New user, problem plate solving and PA

mickp

Just purchased the asiair kit with mini camera and guide scope to use on sky watcher az eq6 pro with canon 750d and WO apo61 with field flattener.

Really easy to set up and install software and opted for eqmod cable.

I've never done guiding before but within minutes of manual PA was guiding at 10 mins really impressive bit of kit. My problem is that I can not plate solve and therefore are unable to use asiair to PA. When I attempt to plate solve I get healthy amount of stars found but it just keeps running for about 90 secs then stops saying unable to solve.

I've tried my focal length set at 360 for the WO apo61 and at 576mm which is the suggested focal length with a canon 600 according to a review on astrobackyard, which I presume is the same crop sensor as in the 750d.

Should I be looking at focal length as a solution or have I missed on something else in my setup ?

w7ay

To plate solve, the software needs to know the plate scale (arc seconds per pixel) for the lens/sensor combination. "Plate" is the term from the old days of photographic plates.

I believe the ASIAIR is already set up with a database of DSLR that it supports to know the camera sensor's scale (micrometers per pixel), so that part is taken care of for you, and the only remaining unknown to solve for the plate scale is the focal length of the lens.

From their web page, the William Optics Zenithstar 61 APO has a native focal length of 360mm.

You also mentioned a flattener (but not which flattener). If you are using the FLAT61, it does not further change the focal length of the optical train, assuming that you have dialed in the correct back-focus. When you use a reducer or flattener, the back-focus is crucial not just to maintain the optimal field curvature, but it also affects the final focal length of the system.

Assuming that you have a FLAT61 set up properly, the focal length that you should tell ASIAIR to use is 360mm.

If 360mm did not work with the FLAT61, perhaps the flattener back focus is not perfectly dialed in (all is not lost, read further).

If you are using some other reducer, say, if the reducer is a 0.8x, you will need to multiply the native focal length by the reducer ratio (e.g., 0.8*360 = 288mm) to get the actual focal length of the lens.

If you are not sure what the true focal length is, enter 0 (yes, zero) into the focal length field of your main camera. When ASIAIR sees a 0, it will try more combinations to plate solve. It will take a little longer to do so, but when it succeeds, it will also change that zero in the focal length field to the actual focal length. This number is the precise focal length of your set up. You might note it down for future reference.

When unsure of the focal length (by entering zero), you might also adjust the exposure so that there are not too many stars detected. Try for perhaps no more than a hundred. Expose for more than 20, but less than 100. Also, reduce gain of camera and increase exposure to compensate, to get a cleaner plate.

You can read more about plate solving at astrometry.net.

Good luck,

Chen

mickp

Thank you so much Chen. Lot of info to take in.

Will try the zero method and see if I can get results

mickp

Ok Chen your suggestion worked now able PA and plate solve thank you so much

w7ay

Great news.

You might take a look at what value ASIAIR populates your focal length with.

If it is not what you expect, you may have a back-focus error. If so, that can cause poorer field curvature correction by the flattener -- stars at the corners might looks like streaks, even when the field rotation is fixed by good polar alignment. If the streaks look like arcs that rotate about the center of your plate, the back-focus is too long, and if the corner stars look like radial lines emitting from the center of the plate, the back-focus is too short.

Enjoy!

Chen

mickp

Yes corners are bad, just checked flattener and for some reason I have a twist lock adaptor on the end which I have now removed and have re-focused so hopefully will resolve issue. once again many thanks Chen

j. taylor

Great information, and will test with the first opportunity if clear skies, given I have a similar scope and flattened.