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DSLR and narrow band imaging

brad11man
22 May, 2020 11:49
Hi all,

Looking for some help, I am very new to all of this (3 weeks in) and sometimes it is really difficult to see through all of the info available online. My setup is a skywatcher Mak 127 with a focal length of 1500mm, a skywatcher EQ5 pro goto mount, using a guidescope and camera (Omega 50mm Touptek cam).  I use a generic skywatcher light pollution filter (I am sure there are better ones out there). My imaging camera is an EOS 7d (no astro mod).

I am currently working on M81  (1st attemot was using my AZ mount) and have 7 hrs of colour data (I will upload what I have), and am wondering whether its worth buying an H-alpha filter and get extra data using that to help bring out detail in the galaxy. Any thoughts on this are welcome as. Is it even worth thinking about?

Also I am wondering whether its worth trying full RGB narrowband imaging, can this be done effectively with an unmodded camera?

Thanks and go easy on me, I am a real newbie! smile
dkamen
22 May, 2020 15:55
Hi,

It's emission  nebulae where Ha gives you detail in the sense that we usually have in mind when discussing narrowband pics.

Galaxies are the very definition of a broadband target since they contain stars of all spectral types plus all the other deep space objects. They are also thousands of times farther than nebulae, and almost all of them (including M81) are really, really faint.

Granted, some of them will have really prominent Ha regions (M101 and M33 spring to mind) but they will show up as bright red or purple splodges hanging from the spiral arms, kind of like the lights on a Christmas tree. You would not really be able to resolve the detail in Ha like people do with a nearby emission nebula, unless you had a scientific grade telescope/observatory combo. The only

The detail on galaxies comes primarily from dust lanes.

Now, narrowband in general is a bad idea with an one-shot-colour camera and it does not matter much if it is modified or not. Because you are collecting very little light to begin with and only one out of every four pixels in your sensor (the red pixel) is sensitive to it. It is not impossible and can even be rewarding with a bright source such as the Lagoon, but for a galaxy you need so much exposure to register anything above your offset, that thermal noise and other factors associated with ultra long exposure (random breezes etc) become significant enough to render the whole exercise futile.

Cheers,
D.
brad11man
23 May, 2020 07:35
Thanks D. That’s kind of really what I wanted to hear. It at least gives me a view on where I need to go with my equipment to get the data I want in the future.

So with my setup, to see the dust lanes in detail am I looking at 30 hours plus of light frame data. I look at some images where the f ratio is pretty low so I presume I just need to add the extra hours. Is there a way of working out exposure time to get the same results as others?

thanks and sorry for my complete inexperience

Brad
 
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