Changes to FITS header

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garyhawkins
Posts: 54
Joined: Sun Dec 22, 2019 6:59 pm

Changes to FITS header

#1

Post by garyhawkins »

Hi Robin,

Can you confirm that the FITS header has been modified to include DATE-END and DATE-AVG, in addition to DATE-OBS? Have any additional changes been made to the FITS header in recent versions of SharpCap?

Best regards,

Gary
ChrisR Oz
Posts: 158
Joined: Tue Sep 28, 2021 11:03 am
Location: Melbourne

Re: Changes to FITS header

#2

Post by ChrisR Oz »

Hi Robin,

A related question, if I may:

When you slew to a target and then "Plates-solve and sync", what values for RA, DEC (or OBJTCRA, OBJCTDEC) end up in the FITS header? Is it the text book coordinate requested (after all, we just synced to them), or the somewhat erroneous position requiring a sync/adjust?

I am seeing some differences, which can cause PCC in PixInsight to "spit the dummy" complaining that the plate-solve failed, which is only happening because the RA/DEC from the FITS header are off by a few minutes of arc. I can fix that. But it would be nice to know how to get the RA/DEC correct in the header. Returning to targets using plate-solve and sync always aligns them well with previous visits, so all is working well. But evidently the FITS tags are not consistent.

Cheers, Chris.
Celestron EdgeHD 8, reducer 0.7x, Star Sense, CGX-L mount, Focuser, CPWI; Starlight Xpress AO, OAG and Filter Wheel; ZWO 294MC/294MM Pro and 174MM mini; SharpCap Pro, PHD2, Powermate 2x, Baader Neodymium, Astronomik CLS-CCD, ZWO UV/IR, Duoband filters.
ChrisR Oz
Posts: 158
Joined: Tue Sep 28, 2021 11:03 am
Location: Melbourne

Re: Changes to FITS header

#3

Post by ChrisR Oz »

... of course, if I ever get some clear skies I can do a few tests/experiments. But it's probably quicker to ask, with the way the weather is down here.

Cheers, Chris.
Celestron EdgeHD 8, reducer 0.7x, Star Sense, CGX-L mount, Focuser, CPWI; Starlight Xpress AO, OAG and Filter Wheel; ZWO 294MC/294MM Pro and 174MM mini; SharpCap Pro, PHD2, Powermate 2x, Baader Neodymium, Astronomik CLS-CCD, ZWO UV/IR, Duoband filters.
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admin
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Re: Changes to FITS header

#4

Post by admin »

Hi folks,

DATE-END and DATE-AVG have been in the fits headers for a while now. Other recent changes include correcting the pixel size headers for binning and correcting a bug where the timestamp headers were being rounded to the nearest millisecond.

The RA/DEC/OBJCTRA/OBJCTDEC headers are values read from the mount position at the time the frame is captured. That means they will depend on what settings you have used for plate solve/resync. If you have selected the 'offset goto' option then they will not match the target co-ordinates from the sky catalog. They will also be JNOW co-ordinates in most cases where the catalog co-ordinates will be J2000. As far as I can see there are simply too many things that can happen to the mount position after the initial GOTO and before image capture (framing, solve/recenter, dithering, etc) to try to do anything other than use the current mount co-ordinates.

cheers,

Robin
ChrisR Oz
Posts: 158
Joined: Tue Sep 28, 2021 11:03 am
Location: Melbourne

Re: Changes to FITS header

#5

Post by ChrisR Oz »

Hi Robin,

Those are all good points. I guess I was just focusing on the case of saving an image immediately after a "plate-solve and sync". But it seems to me that the problem is in PixInsight.

It seems that PixInsight is not recognizing your "EQUINOX" tag in the header to flag JNOW coordinates. It seems to accept the RA/DEC as J2000, which throws off the plate-solve in PCC.

I have logged a couple of questions on the PI forum to straighten this out, and to ask what rules they follow for decoding FITS header tags for coordinates.

Cheers, Chris.
Celestron EdgeHD 8, reducer 0.7x, Star Sense, CGX-L mount, Focuser, CPWI; Starlight Xpress AO, OAG and Filter Wheel; ZWO 294MC/294MM Pro and 174MM mini; SharpCap Pro, PHD2, Powermate 2x, Baader Neodymium, Astronomik CLS-CCD, ZWO UV/IR, Duoband filters.
ChrisR Oz
Posts: 158
Joined: Tue Sep 28, 2021 11:03 am
Location: Melbourne

Re: Changes to FITS header

#6

Post by ChrisR Oz »

Hi Robin,

Reply from the PI forum confirms that PI does not recognise EQUINOX at present, but they will add that for next update.

On export as FITS, after plate-solving in Photometric colour calibration, PI outputs in J2000 with the tag “RADESYS” = “ICRS”.

Thought you’d want to know that, so that SC can use such frames in the new image plate solve and goto function coming soon …
Cheers, Chris.
Celestron EdgeHD 8, reducer 0.7x, Star Sense, CGX-L mount, Focuser, CPWI; Starlight Xpress AO, OAG and Filter Wheel; ZWO 294MC/294MM Pro and 174MM mini; SharpCap Pro, PHD2, Powermate 2x, Baader Neodymium, Astronomik CLS-CCD, ZWO UV/IR, Duoband filters.
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Re: Changes to FITS header

#7

Post by admin »

Hi Chris,

thanks for the update on that - I'm not sure if I need to worry about the co-ordinate system too much, since SharpCap will automatically trigger a plate solve on the image (with a 15 degree search zone), so the difference between JNOW/J2000/etc in the header data will be insignificant (and the plate solve results are always J2000). I do hope that no-one is planning a J2050 system too soon though!

cheers,

Robin
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