The Rosette Nebula

This has been my great white whale for the last couple of months. It’s in the south in the winter which makes it tricky from my back yard. Last night I went to a nearby park where I have a view to the south over farmland. With the snow gone I was able to get 30 100 second subs at 800 ISO through the william optics z61(360mm f/6). I had to stand for the better part of an hour blocking a streetlight but it seemed worth it. There was a pretty strong gradient in the subs but I found a new siril script that seems to do a good job knocking down gradients. Probably a few weeks ago at the last new moon would have been better but i’ll take this for now. The blotch at around 2 o’clock is dust that wasn’t mitigated by my flats. Besides the script I would have used siril’s auto-stretch histogram, the asinh function to raise the blackpoint and colour saturation to boost the colour.

I’ve been looking lately at all-in-one astrophotography solutions like stellina or the EVscope. It’s not that i can’t get decent results but it’s a lot of hacking around in the field and after the fact.


Inverting the DTR Signal for Stellarmate

I’m trying to install Stellarmate OS on a raspberry Pi to simplify my setup and let me stay indoors on these cold canadian nights. The first thing I had to do was download the FT_PROG tool from, and invert the DTR RS232 signal of a chip inside the IEXOS-100 mount. This seems to be a pretty standard thing to do and doesn’t seem to interfere with POTH’s managing the mount.

A Tiny Perfect Ring Nebula

I’m delighted with this image of M57, the ring nebula in Lyra. I wasn’t sure I could see it at all, it’s very small compared to the galaxy targets I’ve been shooting. The ring is only about 15 pixels across in the final image but it seems perfectly clear to me. This is 11 lights at 25 sec ISO 800 shot through the Takumar 200mm lens wide open at f/4 combined with darks and biases, no flats. Processed in Siril then cropped to about 1/10 of the original. After Siril I used Paint Shop Pro and reduced brightness -15 and boosted contrast +26.

Wikipedia gives the dimensions as 230″X230″ (arcseconds) so about 4’X4′ (arcminutes) I think my camera/lens is supposed to be 2.2 arcsec/pixel so that would be only 10 px where my image is about 15px. Maybe bad focus or CA fuzziness are helping me out for a change! Jupiter at opposition is only about 50″.

UPDATE: My images are about 4.4 arcsec/pixel so 15px is 66 arcsec. NASA gives the size as 1.3 arcmin so i’m missing the red part – that’s fine.

The actual object M57 is about 2500 light years away and almost a light year across. which is around the 1.3 arcmin NASA gives.

For processing I am trying to centralize my calibration images – darks, biases, flats – and use symlinks to make Siril think they’re in the directory with the lights.

Reducing Cone Error(Centering Polaris)

Cone error is the divergence between where the mount is pointing and where the attached camera or telescope is pointing. I decided to tackle this with the mount and camera in the polar home position – i.e. polaris should be close to the center of the image. Horizontal error is just a matter of rotating the camera around the declination axis – orthogonal to the polar axis, but vertical error is trickier. A telescope would have a means for adjusting it but with the camera on a dovetail bar attached to the mount there’s no obvious approach. When I first measured it in the left image above, Polaris was at 2810 vertically instead of the 1728 which would be the midpoint. Since the FOV was about 4.3 degrees, the error is about 1.34 degrees. After shimming the attachment plate Polaris last night was at about 1860, so still about .16 degree low.

In the picture below you can see the shims in place. I’ll try moving them halfway to the mounting bolt. I’ve tried doing calculations but they never work out. I’m a bit worried about making the attachment unstable but i might as well try.

In terms of reducing Chromatic Aberration I don’t think I’ve done much. The two images of Polaris below were before refocusing and stopping down on the right and after on the left. The left image is a bit better but not specifically the CA and there’s a lot of other stuff going on that affects the image.

UPDATE -here’s my cone error 20-05-18 – still a bit below center. I make it .2 degrees below center. I’m not sure I can do any better than that.

Explore Scientific PMC-Eight Mount

I bought a new mount for astrophotography. Besides passively tracking to keep my camera steady it has “goto” capability for locating objects in the sky. It was definitely a big decision, even on sale at C$500.

I’ve had it out twice on our balcony overlooking the beach in a rented Florida condo. I have a good view to the east and north but limited to maybe 35 degrees altitude and no view to overhead or the west. It’s still pretty brilliant though to be outside at night without being cold or assaulted by neighbours’ yard lights.

Homemade alignment assist.

The first challenge is to align the mount with Polaris. The mount has a borehole in the Polar axis that you are supposed to sight through but the field of view is very small. I made a prealignment tool from a rolled up paper stick to the side of the mount that gives me a six degree view. With the North Star centered in that it’s much easier to nudge it into the borehole.

The paper tube and borehole got me roughly centered on Polaris and I went on to poke around the sky and take a few test pictures. At the end of the session I remembered reading about Photo Polar Alignment and I took two specific pictures of the Polaris area with the scope slewed 90 degrees between pictures. I used to solve the two images to see how I had done. The centers of the two images were about 1/2 degree apart which would probably be great for visual work but not great for astrophotography.

I downloaded a Python script called photopolaralign(PPA.PY) and ran it on my two images. This indicated that I needed to shift the mount 40′(arcminutes) – 2/3 degree laterally; and 18′ – 1/3 degree vertically. I’ll try it tonight in realtime – it’s supposed to be clear.

Photopolaralign: The X is where I am pointing, the green crosshair is where I should be.