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PostPosted: Wed Mar 28, 2012 3:01 pm 

Joined: Sat Oct 11, 2008 8:27 pm
Posts: 404
Location: Bristol, UK
Hi Guys,

Its been a while since I posted an image here, but I felt this was an improvement which I have been struggling to get lately. Its all starting to fall into place! BTW the forum looks different, good different that is!

I've been having trouble with my last efforts (M45), where the towel almost entered the ring, but I think I am getting there. Last night was about getting the smaller details right and trying out a few things.

I was actually using my scope rather than lenses on my DSLR which I have been doing lately. I wanted to get my focus technique sorted for my impending leap into CCD imaging. To aid focussing I used a home made 'Y' mask which I made at the weekend. For those who don't know about 'Y' masks they are touted as a good alternative to Bhatinov masks which can be fiddly to self construct. Simply put they are a mask in the shape of a 'Y' used to create the diffraction spikes on a star to allow accurate focussing. The width of the 'limbs' can be the same order as that used in a Bhatinov mask and an angle of around 20 degrees, from the vertical, is also good.

My mask, not having the equipment to measure angles was constructed with the upper parts of the 'Y' each at 45 degrees from the vertical i.e. at right angles to each other. The width of each limb was also set at 10mm, which is wider than the calculations for a Bhatinov mask suggest making is around a third order mask. I primarily did this for stability of the 'Y'.

Well, I can say it worked a treat. Using the live focus on my DSLR I could clearly see the additional diffraction spikes and managed to focus well using Venus as a reference as it wasnt quite dark enough for anything else. It was also useful to see the diffraction spikes created by the spider vanes of the secondary which, when out of focus, doubled up and when in focus went to single spikes.

So it started well, focus sorted and locked off (not done this before!), polar aligned and star aligned. I had a bit of trouble trying to calibrate PHD (thanks guys last night for the tips) but I got guiding by about 9:15 and ran a quick imaging session to just after midnight on the double cluster.

I also wanted to try and nail my flat frames as whenever I use my DSLR I get nasty vignettes. So today, having left everything in place on the scope, I took some flats using an embroidery ring and pillow case. These seemed to have the desired effect! Another tick in the box.

The image below is a little noisy due to no darks and the data possibly being stretched more than it should, but Im pleased with the results. Remember this is taken with a Nikon D7000 non modified DSLR.

Stacked in DSS, edited in PI
29 x 5min subs and flats.


Lee Diggle
My Astronomy Blog | My Photography Gallery

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