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Stub Mandrel

Producing Narrowband FITS for Photometry from DSLR Images

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I'm taking part in the HOYS-CAPS project.

http://astro.kent.ac.uk/~df/hoyscaps/index.html

The challenge for me has been producing narrowband FITS images suitable for photometry. That means not applying any transformations to the data, but as I use a DSLR to produce a HA image I have to extract the red layer and use it to produce  a 'flat' greyscale image. I can't simply save suitable a FITS file in DSS.

Following some discussion and advice from Dirk Froebrich, who is running this Citizen Science project, I have come up with a workflow for Deep Skay Stacker and Photoshop that produces a suitable FITS file. I've also used FV, a free program from NASA, to place date, filter and exposure data into the final FITS image. Dirk suggested I put it on the project Facebook group, but I'm allergic to Facebook, so I'm posting it here as some folks may find it useful for the project or indeed any other photometry project.

 

Stack in Deep Sky Stacker using control frames. Use super pixel mode to prevent debayering.

Open the resulting 32-bit autosave file in Photoshop:

  • Open the channels tab and select the red channel (for Ha)

  • <ctrl>-A to select the layer

  • <ctrl>-C to copy the layer

  • File --> New --> enter (should create a 16-bit greyscale image)

  • <ctrl-V) to paste the red layer as a (rather dark) greyscale image

  • Flatten the image and save it as a 16-bit grayscale tiff with a suitable name

In DSS:

  • Open a picture file dialogue, choose the new file just created

  • Save as a 16-bit FITS without applying any adjustments

  • The red channel only is now in a FITS file without any stretch or transformation having been applied but the FITS header contains no image specific data.

The FV utility from NASA is very user unfriendly but can be used to add the FITS 'keys':

  • Use this format, key name, 'data in single quotes', /comment

  • Date = 'YYYY-MM-DD HH:MM:SS' /UTC (or GMT etc.)

  • Filter = 'Filter type' /note   (e.g. 'Ha' /Baaader 7nm)

  • Exposure = NNNN /in seconds

  • Save the header, then save the whole FITS file.

 

You can now upload the file. You will need to tell the upload site it is Julian date, format  'date and time combined'

 

RGB DSLR images can be directly saved as FITS in DSS (without adjustments), you can use FV to add any missing data.

It is also possible to add missing FITS data directly through the upload site.

 

The main point is using Photoshop to extract the narrowband data and using DSS to covert it to a FITS - Google suggests TIFF to FITS converters are virtually unknown!

 

Additional note on FITS to TIFF:

Unlike TIFF to FITS, thr conversion from FITS to TIFF is supported by many utilities. If you use 'FITS Liberator', also free from NASA, you can save your images as an unadjusted FITS in DSS, then apply a range of mathematical stretches (much as you might do in Pixinsight) to your data and save it as a 16-bit tiff. This means you are maximising the data available to be processed in Photoshop bin the 16 bit file.

My brief experience of this is that it is far superior to applying a stretch to 32-bit data in DSS.

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Hi,

I was wondering if you've seen this:

I used this to extract the r,g & b channels from my Canon raw CR2 files. It was a bit tricky to get it set up and working under Ubuntu Astronomy (running under VM VirtualBox on an Intel NUC PC) largely due to my total lack of Linux knowledge but it did work and I was able to extract each channel as FITs files and then process them more easily in PixInsight.

Apologies if I am off the mark with what you are doing so hope I'm not wasting your time.

Adrian

P.S. Oleg was really helpful in helping me to get the whole thing up and running.

Edited by Adreneline
P.S. added
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Although mainly designed for spectroscopy, ISIS for example

http://www.astrosurf.com/buil/isis-software.html

has a tool to convert raw DSLR images direct to 3 separate fits files. See version 5.8.0 update here

http://www.astrosurf.com/buil/isis/new/release.html

(It probably exists in IRIS somewhere too)

The AAVSO also have a detailed manual covering DSLR photometry which describes the full procedure.

https://www.aavso.org/dslr-observing-manual

Cheers

Robin

 

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Thanks both.

I think DSS's super-pixel mode has the edge as converting RAWs to three separate files then stacking is three times as many files as stacking then extracting the red channel. I can see both routes avoid any interpolation.

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