Plate solve with short focal lengths

Hi, I’m learning step by step about the great features of Prism. But plate solving with a telephoto lens gives me insurmountable problems. What star catalog and settings are best to use with a 200mm lens. I am glad about any help.

1 Like


200mm lens, is big field but should still be possible.
Take the GSC-ACT catalogue for this widefield.

The UCAC 4 and GAIA would take to much to stars for this big field and so more difficult to plate solve.
If you could pass a link containing one image and give me the details about equipment:
200mm focal length, and pixel size of the camera in micron, i will check for myself if i can platesolve it.

Also what are the center coördinates ( approx ) and possible angle of the image…

1 Like

Thank you very much for your help,
here is the link:

In the FITS header is all the data you need.
Hopefully you will be successful.

excellent response @ObservatoryGromme , the GSC-ACT will do it. Also, it has a lot to do with your pixel scale, I recommend bookmarking this new page that I am building to be the astronomer’s one-stop-shop utilities page : Astronomer Tools - Hyperion Astronomy

Be sure you have set up your FL and f ratio under Hardware setup / camera. also as pointed out be sure your pixel size is correct (this is generally set when you select properties for the driver.
To plate solve an image I right click on the image and select the last of three options for plate solving about 3/4 down the menu options. I use GSC-ACT. My set up is a 9x50 finder scope with an ASI 120 which works out to a fl of 175.9 fl at f1.9 and 3.75 pixels so it should work for you.

1 Like


Okay, first things first:
Platesolving a 99mb image is just to much… This requests a lot of solving time.
I had never got this working as i eventually gave up because of the size in mb.
I binned the image 3x3 so i had an size of 2084 on 1392 pixels.

I had a succes using the application in the 32bit version.
The ascom is not installed in the 64 bit version. Maybe a V11 feature?

The information retrieved after a succesfull platesolve with
Annotated Jpeg :
Plate solve image retrieval pending...
Retrieved plate solved image
Center image coordinates (eq 2000): RA=11h13m44.939s  DEC=+57°51'22.49''
Focal before computation : 195.2 mm, after computation is 197.4 mm
Image angle : 210.8 °

This gave me information that i could use on the “single image platesolve”
Then i got succes using GSC-ACT catalogue giving me this result:

66 stars after test, 120 before test of centroid...
Number of stars validated by centroid :
Plate Solve -> Final computation, step #1
Automatic computations are completed
Plate Solve -> Final computation, step #2
Plate Solve successful
SolAlpha (+3.5441653E-01,+5.1682072E-01,+8.6360176E-01,+6.9255902E-07,+7.1706923E-07,+6.0856405E-07)
SolDelta (-1.5490051E+01,-8.6389984E-01,+5.1687958E-01,+6.9897743E-08,-5.2572951E-07,-6.0025231E-07)
SolX     (+7.5989582E+00,+5.1008856E-01,+8.5264875E-01,+1.7834078E-07,-5.7705041E-07,+2.8794823E-08)
SolY     (-1.3383711E+01,-8.5237245E-01,+5.1014912E-01,+1.4387648E-07,-9.3465876E-07,-6.8197229E-07)
Star used: 66, final residuals 1.057", polynomial degree: 2
Computed Focal length : 196.1 mm
X+ CCD Axis and RA+ angle 210.9 °
DEC+ Axis wrt X+ CCD angle: 300.9 °
Direct orientation
Image Center Coordinates, RA=11h13m43.474s, DEC=+57°48'06.51''

But for some reason unknown to me i cannot repeat this plate solving with succes again…

I suggest you take 3x3 binning images if your platesolve is for finetuning the pointing accuracy.

Thank you both very much for the help. The problem is probably the much too large amount of data. Because I have set the setup as amstpils suggested. Helpful is the suggested use of GSC-ACT. But I had hoped to be able to make special settings in the catalog options. Unfortunately, with the QHY268 I can only set a 2x2 binning. Perhaps that will be sufficient. In the next clear nights I will try this. Is there a way to downsample an existing FITS image for testing (2x2 binning)?

Absolutely you can, it is under Edit > Resample

@ObservatoryGromme : What do you think about using a smaller Windows frame in the CameraControls to keep the image size for the Plate solve small?

1 Like

that would actually work better as peter noted, please see this screenshot

1 Like

Like @Hamza said: Try indeed to use a smaller (1/2 or 1/4) with binning 2x2 to platesolve.
What also helps is to have 0° degree north on top or bottom image. This is easier / faster to platesolve.

Yes, the combination of 1/2 or 1/4 field and a 2x2 binning will lead to success. I am convinced of this by your advice. Aligning the camera angle will be helpful but not always possible. For example, if a certain arrangement is to be created. :+1: Now I just need clear skies…

a pro tip: test your stars after focus. IF they are bigger than 5 pixels, absolutely bin. If the are low, 1 pixel or so, do not bin.