How do I plate solve correctly?

[VinnyV84]

Hi, I have an Evolution 8 SCT and use a ZWO ASI678MC to do planetary imaging. I was interested in bypassing my StarSense auto align camera routine in favor of trying plate solving with Sharpcap however every time I aim at a nearby group of stars and hit plate solve it fails every time. I am in a Bortle 8 so not sure if that plays into it.

I’m thinking the chip in my camera may be just too small? Or am I missing a critical step prior to hitting the plate solve button? Any help would be great. Thanks!

[admin]

Hi,

yes, you're quite right - the field of view from the combination of your 8" SCT and your ASI678 works out to be 0.22x0.12 degrees (you can use this web site to do field of view calculations : https://astronomy.tools/calculators/field_of_view/). That's really pushing the limit for plate solving, certainly too small for SharpCap's built in SharpSolve (OK down to about 0.25x0.25 or above). You might just get some success with ASTAP with the highest resolution D80 database, but even that is on the edge and will need long exposures to get enough stars and be slow to calculate results.

To get things working nicely you would need to increase field of view, which would really mean a larger sensor camera - at least twice as big in each direction as the 678. Other options would be a shorter focal length telescope (but then you would be aligning the shorter telescope, not the SCT8) or a focal reducer (but a .66x focal reducer would not be enough and the 0.5x ones are not usually optically very good)...

cheers,

Robin

[VinnyV84]

Hi,

yes, you're quite right - the field of view from the combination of your 8" SCT and your ASI678 works out to be 0.22x0.12 degrees (you can use this web site to do field of view calculations : https://astronomy.tools/calculators/field_of_view/). That's really pushing the limit for plate solving, certainly too small for SharpCap's built in SharpSolve (OK down to about 0.25x0.25 or above). You might just get some success with ASTAP with the highest resolution D80 database, but even that is on the edge and will need long exposures to get enough stars and be slow to calculate results.

To get things working nicely you would need to increase field of view, which would really mean a larger sensor camera - at least twice as big in each direction as the 678. Other options would be a shorter focal length telescope (but then you would be aligning the shorter telescope, not the SCT8) or a focal reducer (but a .66x focal reducer would not be enough and the 0.5x ones are not usually optically very good)...

cheers,

Robin

Thanks Robin! Makes complete sense. I have focal reducer (F/.63 I plan on using when doing some EAA but sounds like my best bet is continuing to use my StarSense Auto Align to to do my alignments. I wouldn’t mind investing in a proper guide camera but seeing as how I’m on an Alt/Az it’s only real function would be to plate solve as the mount won’t actually “guide”. I appreciate the insight!

[Chuckwagon]

Just curious, I don't own a Starsense so I don't know, but does the Starsense camera show up as a camera on your computer that Sharpcap could use? The specs say it uses an MT9M034 sensor, which is what the QHYCCD 5L-IIM use I think. So I'd suspect a driver exists that should work with it, unless Celestron does some weird fiddling to keep it hidden. If it can be seen in Sharpcap I'd think you could use the Starsense for PA in much the same way that a Polemaster can be used in Sharpcap. I use my Polemaster with Sharpcap when I have too much focal length to use the main camera and it works great. Maybe that would work.

Chuck

[kaymann]

Robin my C9.25 straight (f/10) with my 533 MC Pro on the site you listed above gives a Field of View: 0.28° x 0.28°. This FOV seems to be within your SharpSolve requirement of 0.25°.

I too can never get it to solve, even using your 0.25°optional database. Any suggestions to improve probability of success or is this just too close to the limit?

[admin]

Hi,

I'm afraid I have no idea if the starsense can connect to a PC. My guess is not, but maybe I'm wrong.

On plate solving at 0.28x0.28 degrees - it ought to work with enough stars being detected, which at that field of view will require longer exposures (10s or more), high gain, 16 bit mode and possibly binning too. If you are getting a good visual selection of stars and still no luck, feel free to post an image or two and I will see if I can work out what is going on.

I have been getting working plate solves with an IMX571 camera and a 1836mm focal length Classical Cassegrain + 2x barlow (0.37x0.24 degree FOV). I did notice the other day that it didn't work when there was a very bright star in view though, which I think was throwing off the detection of the fainter stars.

cheers,

Robin

[kaymann]

Robin, I will give those settings a go and see what happens. I will keep in might the abnormally bright star throwing things off...

BTW because plate solve fails, I resort to plate solving on my guide scope. However, I would much prefer to plate solve the main scope in case the guide scope got bumped or moved during guide scope focusing or other setup procedure. With five thumbed me that can easily happen.

[shoste]

Dear All,
All my astrophoto double star work is done with Sharpcap. Superb!
However, I have NEVER succeeded in plate solving even once, using ASTAP and the D50 database. I use the WCS method in AstroimageJ. Works 95% of the time but is slow. When I load any FITS file in ASTAP directly, it solves them instantly! I diligently loaded ASTAP and tested it in the Sharpcap settings. I use a Mewlon 180 at F/11, so admittedly there are not too many stars present, but they seem more than sufficient for standalone ASTAP. What am I doing wrong?
Yours truly, Serge

[admin]

Hi Serge,

probably the best thing to do is to share two things with me

1) The log from SharpCap after a failed attempt to plate solve

2) A copy of the image file (saved as FITS) that failed to solve.

That lets me see what options are being sent to ASTAP and whether there are any problems with the configuration, and also test myself with the image file and see if I can see what might make it work.

The image file will be too large to upload here (we have an attachment size limit of 1Mb) - please upload to Google Drive, DropBox, OneDrive or similar and send a sharing download link for me to download the file.

thanks,

Robin

[shoste]

Dear Robin, Thanks for your kind reply and offer. As by magic the proprietary plate solver in Sharpcap works really well on 60 seconds exposures with my f/12 Takahashi Mewlon. Problem solved.

BUT, another issue came up. In measuring eclips variables (e.g. a sequence 200 60 sec exposures), exposures taken after midnight automatically go into a new folder and are renumbered from 0h0m0s onwards. That means that the light curve is being split in two parts and rejoining them is not straightforward (apart for copying them to the directory of the day before, but then the hours are not consecutive any more...). Would it be possible to save sequences of files simply using Julian date and time (eg as a string with 5 digits after the comma) for the filename? That would put any sequence automatically in perfect chronological order. What about using the JDaverage time from the FITS header. Thanks for considering this.Do you have a better suggestion?
PS: I know that you can append yesterday's date {Date:Y} but that still renames the new files beginning with 0-0-0, and thus loosing the continuity again. Actually, sorting these would then place them in the wrong order.