M27 Dumbell Nebula with EOS600D despite Backlash

Posted by XTSee on 15th October , 2012

A couple of nights ago I had another go at learning to use my un-modified Canon EOS 600D DSLR to hone my techniques in astrophotography with my C6-SGT scope. Now I’m starting to feel like I’m actually getting somewhere!

M27 Dumbbell Nebula

M27 Dumbbell Nebula

Even so there were a few things wrong with trying to get this image…..

For instance both the DEC and RA axis have quite a bit of backlash on my Synscan EQ5 mount (it needs adjusting) and because I didn’t give a bit of weight bias to make the RA axis motor work to pull the counter-weight “uphill”, so to speak, the gears were pushing the mount to a certain tipping point, then gravity caused the weight on the scope+camera side to “topple” downhill. This was only by the tiniest amount of the backlash distance in the RA worm gear as the RA mount gears teeth shifted between the worm gear teeth, but this was sufficient and regular enough to cause all the stars to suddenly create a streak during the 2 minute exposures I was using for this image. I didn’t realise this until the next day when I went to process the images.

I think the same backlash problem is also contributing to poor polar alignment when I try to use Alignmaster because when trying to perform the aligning process as instructed by Alignmaster, I cannot re-position the chosen alignment stars in the centre of the reticule properly (I use a webcam and Sharpcap for this purpose). So this contributes to some drift in this image (slightly elongated stars), because as yet I don’t have the kit to perform proper autoguiding, although I have done some experimentation with the free PHD software.

Out of the 11 shots I had taken (using my recently acquired Backyard EOS to control the camera), I found that only 4 of the set could actually be used for pre and post-processing in Nebulosity 3. Nevertheless these 4 x 120 seconds images at ISO1600 pre-processed with 5×120sec Darks have given quite a pleasing image.

I used Nebulosity 3 alone for all the image processing. This included; Background Colour Offset, plenty of Levels and Curves work to pull out the faint detail of the nebula, GREYCstoration noise reduction, and Gaussian Blur to correct some of the mottling caused by the Hue and Saturation when I tried to pull out a bit more colour.

Since taking this photo I have taken my GEQ5 mount axis drive motors off (to get at the worm gear tension adjustment screws) and adjusted the worm gears to engage properly with the big RA gear so as to remove as much backlash as possible without adding too much tension for the drive motors. I’m hoping for another good night soon to try this out and see if the aforementioned problems have been improved. Also I have learnt that a bit of weight bias is required to prevent the tip and topple affect of the mount.

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