Newbie question- why use separate guidescope?

EFRIDAY

I have been using my Celestron 8 with a Focal Reducer / ASI224 camera to get OK results in Plate Solving. I guess the 0.2 degree X 0.18 degree FOV just barely meets ASIAIR's requiements.
Why would one use a larger FOV scope (like a ZWO Mini 30 mm) for guiding?
Wouldn't guiding be much more sensitive to background star motion if the guidance image came from the main camera ?

w7ay

EFRIDAY Why would one use a larger FOV scope (like a ZWO Mini 30 mm) for guiding?

The general answer is that the larger FOV will gain you many more guide stars. This is a huge advantage, especially in the case of multi-centroid guiding, where you want to have as many as 12 guide stars available to average out the "seeing."

A further advantage of a separate guide scope is that when you use an off axis guider (OAG) to pick up guide stars on your main scope, the OAG's prism is picking up photons that are well away from the optical axis. The guide stars can look like birds with wings instead of circular disks. That will give erratic guiding.

The prisms are also small and pick up very little light -- that again, will reduce the number of available stars.

However, one thing you have to make sure to check is that there is less than an arc second of differential flexure between the imaging scope and the guide scope as you track across the sky. And even stiffer mounting for longer focal lengths like your SCT. If you cannot maintain that kind of mechanical integrity, you will need to use a guide camera in conjunction with an OAG.

Finally, do yourself a favor and find a better quality guide scope than the ZWO Mini 30. If you are using an 8" SCT, your guide scope should also ideally have a focal length that is longer than 500mm.

Chen

enaiman

Your post is a little bit confusing.
You can't use your main camera for imaging AND guiding (you can only do 1 thing).
Guiding will always be done by a separate camera (guide camera) which will either be connected to an off axis guider (OAG) or to a separate guide scope.

EFRIDAY

Chen,
Thanks for your thoughtful reply.
I had another thought while it is overcast here in Florida.
My C11 Edge has much too small a FOV for the ASIAIR to plate solve directly. Is it possible to load RADEC coordinates directly into my AM5 mount (obtained, say, from Astrometry.net) and force the mount to Sync ?
In other words offload Plate Solving to Astronomy.net ?

w7ay

EFRIDAY My C11 Edge has much too small a FOV for the ASIAIR to plate solve directly. Is it possible to load RADEC coordinates directly into my AM5 mount (obtained, say, from Astrometry.net) and force the mount to Sync ?

Remember that plate solving has nothing to do with the mount.

"Plate" refers to a photographic plate that is used by astronomers even not too long ago, like the plates on the View Cameras.

"Solve" simply means that you use the pattern ("asterisms") of the stars in the plate and use them to match with the asterisms in a database. ASIAIR itself uses a digested version of the database from atrometry.net -- but the digest is not the full database, and don't have sufficient asterisms to solve for a smaller area of the sky (i.e., smaller FOV of a small sensor/longer focal length telescope).

So, yes, you can take a JPEG, PNG, TIFF or FITS file from your camera, and send it to astrometry,net so they can use their full database. If you are not a paid user, you need to be willing to stand in line for their limited computer resource, and it could take 30 seconds to a minute to plate solve. I would suggest a small 8 bit PNG or JPEG to make uploading faster.

The plate solve tell you what the center of your camera sensor is pointed to in the sky.

The solved coordinates are useful for a number of things (if you want to tell someone else of a new comet that you have discovered, for example). It includes telling you mount where the camera is actually pointed (that is called "syncing the mount.")

You can sync your mount to the coordinates that are solved by astrometry.net. Depending on the quality of your mount, you may have to repeat this a number of times to get arc-second type accuracy.

Chen