pull the camera out 16mm.

A 20mm extender would be about right I think. The T2-1.25" adapter would have to move 4mm up the stalk, and I can place the par focal ring on the eye piece to extend it 3.5mm to match the guide camera. Hopefully the T2 adapters don't add anything further or I'm getting the file out!

-Dave