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Imaging a Galaxy from a Fixed Tripod

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#1 james7ca

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Posted 10 October 2018 - 07:13 AM

There was a tread from last week that asked about using a simple alt-azimuth mount to do imaging of galaxies. There were comments suggesting yea or nay and a discussion about the limits imposed by field rotation. A further complication was added when the OP reported that the mount in question had no motorized tracking or guiding.

 

So, the question then became is it possible to get an acceptable image of a galaxy using a fixed, alt-azm tripod? One can certain find wide-field images that include the Andromeda Galaxy (M31) or the Magellanic Clouds that were taken with a single, short exposure with a fast camera lens but those generally don't reveal much detail (at least not in a way that might be termed an "acceptable" astro photograph, pretty yes, but maybe not that recognizable as a galaxy).

 

To be inclusive it should be noted that before computer-guided mounts and digital cameras it was fairly commonplace to manually guide equatorial mounts when imaging with film-based cameras and a lot of really fine images were produced that way. In fact, many years ago I strapped a 35mm film camera with a 50mm f/1.4 lens to the side of an alt-azimuth-mounted, department-store refractor that was used to manually track a star for several minutes to create an image of the center of the Milky Way (no slow-motion controls, just my hands on the body of the optical tube). So, yes, manually guided images are nothing new. 

 

In any case, I eventually posted an image of M31 to the original topic that I took using a 105mm camera lens from a fixed tripod using a series of 0.5 second exposures with a cooled ZWO ASI178MM camera. At that time I stated that with some DSO targets it would be possible to get an acceptable image but that it might not be that practical to do so (most of us probably don't have the patience to sit by the telescope for hours at a time to manually track a target, particularly when restricted to a non-motorized alt-azm mount).

 

My original image was done rather quickly under red/orange zone light pollution with a total integration time of 12 minutes and several days later I set out to see if something better could be done. Well, I had a relatively nice evening this past Monday and I sat outside for about one hour and captured just over 5000 subs using that same 105mm lens and ASI178MM camera that was mounted on a manually positioned Vixen Porta II alt-azm mount (no motors or auto guiding, I just let the field drift for one minute and then repositioned the framing).

 

Herein is the result from combining those 5000+ subs that were each exposed for one half second (luminance only with a Baader Fringe Killer filter). The total integration time was 42m 50s (5140 x 0.5s, lowest read noise gain setting in SharpCap). Image processing was done in PixInsight and Photoshop CC2017 and I was still working under red/orange zone light pollution. The Nikon 105mm AI-S lens was stopped down to about f/3 using a front-mounted, step-down filter ring and I focused as best as I could using a 52mm diameter Bahtinov mask. When I say "best as I could" I need to add that this Nikon lens has a fair amount of chromatic aberration which is plainly detectable given the 2.4um pixel size of the Sony IMX178 sensor. This same lens seems to do fairly well with narrow band, not so much for one-shot-color or luminance.

 

I think this image is okay, but I really wonder how good you could get under dark skies with a better lens and when working with a camera like the color QHY5III-178C. Perhaps someone already has such an image that they would like to share (requirements, fixed, non-motorized alt-azm mount and reasonable image scale).

 

Lastly, there are three galaxies in this shot, the Great Andromeda Galaxy (M31), M32, and M101 (lower right). Given those two smaller galaxies I'm fairly sure that other galaxies could be imaged with similar techniques (if you have the time or patience to do so).

Attached Thumbnails

  • M31 with a 105mm Nikkor on a Fixed Tripod (Small).jpg

  • dciobota, deepwoods1, tomwall and 6 others like this

#2 james7ca

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Posted 10 October 2018 - 07:55 AM

Here is the setup that took the above image. The wires running down to the ground were for the camera (USB3), the camera cooler (12VDC), and a dew heater (12VDC).

 

As I mentioned above, I used a front-mounted step-down filter ring to produce something close to an f/3 system as I didn't want to introduce diffraction spikes by using the aperture stop that is built into the lens. The lens was a 105mm f/2.5 (native) Nikkor AI-S manual focus product that I purchased used on eBay several years ago. On hindsight I probably should have used an aperture closer to f/4, but that would have reduced my signal (effective exposure) quite significantly.

 

As for the base exposure (0.5 seconds or 500ms), I probably could have gone a little longer but even with that short exposure you could definitely see drift between each frame (as was revealed when I registered the images, more than 1 pixel of movement between each frame and the median FWHM on the linear master was 2.222 pixels or 10.5 arc seconds).

Attached Thumbnails

  • Alt-Azm Setup.jpg

Edited by james7ca, 10 October 2018 - 10:44 AM.


#3 james7ca

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Posted 10 October 2018 - 08:33 AM

Here is the high-value pixel rejection map for the images that were captured during the first 30 minutes of the session (about one half of the total 5140 subs, my computer and PixInsight couldn't handle the integration of the full count of subs in one run). What you see here is an indication of the field rotation that happens when using a simple alt-azm mount. PixInsight can align the individual subs to remove this rotation but if you expose each sub for too long of a period of time the rotation will show up in the finished image. I'm not sure what that exact time limit would be as it varies with the location of the target in the sky, but you'd probably be limited to exposures of not more than a handful of seconds (and also depending upon your image scale and the field of view).

 

If you simply registered the images with only x and y translations (without rotation) then the final image might look like a circular blur with the only sharp part of the image toward the center (either that or the entire image would appear blurred or maybe the registration would just fail).

Attached Thumbnails

  • Pixel Rejection Map.jpg

Edited by james7ca, 10 October 2018 - 10:54 AM.


#4 james7ca

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Posted 10 October 2018 - 08:47 AM

Here are my FWHM and eccentricity plots for the finished, linear master (all 5140 subs). They aren't too bad although I can definitely see distortion in some of the stars (optical aberration from the lens). The image scale for this lens and camera combination was 4.71 arc seconds per pixel.

 

I had to crop the top, bottom, and sides because the field rotation didn't allow all of the subs to cover those areas. You can see that lack of coverage in this pixel rejection map (showing the low pixel value rejection, white means high rejection because those areas received less exposure and were much more noisy). I used this map as a guide for the crop. If you imaged for a very long time you'd probably only be able to use the central, circular part of the image, so you'd end up with a square looking format (or round, if that is what you'd want).

 

During this approximately hour long session I definitely noticed the rotation while I was taking the subs, since it changed which stars were visible in the corners of the field. It also changed the amount of movement in the altitude and azimuth that was required to recenter the galaxy. As the session progressed I needed more and more movement in one axis and less in the other.

Attached Thumbnails

  • FWHM and Eccentricity.jpg
  • Low Rejection Map (Used for Crop).jpg

Edited by james7ca, 10 October 2018 - 10:25 AM.



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