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I apologise for asking this question again, but I still don't understand why the edges of images like this one are unevenly focused. If all the corners were similarly out of focus, I could understand it, but I don't understand why the middle at the top has the best focus and the lower right corner is the worst, with the other corners not being as bad. This is a single three minute exposure using K3iii, HDDA 16-85 and Astrotracer 3. Yes, I understand that the mountain range and the trees are blurred by Astrotracer.



Looking West by RobGeraghty, on Flickr

Next time, I'll take the HDDA*16-50.

Others will probably explain it better but I'm sure it's distortion caused by the way AT and lens react to each other.

Another words, if you used a proper mount you would move the whole camera and lens together while with AT you are moving the sensor which behaves differently due to the curvature of the lens.


It's a great image though for a 3 min exposure

OK, if the sensor is being tilted not just rotated, the edge focus might be affected more than at the centre. Thanks.


Thank you! I was amazed by the number of stars that the sensor has resolved as distinct points. I'll have to crop away a portion of the right hand edge and lower edge. The K3iii seems to be a much better Astro camera than the K3.
Fine focus is still driving me crazy though.

I have the K1 ii , which is a fantastic camera but I must admit that currently daytime or linear photography is driving me a little nuts. For astro it's great though the version of AT is far more limited then the one on the K3 iii.


If you can get into shooting some DSO's (deep space objects) like galaxy's and nebula, definitely give it a shot. You won't be disappointed

Nice! The K1ii has a great reputation. Most of the photography I do is wildlife, so I like the extra "magnification" I get with APSC.

I have just tried the Orion nebula and 47 Tucanae with the DFA 150-450 and was happy with the nebula, but not the cluster. I just need to be able to focus on a dark night like we had on the weekend, and using live view isn't working so well for me. I might have to find a bahtinov mask and try that.

As has already been noted, the corner smearing is a function of using astrotracer.

In order for astrotracer to "fully simulate" an equatorial mount, the sensor needs to tilt the sensor away from the orthogonal to the mount (not just translate it in x/y). If you have ever used a camera with a tilted sensor plane you'll know that the main symptom is that the corners sharpness is obliterated when using a shallow depth of field. Depending on the tilt, one corner or side can be much worse than the others.

IfIi where taking your photo and targeting a 3 minute exposure, I'd split the time across 12ish 15seconds exposures and stack (integrate) the individual frames. The corner smearing will the greatly reduced over a 15 second exposure too.

Final tip would be use the Sigma 18-35mm, it will blitz the native Pentax lenses for this application.

OK, this makes sense, thanks.

I'll give that a try. Getting the interval timer to work with Astrotracer is a bit of a pain, but can be done.

Interesting! The only contemporary wide Sigma lens I have is the 8-16, which I'm tempted to try for the heck of it, although I wouldn't expect is to work with Astrotracer.

FWIW, here's the Orion Nebula taken on the same night with the DFA 150-450.


Orion Nebula by RobGeraghty, on Flickr

It's nice but I'm sure you can get more detail out of that image.

The bottom of M42 hasn't come out so I would try reprocessing and see if you can bring out the fine dust that surrounds the nebulosity, it should come out as dark dust.


Hopefully the permissions are set to public but here's one I did during our winter season:

Astro

I could probably get more detail if I lifted the shadows, but I prefer to have the background close to black. I'll have to wait for another opportunity to get more images to stack. I was struggling a bit with the tripod to get the nebula at the top of the frame at the beginning of the stack so that most of it was still in the frame by the end of the stack (using the intervalometer).

Does your software support asinh transformation and masking? That will help will the background while not loosing detail in the subject. If the software doesn't then I think something like AstroPixelProcessor comes in at quite a bargain... something like $50, if I remember correctly??


As for the tripod, I guess you're using a ball head? I would adjust the shot count to suite AT. In otherwords, make sure that the target stays in frame then move the camera to reframe. It can become tedious, when I tried M31 (Andromeda), I reframed every 20 shots. Total shot count 1400.... on that one.

I'm using Sequator. Astro Pixel Processor is EUR165 which is AUD$243. I don't think I do enough Astrophotography to justify that cost.

Andromeda must be small, or you're using a much wider lens. I needed to reframe Orion after about 6 frames at 450mm, especially given frequent failures of the pre-image processing by the K3iii. Thanks for the suggestions!

Have you seen Siril? It is free opensource software. There maybe some other ones out there too that run on the common platforms. I use Linux so I don't know much about the other stuff.


Andromeda is actually quite large, it's a good size for my 300mm. The issue I have is that I live in the center of a large city and the light pollution kills the sky. I can only realistically get 3 targets using my K1. If I had a dedicated mono astro cam and tracking mount, things would different as they are immune to LP. Especially when you start looking at the wavelengths beyond the standard visible EM spectrum.