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Narrowband

Interesting Experiment with Ha and lEnhance Filters


Catanonia

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I have a ZWO 2600 MC Pro OSC camera and normally image with the Optolong lEnhance filter that gives RGB images along with Ha and OIII data.

Specs are here

The Major Emission Lines of Nebulae: H-α 656.3nm, H-β486.1nm, OIII 495.9nm, OIII 500.7nm 
The Major Emission Lines of Artificial Light Pollution: Hg 435.8nm, 546.1nm, 577nm, 578.1nm, Na 598nm, 589.6nm, 615.4nm, 616.1nm

Now with the recent moonlit nights, even this filter gets washed out by the moon.

So I decided to replace the lEnhance filter with a generic Ha 7nm filter and take some images with the hope of combining this with the original data.

As I started to process the images, I noticed that the Ha data and the Red channel of the lEnhance data were basically the same. Ie no difference and if anything the lEnhance filter Red channel was better than the Ha 7nm data

Here is a picture explaining it.

  • Left is Ha data in 25x 5min subs @bin2  (needed this length of sub to get a decent signal) - Rasa 8 @F2 - warning that the filter is not working with the Rasa @F2 ?
  • Right is the lEnhance Red channel in 74x 1min @bin2 (needed shorter subs to achieve same signal) - Rasa 8 @F2
  • Both images have had exactly the same cropping, DBE, stretching done to them on starless images to make it easier to see.

As you can see, the lEnhance Red channel is just as good if not better than the 2+ hours of Ha 7nm data

So in summary, no point whatsoever in using the Ha 7nm filter to gather extra if you have a lEnhance filter. If you want Ha data on a moonlit night, use the lEnhance filter and extract out the Red channel.

Also you don't get those horrid halos that some Ha filters give you.

Either that or the 7nm Ha filter I am using is blocking too much of the sharply angled fast F2 light the Rasa delivers - Thoughts ?

 

edit

After some research, found this about highspeed F2 filters

Baader f/2 Highspeed H-Alpha Filters

 

Available in 1 ¼", 2", H-alpha / O III and S II are designed especially for the delicate requirements of extremely fast astrograph optics, in particular Hyperstar and Celestron's New RASA.

Conventional narrowband filters cause a heavy loss in transmission due to the strong CWL shift (CWL= Center wavelength).

In extreme cases the CWL even does shift out of the FWHM (FWHM= full width half maximum).

Therefore these filters have a CWL preshift which matches f/2 to f/3 perfectly.
Also the FWHM is optimized. In spite of the typical line broadening with such fast optical trains they are able to deliver maximum contrast. For the first time these filters allow to use fast optics really effectively with high contrast emission nebulae imaging. Using these filters between f/1,8 and f/3,5 shows a dramatic improvement compared to any regular set of narrowband-filters.

 

Filter Comparison.jpg

Edited by Catanonia
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