Very Amateur Astronomy

A reasonable successful imaging session last night I think, though I won’t see the results until I can process the frames this evening. However I still have some issues which require further investigation, mainly there is still a small amount of drift and occasionally a noticeable shift where I end up with double stars.

The prime culprit for this is almost certainly going to be differential flexure, that being movement or changes in the optical path of the imaging scope or the guiding scope resulting in drift in the image. So whats the cause and how can we fix it? Well the potential causes are many.

1. Mirror sag – the primary mirror shifts slightly as the scope tracks
2. Focuser sag – the focuser shifts slightly as the scope tracks
3. Camera sag – the camera moves as the scope tracks
4. Tube flex – the optical tube bends of flexes as it tracks
5. Mount flex – the scope array shifts on the mount

The problem here is that  scope  guiding, where the guide camera looks through a different scope to the imaging camera, requires that both scopes remain perfectly in position with no deviation in where they point. The guide scope is technically only guiding itself, if anything moves or changes on the imaging side then the guide software won’t care, as long as it’s camera remains pointing in the right place it doesn’t matter.

My task then is to eliminate this by carefully addressing each potential cause and correcting it, so starting from tonight I’ll be on a mission to :

• Fix the primary mirror, make sure it doesn’t move.
• Tune the focuser, ensure it does not shift with the weight of the camera
• Improve the fit of my tube rings, the felt is thick and will allow shift
• Improve the mounting of the guide scope, it can move to much at present
• Improve cable runs to prevent pulling on cameras
• Improve the fit of the declination head and the clamping on the mounting

Busy, busy.

Once again I am somewhat humbled by the abilities of others. CloudWatcher a poster on the Stargazers Lounge message boards took my jpeg and applied a measure of experience and as far as I can tell a dash of magic and turned it into this.

So much better in every way. I did not even think that I could turn what I had into anything like this, and despite an hour or so mucking about with the hue and saturation controls I still can’t pull out anything quite like this! I shall plod on though, ever time I post a new image I learn something new!

I’ve been having a go with these techniques on my M101 image with the following results, you can compare in the gallary, I think I’ve brought out the depth a bit more.

The skies finally cleared last night allowing me to have a go with my upgrades from the last few weeks. For a target I chose NGC6946, the fireworks galaxy, it’s impossible for me to see it from my site with an eyepiece and even a two minute exposure only brought out a very faint core.

I had everything setup reasonably quickly and managed to locate a guide star after quite a bit of mucking about, the 70mm F12.9 refractor really needs a focal reducer, the field of view if tiny and the web cams tiny chip does not improve it much at all.

I went with 6 minute exposures with auto guiding controlled by PHD guiding. It seemed fine most of the time, but every so often the scope would for some reason move and the guide star would shift just enough for PHD to loose it. This resulted in several lost frames each time it happened, the scopes had slipped pushing things out of balance which may explain the problem.

Still for a first attempt on a faint object with only limited exposure time it’s not terrible. I’ll have to revisit this one in the future and get another couple of hours of data.

My inability to stay up to late on work nights, coupled with the moon being up at the moment means no deep sky photography for me. However, though the moon does stop me going for galaxies, it does fit very nicely on the chip of my Canon DSLR.

My latest and I think my best effort is M101. The pinwheel galaxy  is a face-on spiral galaxy about 27 million light-years away in the constellation Ursa Major.

This images is comprised of 1 hour 38 minutes of data, so 47×2 minute exposures (maths not my strong point) taken at ISO 1600, equipment used as per normal :

• Orion Optics Europa 250mm F4.8 Reflector
• Vixen Sphinx SX Mount Unguided
• Canon EOS 1000D Unmodified
• Skywatcher 2″ Light Pollution Filter

I started imaging at about midnight and got to bed some time around 4am, time I think well spent. I made some modifications to my processing technique this time round, in DSS I increased the saturation to 15% and matched the RGB levels prior to saving the stacked image to tiff.

This made life much easier I found in photoshop to get the black levels correct. Colour however remains a problem. The biggest issue with colour is the loss of reds due to the IR cut filter installed by default into the DSLR camera. Modification is possible but I think it will sit further down the shopping list for now as I’d rather get a Baader MPCC (Multi purpose coma corrector) first to remove the stretching of the image caused by the concave primary mirror.

Tracking remains somewhat of an issue, it’s still not staying as true as I would like. I noticed last night whilst taking some pictures of the moon that the target was visibly shifting in RA between frames, not good. In order to try and improve this I’m going to try the techniques for CCD/DSLR drift alignment as detailed here. Hopefully I should be able to improve things.

I’ll leave this for now, I will update on the improved drift align method after I’ve tried it, also I’m making and improved mounting plate for the scope and I have a Philips SPC900NC webcam on the way, so I’ll update on that when it turns up.