Bad focal in fits header and platesolve

[soulearth]

Hi,
Since around the end of October, sharpcap add an incorrect focal value in header of my fits header( bad focal value = 1260). However, I have correctly configured "Do not use" in the configuration.
This may have been fixed since, but when I specified my true focal length (615mm), the plate resolution have a issue ( crash/bug ).

My conf :

2021-12-13_08h12_18.png (3.59 KiB) Viewed 1389 times

Fits header:

2021-12-13_08h09_06.png (7.22 KiB) Viewed 1389 times

Thank you for your help,
Best regards,
Soulearth

[admin]

Hi,

adding the FOCALLEN header to fits was a fairly recent change. Currently it is *not* turned off by the option in the plate solving settings - that only controls whether the focal length is used to calculate the field of view when SharpCap launches the plate solve operation itself.

The value that you are getting is coming from your ASCOM Telescope driver - somewhere in the setup window for the ASCOM driver will be an option to set the focal length, where you will almost certainly find the value of 1260 is set - you should correct this setting.

thanks,

Robin

[soulearth]

Hi,
Thank you very much. I understand better now.
In the long term, will it be possible to define the value of the focal length?

Best regards,

[TimHaymes]

I have the FL set in ASPS and to ignore any FITS header info (..also set to Do-Not-Use in SC). Is there a recommended way of setting the FL? Just so that im sure i have the best arrangement. (No problems so far)
Thanks very much.
(EQ8, EQMOD, SMP11, SC 4.0.8218)

[admin]

Hi,

I think what I need to do in SharpCap is move the focal length setting from the plate solving page to the hardware page of the settings - then it would be more obvious that it applies to everything, rather than just plate solving. That would also mean that setting it manually would make sense for everything too.

cheers,

Robin

[k6inten]

Hello Robin et al.,

Sorry to bring up an old thread, but I am having this same issue. My scope has a focal length of 2023mm and I enter that into the SharpCap settings. After I plate solve, the focal length is reported as 1793.8mm in SharpCap and in my image FITS headers. You stated that the FL is retrieved from the ASCOM driver settings, but I don't see anywhere to set the focal length of my scope in my driver. I am using EQMOD v2.0w. Any ideas? Anyone else experience this?
The plate solving works fine with the 1793.8mm setting and PixInsight uses that value for it's image solver and it works fine too.
I am not using a reducer.

thanks,

Keith

[admin]

Hi Keith,

when choosing the value for the FOCALLEN header, SharpCap will give preference to the value calculated by plate solving over the one set in the settings - this is because the one from the plate solving calculation should be correct (the only real way for it to be incorrect is if there is an incorrect plate solve or if the pixel size of the camera is wrong).

I had to look this up, but it seems that what you are seeing is normal - see this post on another set of forums which explains that SCT telescopes have variable focal length as that is how they focus - https://theskysearchers.com/viewtopic.p ... f7#p252682

I suspect that if you physically move the camera either further back away from the back of the telescope, or further in, then re-focus, you would find a different result. There is some suggestion in the other forum post that getting close to the 'nominal' 2023mm focal length may give the best results.

cheers,

Robin

[Jean-Francois]

Hello Keith and Robin,

All the telescopes that move the primary mirror (or the secondary mirror in the case of the old Meade RCX)
will change the resulting focal and the back focus position. That is the law of the optic.

In the case of the RC (or Cassegrain) telescopes, they have both mirror fixed.
But the question is ... is it the correct distance ?

So all the telescopes are optimised for a defined distance between the primary and secondary mirrors.
If you change it, then you can refocus, but you will add some optical aberrations (typ. spherical aberration).

Regards,
Jean-Francois

[k6inten]

Thank you Robin and Jean-Francois.

I added an off axis guider to my setup the other night and had to redo all of the focus and now SharpCap is reporting a focal length much closer to the 'nominal' 2023mm. So I can understand where everything is coming from. Plus, as long as it works, I am good!
The only head scratcher is that the reported focal length changed about 200mm with the addition of the OAG but I know the camera distance from the mirror didn't change that much. But like I said, as long as it works...

Thanks again,

Keith

[Jean-Francois]

Hello Keith,

For a standard SCT 200 mm (Celestron or similar) ...

If the primary mirror moves to +1 mm or -1 mm, you have the following changes:

1) back focus moves -24.6 or +26.5 mm from the starting best focus position.
2) FD ratio changes from 9.61 to 10.39

Also, yes a reported focal length change of 200 mm is a lot.
Please note that SharpSolve is not designed for astrometry work. It is for finding the centre of the image view.
Any image distortion will modify the value of the resulting focal.

Regards,
Jean-Francois