Using a ZWO 2600MC Duo with a long focal length scope

I use it with my RC12. Great results. Even with the elongated stars (guide sensor is beyond the APS-C of the main sensor), so stars are not corrected there, guiding is great, typically under 0.5. The main catch, as always with long FL is that sometimes you do not find a star in the field.

I am thinking to buy it too for my C8 edge for galaxies with the native focal lenght and deep sky in general with the 0.7 reducer, . What do you do if you don't find stars? is there any trick or you just give up on this subject?

***You can try rotating your camera if your image framing will allow it. You can usually find a guide star that way. Another possibility is longer guide exposures.  Other typical options are not available with the duo such as moving an OAG prism closer to the imaging sensor or using a larger OAG/Guide cam. ***
CS,
Jeff

You will be fine with any of the larger Celestron SCT's if you have a good choice of mount. 

I use one on my 10" f/12 maksutov (3000mm focal length) which rides on a CQ350 mount. I have posted a few images for you to look at. I'm still only a beginner at this, and using short sessions, but now that the rig works I hope to get some decent long acquisitions soon when the weather breaks. Call me spoilt though, because this scope is a Rumak, with pinpoint stars across a flat-field covering 50mm diameter, so the guide stars look pretty good in the guider. Main camera field is 0.45 x 0.30 degree, with ASIAirPlus controlling it all. 

The big issues with long focal length guiding are - what is your mount, how is the dec backlash, and how steady is your seeing ?

I know the DUO works on a C9 and C11 (some members of my club use a C9 on a Losmandy G11 and C11 on a 10 Micron mount), but on a C8 I would have some reservations about how well the C8 covers the guide sensor, and the sharpness of the guide stars that far off-axis. It should work well enough, though, with a mount like an EQ6.

With the guide camera cycle at 1-2 seconds guiding is mostly around 0.5 arcsec - but I think this is mostly seeing-related because late one night the seeing settled and it guided for about 30 minutes at 0.2 arcsec, resulting in beautifully sharp subs.

As for guide stars it is mind-boggling how sensitive the 224 sensor is. I've never had it fail to find a guide star - and that's without rotating the camera, and that's with an L-Pro filter, so I can believe what ZWO say about the mono version guiding with narrow-band filters at f/7.

Incidentally do not rotate the 2600 DUO to random angles -  for good guiding it is important that the up/down and left/right axes of the guide sensor correspond to N/S or E/W of the mount axes. if you rotate it to some odd angle, guide errors will produce movements in both axes - if the mount has any dec backlash that's going to result in egg-shaped stars.

There are some things to consider though. because long focal length equates to a very long optical lever, and as a closed-loop feedback system the net gain may be too high. When the guiding starts, the mount may oscillate, or worse, will not settle and loses the guide star altogether.

a) Try using BIN2 instead of BIN1 on the guide camera, this is equivalent to halving the focal length;
b) Reduce the guiding speed - I use 0.25X sidereal - the default 1X was way too high.
c) Reduce the maximum guide pulse duration. The ASIAir by default uses 1000ms but I found 300-500ms works well.

To tune the mount for good response, what I did is this:

- Start with low values for these parameters so that guiding will settle, even if its not really great.
- Increase either the guiding speed, or the maximum guide pulse duration until you see it starting to oscillate in one axis - zigzagging back and forth a little.
- Back off that setting one notch (guide speed) or by 30% (pulse duration) and it should be stable, yet respond well to errors.

Good luck !

PS since getting it all to work with the 2600 DUO, I would not consider OAG or piggyback guide scopes again, and I have even ditched the finder scope.
The 2600 DUO  really is THAT good.

Now, if only all this rain and cloud would just go away…

I have a friend with a C14 edge with reducer using the Duo. He is having a heck of time finding guide stars. It's A LOT of focal length and the guide chip is relatively small compared to the 174mm. A legit OAG and larger cam than the 220 is probably required for the C14.

+1 for binning the guide camera. For high focal length telescopes it can be a real pain in the arse finding guide stars via Off-/On-Axis guiding, at least at the native image scale of the guide sensor. I shoot at 2000mm and struggled with the 290m sensor at 2.9µm pixel size. It was just too dim (on top of it being a small chip size). Now with BIN3 or even BIN4 its better and with the sub pixel guiding the guider image scale is still sufficient for getting round stars (average middle european seeing). 

CS 
Tobi

Yes, I agree the scope + flattener/reducer should support the image circle also. Don't know about the C14 and the reducer. Many scopes support "full frame" image circles but with a reduced illumination, which would make the whole OAG topic worse 

CS
tobi

Ok, thanks again for the feedback!

Andy:

Jason Fleming:
I use it with a C9.25 and it works great so far.  I got great results hitting the crab nebula with it. With Galaxy season coming, the challenge may be finding guide stars at such a long focal length off the galactic plain, so I guess we'll see.

Are you using the duo with or without the reducer?

Both (or all three when you include Hyperstar). No reducer with the smaller targets, but of course great seeing is needed then.  But I haven't had trouble finding guide stars.