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Old 07-01-2019, 08:27 PM
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silv (Annette)
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Join Date: Apr 2012
Location: Germany 54°N
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tl;dr The index files contain star catalogs according to field of view, 2-MASS was used for the 42xx index files and Tycho-2 for the 41xx.

42xx index files are installed according to the field of view your gear images at. Choose the index files as listed in the screenshot (that's taken from a different program) and with the index files cover a range from 10-20% to 100% of your FOV.
You could download every index file but that leads to long processing time during blind solving.
42xx files are for all sorts of photos, wide and narrow fields.
41xx files are only good for wide field photos.

end_of tl;dr


From the FAQ of another astrometry app called AstroTortilla:
“Which index-files do I need?
Each index-file covers a section of sky at a certain resolution, it is recommended you download the index-files that cover features from your FoV down to 1/10th of the FoV. If you use multiple setups, you can download from the largest FoV to 1/10th of your smallest FoV.”


The index files are from nova.astrometry.net, a US tax funded service. Almost every plate solving app uses the same index repository. Once you know the index files numbers fit for your gear, that knowledge can be applied to all plate solving software running on the “astrometry.net engine” .
The index files contain the star catalog for a given Field of View and pixel details - which depends on your telescope’s focal length, the pixel size of your camera, and its used resolution.
As marked in bold in the previous paragraph, you need a range of FOV-indeces, covering the range of 10-20% up to 100% of your FOV.

Basically, plate solving first looks at the image and then cycles through all downloaded index files one after the other until it finds a match.
That means, the less index files you download, the quicker the solving. Downloading all index files is certainly possible for plate solving and blind solving but could potentially lead to a veeeery long processing time for your image. Free hard drive space is also a consideration for some computers: all indeces take up more than 10GB of space.
Sophisticated plate solving software determines the FOV and the likely index file to start with by settings you entered abut your gear.
Cloudmaker’s Astrometry.app has no program settings per se because it’s really meant to run as a background service to Cloudmaker’s software package. In stand-alone mode, it requires you to know your FOV, and to choose and download the matching indeces accordingly.
I don’t know if one of the other Cloudmaker programs is more sophisticated and automates the first index-file determination for cycling through the possibles to find a match. I haven’t tried them.

Of the several Windows apps I tried, “All Sky Plate Solver”, ASPS, http://www.astrogb.com/astrogb/All_S...te_Solver.html
to me was the most descriptive in the task of understanding which index files to download. Although not complete, as I’ll describe later.
Attached is a screenshot of the Index Installation Wizard. You open that wizard from the main program menu “Indexes”.

I entered my scope’s focal length, 500, the “pixel size”, 4.72 (which is actually the “pixel pitch” = the distance from the centre of 1 pixel to the centre of its neighboring pixel. But the app calls this value “pixel size”) and the image’s resolution, 4912 x 3264.
From this, the app calculates on the fly my FOV as 158’ x 110’.
And it highlights in yellow which index files to download. Neat, isn’t it?

The first highlighted index is “FOV 30’ to FOV 42’ index-4208”.
30 arcminutes translate to the recommended ~20% of my actual FOV of 158’.
The other highlighted indeces state the FOV in degrees * instead of in arcminutes.
The last yellow file covers the range from 2.0* to 2.8*. Which includes my 100% FOV of 158’ = 2.6 degrees.

If you know your gear’s FOV in arc minutes, you can skip installing ASPS and just decide from the screenshot which index files from the 42xx range to install in Cloudmaker’s Astrometry.app.
Installing ASPS and the software it depends on was a pain [amount of pain is relative to the actual person in front of the computer ]. So if you can get the information re arcminute FOV resolution (10% or 20% to 100%) for your gear from somewhere else and then refer to the screenshot for the 42xx-range of index files, you’d save yourself a few grey hairs. For example use SkySafari and enter your scope, barlow, and camera details in the “Equipment” settings to get the FOV in degrees.
Here you can convert from arcminutes to degree:
https://www.advancedconverter.com/un...tes-to-degrees
from degrees to arcminutes:
https://www.advancedconverter.com/un...-to-arcminutes


As mentioned previously, this index suggestion in the 42xx range turned out to be incomplete for my particular case/image. It’s a true widefield shot and that’s where the 41xx index range steps in. All 41xx indeces combined are ~400MB in size so size matters not. But processing time does.
I have found no explanation for the different 41xx index files and which of them to choose. So far, Astrometry.app was able to solve my photos by using the indeces 4107, 4011 and 4012 so I installed all indeces from 4107 - 4012.


Astrometry.app is picky regarding the image quality. For example, a photo of 15 seconds with a lot of light pollution noise is processed forever and then fails to get solved. When I adjust the RGB curves and reduce noise before feeding it to the app it gets through, but possibly taking much longer than the mentioned 37 seconds.
The “Index Manager” in Astrometry.app for installing the indeces is in the menu “Window”.


I need to add that I photograph manually using a Sony NEX-5N and have to convert the photo to a fits-file using GIMP. My fits-format lacks all the information a true astro-imaging software might add to a fits header. So your mileage might vary.
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