I recently started taking some deep sky images of broadband sources from our location in central London. This is a Bortles class 9 location, a so-called white zone. You would think it would be impossible to take images of dim objects like galaxies when you can barely even see any stars in the sky. But an interesting technique that I learned about recently allows taking broadband images (rather than narrowband images like H-alpha) from the most extremely light polluted areas.
The light pollution problem has become worse from an astronomical perspective as narrowband light sources like low pressure and high pressure sodium lamps are replaced by LED lighting. The narrowband sources had distinctive lines associated with sodium and mercury that could be filtered, but with LED lighting, this approach is not useful.
In the past, amateur astronomers have used deep red filters (like the Wratten #29) to push the imaging into the near infrared. More recently, some amateurs in Tokyo https://satakagi.github.io/IR-imaging-o ... pollution/ have started to use the Astronomik ProPlanet 642 BP filter, which has a very useful 200nm bandwidth of approximately 642-842nm. In this area, CMOS/CCD sensors are typically very sensitive and LED lighting has fairly little output. It also encompasses the H-alpha line.
Another advantage of imaging in this wavelength band is minimising the scattered light from the Moon. Blue light of course scatters more than near infrared and so sky glow from the Moon is much diminished.
The images below were taken with a Vixen 140 NA (140mm diameter, 800mm FL), a unique telescope that has a bit of cult following -- it is an achromat with a Petzval design, so a very flat field and quite low F ratio. It turns out, this (relatively inexpensive) telescope is very suitable for this kind of bandpass imaging. The ProPlanet 642 BP was used with an Atik 460EX monochrome camera. The mount was an HEQ5 Pro, the guide scope was a 100mm f4 Pentax lens, and the guide camera was an ASI 174MM. A red 25A filter was used on the guide scope to improve guiding performance.
I used 1 min exposures and did not have much time to take many of them since in London at the end of May, the nights are very short. Instead of Deep Sky Stacker, I tried out the ASI DeepStack that comes with ASI Studio v1.4. It's probably not as good as DSS, but it's very simple to use. Its best feature is it has a very good built in FITS viewer so you can identify and remove poor quality frames easily. In general, the ASI FitsView is very good and I think a lot people now use it as a default FITS viewer. Knowing ZWO, I'm sure their stacking software will improve quickly (I think this is their first version).
In terms of image processing, I just did a little GIMP and Photoshop Elements (nothing fancy like star masks). This was more of a proof of principle. I had previously only done planetary and solar imaging given the extreme light pollution here. Now I see deep sky is also possible.
Deep Sky images from a Bortles Class 9 white zone
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Re: Deep Sky images from a Bortles Class 9 white zone
These are amazing results for wideband sources from your location, Douglas!
Perhaps I’ll try my hand at some deep space imaging with this type of filter.
Thanks for posting your results.
Cheers.
Peter
Perhaps I’ll try my hand at some deep space imaging with this type of filter.
Thanks for posting your results.
Cheers.
Peter
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Re: Deep Sky images from a Bortles Class 9 white zone
Have a read of the Japanese website I mentioned. He goes through some good analysis of the spectrum of light pollution, etc. It's a really nice filter and not very expensive. I got the 1.25 inch version and put it into a filter wheel. I already have the Astronomik H-alpha and OIII so maybe will use the ProPlanet as L and try adding some colour.
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Re: Deep Sky images from a Bortles Class 9 white zone
Very impressive results.
While I have a remote setup under class 2 skies to satisfy my deepsky needs, it might be fun to play with this a bit at home (class 6) as well. Will read up on that filter; thanks for sharing.
While I have a remote setup under class 2 skies to satisfy my deepsky needs, it might be fun to play with this a bit at home (class 6) as well. Will read up on that filter; thanks for sharing.
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Re: Deep Sky images from a Bortles Class 9 white zone
Very interesting, and wonderful results too hamster: I would never have imaged you could get such good images from London and in summer time too
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Re: Deep Sky images from a Bortles Class 9 white zone
Good information and great images.
Stu.
Stu.
H-alpha, WL and Ca II K imaging kit for various image scales.
Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
More images at http://www.flickr.com/photos/solarcarbon60/
Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
More images at http://www.flickr.com/photos/solarcarbon60/
Re: Deep Sky images from a Bortles Class 9 white zone
I'm also fond of photos, but how do you get them like this?