Andrew’s Astronomy Blog

Reviewing one of the folders of images that I took in La Palma I came across this one that I’d stacked, mis-labelled and forgotton about. Once I started to look at it a little closer I realised that the stars were very misshapen with ones at the bottom of the frame appearing as small eclipsed disks. Vignettting was also very pronounced and uneven. Checking the focal length revealed it was taken with the Canon 18-55mm kit lens. I really must replace this lens at the first opportunity!

This is a severely cropped version of the image, showing the area from the clusters M6 & M7 at the right to M20 & M21 at the left.

10 90 second frames at ISO 800 with the Canon 350D.

Our old friend the Bubble Nebula in Cassiopeia again, this time on the society’s 20″ with the ATIK 383L camera.

By the time I was imaging this, it was almost at the zenith with all the issues that brings when using the Alt/Az dobsonian. Can anyone say ‘field rotation’?

This image is a stack of 30 second LRGB frames. Luminance are binned 2×2 and the colours 4×4.

Whilst at the Kelling star party I had a talk with Bernard from Modern Astronomy about cameras and specifically the Kodak KAF-8300 chipped models. I’d been thinking about a single shot colour for the astrotrac setup and the QHY9 looked a good match for a very good price. I placed an order and delivery was via Fedex within 2 days.

I’d had the idea in my head that the cooled colour CCD camera would be a low noise, high sensitivity version of the DSLRs I’ve been using but some test shots revealed that this isn’t entirely true. Initial pictures were a little disappointing and it wasn’t until I was reading an article on setting the offset and gain levels for the camera (http://www.stark-labs.com/blog/files/GainAndOffset.php#unique-entry-id-6) that the pieces fitted together and illumination dawned.

You can get surprisingly good pictures with short exposures on DSLRs by increasing the ISO rating. This is effectively increasing the gain and it’s not a free lunch. Noise is amplified as well and dynamic range is reduced. With the CCD, once you’ve calculated the optimal gain you set it and never alter it again. This level will be a lot lower than the DSLRs value and correspondingly longer exposures are required to fully exploit the increased dynamic range available.

With my Zenithstar 70mm Astrotrac mounted on a Manfrotto tripod I’m limited to 5 minute exposures without trailing so I picked a bright target for a test. M31 is ideally placed for this  at the moment and here’s the result resized to about half the original.

12 5 minute exposures

Zenithstar 70mm

William Optic FFIII field flattener

Last night yielded a couple of clear spells without a moon in the sky, (the first for a while) and with the nights drawing in rather earlier as we approach autumn I made the most of the opportunity for a final run through of equipment and techniques before the Kelling Heath star party.

It was just as well that I did because I discovered that my T adapter for the Canon had worked loose and needed a tweak with a screwdriver. The weather has been so bad this summer that I’ve barely had the telescope out since the La Palma trip.

M52 is an easy target to find and with the Zenithstar, if you’ve got it in the middle of the frame then you’ll also capture the Bubble nebula as well. After a short interlude waiting for cloud to clear, I took 5 3 minute exposures at ISO 800 with the unmodified Canon 350D.

Friday night was the first opportunity I’ve had to take some pictures of this. Thursday night was also clear but I was busy imaging comet Garradd again, this time passing close to the Coathanger cluster.

Using the club’s ATIK on the 20″ we took 41 30 second exposures binned 3×3 and also 3 each of red, green and blue binned 6×6. This really wasn’t enough colour frames and after a lot of effort I gave up trying to get a colour image and just stacked the luminance frames.

Sn2011fe is a type 1a supernova a mere 21 million light years away.

C2009/P1 made it’s closest approach to M71, a globular cluster in Sagitta on Friday night. While the weather people were predicting a gap in the cloud for mid evening, the gap turned out to be only 5 minutes in length!

Checking the star charts for the following evening showed the comet still fairly close and within the frame for the 70mm ZenithStar and Canon combination. The weather didn’t start too promising but cleared late evening for long enough to get a five pictures before it clouded over again.

The comet core has trailed in this stack as I didn’t have enough images to process the comet and background stars separately and then recombine them. The two bright orange stars in the frame corners are Gamma and Delta Sagittae which make it really easy to find the cluster.

Image comprised of 5 90 second exposures at ISO 800

ZenithStar 70 with WO Field flattener III

Looking back on our time in La Palma it seems that the time passed all too quickly. Certainly, I would have liked to have spent more time doing LRGB imaging on the 40cm. However, this type of photography consumes time at an impressive rate and there were a lot of other things that we needed to do. This image of M17 is comprised of single 10 minute exposures with red, green, blue and luminance filters, the colours being binned 2×2.

One of the treats of the trip to AstoPalma was the chance to use the 40cm relay cassegrain. This is an interesting design where all the optical surfaces are spherical and therefore easy to make but, there are rather more of them than in a conventional design. The focal point is inside the telescope tube and a small lens is used to ‘relay’ this point back to the eyepiece. Details of the the optical design are here: http://www.astropalma.com/Cass_Relay_Componentes_p.jpg

The main camera on this telescope is an SBIG ST8XE using the SBIG Adaptive Optics unit for guiding. Limited time meant that we only acquired a single 10 minute frame for each colour but we did get 4 luminance frames of the same length. The result of this was some pretty bad colour artifacts which required judicious use of the spot healing brush in PhotoShop. So, the result is here.

Edit: Adjusted the colour balance to make the spiral arms a little less green.

Living within a spiral galaxy gives us the chance to get an up close view of the dust clouds that fill the inter-stellar space in the spiral arms. If it wasn’t for this dust obscuring the starlight from millions of stars then our night sky would look very different.

This image is a wide-field view of the area around Cygnus. Six exposures of 3 minutes each at ISO 800 and f/3.5 with my Sigma 10-20mm zoom.

One of the main attractions of La Palma was it’s views of the Milky Way. It’s southerly latitude opens up views of our galaxy’s centre that just aren’t possible from the UK. This picture is a mosaic of two images taken with the Sigma 10-20mm  set at 10mm on my Canon 350D. Each image is a stack of three 3 minute exposures at ISO 800 and f/3.5.

Cygnus and Lyra are at the top of the picture, with Scorpius and Sagittarius at the bottom.