Guide camera and plate solving

[JohnA]

Hello all,

Another newbie question.

I was looking into getting a guide camera to add to my setup.

My current rig is the celestron c925 with the asi 678 planetary camera.

Now with this combination, the FOV is so narrow i can't plate solve at all, fails every time, so I though to add a guide camera, which I can use to plate solve and to of course guide with.

From all the reading I have done, it would seem an oag is the only way to go with my scope.

So would this work OK.

C925, 6.3 reducer, filter draw, zwo oag-l 178 mini guide camera and my 678, and of course any spacers required for the correct back focusing.

Down the track I would like to also add a cooled camera.

Am I on the right path, will this all work, or should I just leave it simple with the 678 and no reducer for planetary, and have another combination I can just swap out for DSO

Thank you

[Cosmic]

Hi John,

Just take my advice with a grain of salt, I'm no specialist :)

I feel like you need to pick your poison. If your doing planetary stuff, is there really any need for plate solving anyway. Just do a good 3 star alignment and try stay on target manually? Not sure how tedious it is haha I'm a DSO man myself.

For DSOs then a camera to match in turn with the reducer to help plate solve with your eyes closed...some of those galaxies can be hard to find otherwise. If you go down this road then grab a guide scope + camera accordingly which makes sense. The OAG sounds good and might be the go as well but I have no exposure to using one sorry. I feel at those focal lengths finding a star might be very hard. I only have experience with Orion StarShoot and scope @400mm. I know the StarShoot combination will work but in your case you need fairly good guiding as well, total rms of .67 arc sec. All the best, hope you can find a solution for guiding to make life easier. :)

I hope this helps in some small way.

Regards,
Dan

[gb44]

John,
You'll need to take out the focal reducer and install a barlow 2x - 4x for planets, and maybe just use the focal reducer on bigger nebulae. Powermates are preferred over barlows.

New ZWO filter drawers are redesigned to stop light leaking in.

SharpCap pro can do the whole job of acqusition and plate solve / mount control. Astrosurface can do the processing simply.

There is a formula for calculating guidescope suitability. Lots of people gave up on OAG.

GlennB

[Drac0]

Hi John,

You don't mention the software you're using for plate solving. With the combination of the small sensor & long focal length most plate solving software will fail with their default star database. Your FoV is only about 11x5 arc minutes, you need a star database that can work with it.


Adding an OAG won't help with plate solving at all, that's not what it's for. It will be an even smaller FoV, picking off just a tiny section of the FoV of the telescope.

Cheers,
Mark

[By.Jove]

1. Platesolving with ASIAir requires a field of half a degree or so. Using your C9.25 you will need an APS-C-sized sensor such as a DSLR or the ASI2600.

2. For guiding there are two issues:
- the image scale of the guide camera (in arcsec/pixel) should be no more than 3X that of the main scope+camera.
- for piggyback setups, flexure between the guide camera and main camera will be your main enemy. This results in egg-shaped stars.

For example using the C9.25 at its native f/10 with a sensor with 3.7 micron pixels, the image scale is 0.32 arcsec/pixel. The guidescope+camera should be no more than 1 arcsec/pixel, which suggests something with a focal length around 750-800mm, which will need to be pretty robustly bolted on the C9.25 for example rings + a Vixen or ADM MDS dovetail bar + radius blocks.

A little ZWO 30mm guidescope sitting in a finderscope bracket is ok for the little refractor crowd, but inadequate to guide a C9.25.

The snag with an OAG is finding guide stars; the field of the guider is tiny.