First light for the Megalens
First light for the Megalens
If you’ve been following along, you’ll be aware that I constructed a telescope mount and auto-focus setup for my 1000mm camera lens. (See here for the write-up.)
I finally had some clear skies (well, mostly clear skies!) and jumped at the chance to test it out - and to see just where the focal point was for the lens and my usual camera setup.
I hadn’t planned on setting up the rig, as the entire day had been cloudy and overcast - definitely not good skies for astro. At about 8pm two of my three forecast sites were saying it was clear, so I stuck my head outside and saw that indeed, it was clear.
So I quickly got everything assembled and polar aligned. (I can’t wait until I have some sort of permanent setup - this constantly assembling and tearing down every time I want to image is not much fun.) I could see clouds rolling in, so I needed to be quick and sort things out.
When I tested the EAF (Electronic Automated Focuser) on the bench, I determined that I had a movement range of about 21000 steps. So where in the movement was the best focal position?
I started by taking a 5 second image, moving the EAF by 1000 steps and taking another image. At about 3000 steps in, I could see faint images of stars appearing. At 4000 steps, they had vanished again. A quick glance upwards showed me that nope, no clouds obscuring the stars…
I eventually narrowed it down to step 3650 gave me the best focus. I really do need to get this tuned correctly - there are a million options in Ekos to work with. Some of them are designed specifically for lenses like this, with a large central obstruction.
I initially chose the star Hadar as my first target because it’s bright and clear (and I knew where in the sky it was). As the image below shows, I had the focus pretty well sorted out. Then I saw the huge reflection / halo extending down and right from Hadar. I don’t know if this is caused by the UV-IR cut filter (I kind of doubt it, as nothing like this has ever appeared before) or some strange internal reflection within the lens. Given that the lens appears to have been dropped somewhere in it’s life, I’m guessing that something is a little bit out of alignment.
For now, I’ll just avoid the really big, really bright stars.
A total of 2 frames, for 50 seconds (20 seconds + 30 seconds) of imaging time. I don’t think I’ll be imaging bright stars like this again - or at least not with exposures this long.
The star Hadar
The second and third images below show the ‘Keyhole Nebula’. Part of the Eta Carina nebula, which every astrophotographer targets first. It’s big, it’s bright, and we all know what it looks like. So it makes a very cliched sense to image it as your first target.
Initially the Keyhole wasn’t in the centre of the frame, so I recentred the frame on it and kicked off a sequence of 30 second exposures. What I forgot to do was restart the guiding. I’m still kicking myself here for forgetting what is a very important step - especially at longer focal lengths.
I am impressed with how well my mount tracked, though. 30 seconds at 1000mm would leave massive star trails on a badly performing mount.
As good as it was, Siril still kicked up a stink and rejected about 2/3 of my images when I came to stack them. So these images are the result of about 20 minutes of data. Yes, they are slightly out of focus and the stars are a little stretched. They’re faults I can work and improve.
One thing I did not find was the ‘V’ shaped stars in the image corners. It looks like my hypothesis of having a small camera sensor (only 13.2 x 8.8mm) paid off and the distorted stars are outside the sensor area. Yay! Or the ‘V’ stars in my previous test images were a result of the DSLR’s shutter moving…
I only had the ‘Dark’ configuration frames to work with. So when I get a way to create the ‘Flat’ frames I’ll be able to properly process the images from the lens. At this stage, I’m happy that I’ve got something recognisable out of it.
I did my usual processing within Siril, splitting the image out to the ‘starless’ and ‘starmask’ with starnet++. The differing results caused by the recombination methods (Affinity Photo vs Siril) are visible below.
Given how well these first images have turned out, the next clear night can’t get here soon enough!
Affinity Photo star recomposition
