C6 SCT, Astro Tech AT6RC and Refractor 120ED?

[ChaosKnight]

My personal experience is only with the C6 SCT with Celestron reducer and OAG.

The reducer should go in front of the OAG. If the OAG goes in before the reducer, you may have problems focusing the guide camera.

Not sure about Meade OAG, but the Celestron version has a removable collar that allows the pick-off prism to go closer to the optical axis. This improves the image quality of the guide star, but the shadow of the prism is more visible on the image. This shadow have to be removed with flat fielding.

And if i'm not wrong, SCT reducers have a female SCT thread and presents a male SCT thread, while OAG for SCTs have a female SCT thread but presents a male T-thread, so the only reasonable way of coupling the two is with the reducer in front.

And if your reducer is also a corrector/flattener, then the image correction is optimum only if the image sensor is at a specific distance from the rear of the reducer. In Celestron's case, this distance is ~105mm. I would think Meade will have a similar distance. The OAG is usually designed to provide this distance.

Finally, i'd like to say after i went through all these, i determined a separate guidescope with a good mount is the way to go. Not being able to find guide stars, especially in a light-polluted environment, is the norm rather than exception. By the time you find a useable guide star for your system, often your telescope will be pointing somewhere other than your target.

[kimo]

I have taken off oag. today i made a small guide scope and its works great. no cooling for ccd.

Here is the test result on 8" SCT.


After done some calculation.
F3.3 = didn't reduce to f/3.3 it came to F/4.9 and still below 1000mm.
F/6.3 = reduce to 1285mm so its ok.

M27= 4 min Exp single raw f/6.3 1285mm
M56= 2.49min single raw f/6.3 1285mm

[kimo]

I took M27 and M15 F3.3, too bad that m27 was too high almost going to invisible due to the window so its not much clear.
M27 and M15, Raw single pic.

[orly_andico]

I used a bunch of spacers and SCT adapters so that I could put the OAG in front of the reducer.

The argument is opposite: most reducers only work with a fixed back focus, which is 55mm for the refractor reducers and 105mm (more or less) for the SCT reducers. Both of the OAG's that I have add too much back focus, so I have to put the OAG in front of the reducer.

If you are using the TS9 OAG then back focus would be less of an issue, and the pick-off prism can also be moved closer to the optical center. But my OAG's (Vixen and Lumicon) didn't have this feature.

Like ChaosKnight, I went with a guide scope and better mount. Wasted time, effort, and money on those multiple OAG's and even my DIY on-axis guider..

[kimo]

I have guide scope ST80 that is too heavy to mount on this scope so i wanted to make it small or even non. I took old 50mm finder scope to make it as a gide scope so its light and small also wider Fov. now i am pleased.

more test will be coming soon.Thanks

[ChaosKnight]

Dude, just get the Orion 50mm Mini Guidescope and be done with it.

[kimo]

last night i took NGC7331 Galaxy. NGC 7331 (also known as Caldwell 30) is a spiral galaxy about 40 megalight-years (12 Mpc) away in the constellation Pegasus.

Telescope: 8" SCT with F/6.3 Reducer, focal length 1285mm
Camera : DSI PRO II

Exposure of 16 X 5.39min

note: Test on nebula. I tried to take some narrowband imaging with 8"SCT F/3.3 and F/6.3. it took very long time, you can say more then 3 times of exposure as i could see with 6" newtonian. our past test result shows that f/3.3 as a F/5 so it should be fast as others F/5 but didn't. so that means F ratio vs Focal length?

[kimo]

compare two pics.

[Duke2012]

check the spacer.

[kimo]

I think i have to re mesure the spacer between reducer and ccd. its funny that F3.3 and F6.3 didn't show correct ratio. probably somewhere compensated the focus at incorrect space.