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03-16-2022, 10:13 AM - 2 Likes   #46
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QuoteOriginally posted by jcshellyg Quote
finally had a chance to test the Astrotracer function. I think I am happy with it for the most part. Not sure what went wrong in some images though. I am guessing since my first ones came out OK this is a result of my not focusing correctly?
Looks like a missed focus, bumped focus, or focus shift as the lens came down to temp. I would guess this was from the upper right corner of the full image given the pattern.

I don't think you went too long and likely could have gone longer. I tried it out on the 1 clear night I had recently (Monday the 7th) and created this 8 image stack with my 400mm where each shot was 40s:

That is the actual field of view with my 400mm when pointed at the Horse Head and Flame nebulae.

03-16-2022, 10:29 AM - 1 Like   #47
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QuoteOriginally posted by jcshellyg Quote
I finally had a chance to test the Astrotracer function. I think I am happy with it for the most part. Not sure what went wrong in some images though. I am guessing since my first ones came out OK this is a result of my not focusing correctly? Or is my exposure too long? I was shooting at F5.6 for 25secs with a 300mm lens and 200iso.
This is a heavily cropped and enlarged image to show the detail more clearly of what I was getting.
That distortion is called "coma" and is an unfortunate characteristic of some lenses that only tends to show itself with bright points of light towards the edges/corners of an image.
Apparently reducing the aperture (and thereby the exposure) can help rectify the issue, but to retain the same exposure time you'll need to increase the ISO by an appropriate amount … 400 or even 800 ISO shouldn't be a problem on a K-3iii, though you may notice the noise-reduction delay kicking in at 800ASA, the camera will take an extra 25sec (or whatever the exposure time was) to save the image.
See https://photographylife.com/what-is-coma for more details.
03-17-2022, 07:20 AM   #48
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QuoteOriginally posted by MossyRocks Quote
Looks like a missed focus, bumped focus, or focus shift as the lens came down to temp. I would guess this was from the upper right corner of the full image given the pattern.

I don't think you went too long and likely could have gone longer. I tried it out on the 1 clear night I had recently (Monday the 7th) and created this 8 image stack with my 400mm where each shot was 40s:

That is the actual field of view with my 400mm when pointed at the Horse Head and Flame nebulae.
Very cool image! Actually it was more middle of the image. Thanks for the help and info that it wasn't my exposure time.

---------- Post added 03-17-22 at 09:21 AM ----------

QuoteOriginally posted by kypfer Quote
That distortion is called "coma" and is an unfortunate characteristic of some lenses that only tends to show itself with bright points of light towards the edges/corners of an image.
Apparently reducing the aperture (and thereby the exposure) can help rectify the issue, but to retain the same exposure time you'll need to increase the ISO by an appropriate amount … 400 or even 800 ISO shouldn't be a problem on a K-3iii, though you may notice the noise-reduction delay kicking in at 800ASA, the camera will take an extra 25sec (or whatever the exposure time was) to save the image.
See https://photographylife.com/what-is-coma for more details.
Ah, that makes sense unfortunately but glad to know there is a work around. I will try some experimenting. Thanks very much for the info and link. Very helpful.
03-17-2022, 07:35 AM   #49
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QuoteOriginally posted by jcshellyg Quote
Actually it was more middle of the image.
Then with coma like that I would say that the focus did shift as that looks to be pretty bad coma in the middle area even if the lens was shot wide open. Missing the focus makes it worse and since you said it was good on the first image it changing is the most likely cause. I use a bahtinov mask for getting a perfect focus and also for checking the focus periodically when shooting. It makes it so getting a perfect infinity focus using magnified live view is super easy and super fast. They work best with longer lenses but I do have one that works somewhat well with lenses as short as 50mm but with those short lenses stopping down a bit more very quickly papers over a slightly missed focus.

03-20-2022, 05:00 AM - 3 Likes   #50
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First attempt with new "astro tracer mode 3", as the sky was clear for the very first since the update was released. Pentax K-3 III, Sigma 120-400 @ 400mm, f/8, 60sec, ISO 1600. Median stack of six of those images to remove noise, edited in PS. Full frame, only cropped to 4:5 ratio. Ah yes - it's the Orion nebula, M42! ��
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03-21-2022, 10:08 AM - 1 Like   #51
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QuoteOriginally posted by clack Quote
. Pentax K-3 III, Sigma 120-400 @ 400mm, f/8, 60sec, ISO 1600. Median stack of six of those images to remove noise, edited in PS. Full frame, only cropped to 4:5 ratio.
I assume the original image was a mosaic as this framing would be wider than what it should be for 400mm lens. This is the field of view I get for M42 with my 400mm lens, and almost 9 hours worth of shots were used which is how I got that level of detail. I cropped the mosaic from stacking back down to the resolution of the K-3 after stacking since I let it drift across the frame some for a few shots and then reframe. The lens I used there was the SMC A* 400mm F/2.8 ED [IF] but after experiencing some issues using it on my first time out and teasing some info out of astrotracer type 3 the other night with some other lenses I wonder if it has an actual focal length slightly different than 400mm like maybe 410 or even 390.
03-21-2022, 05:00 PM   #52
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QuoteOriginally posted by MossyRocks Quote
I wonder if it has an actual focal length slightly different than 400mm like maybe 410 or even 390.
My understanding is that some lenses focal lengths change depending on the distance (resulting in focus breathing)?

03-21-2022, 07:23 PM   #53
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QuoteOriginally posted by RobG Quote
My understanding is that some lenses focal lengths change depending on the distance (resulting in focus breathing)?
Yes but at infinity it shouldn't be the case. Although my understanding is that a number of lenses will claim a standard focal length like 50mm but in actuality be like 48mm or even 52mm. I wondered about the A* 400/2.8 because I got a few data processing error messages when I used it however when interrogating astrotracer type 3 with the A* 200/2.8 I never got that error even when pointing it in a spot where it was exceptionally dark or exceptionally bright. However I did get that error when I started lying to the camera about the focal length and could get it consistently. The only other difference between the 2 nights was that there was more wind when I was out with the 400 so I wonder if that introduced some shake in some calibration shots that caused it problems. I haven't done a formal write up as I do want to do some more testing with it and also see if I can get it to run off of a poorly aligned equatorial for extended long lens tracking (maybe see if I can be really dumb and pull of 5 to 10 minutes at 300mm)
03-21-2022, 07:40 PM   #54
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QuoteOriginally posted by MossyRocks Quote
Yes but at infinity it shouldn't be the case.
OK, I get that it won't change significantly at a single focal length setting but what I meant was that physically the "400mm" lens at infinity focus might not actually be 400mm.
03-22-2022, 03:30 AM - 1 Like   #55
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QuoteOriginally posted by RobG Quote
OK, I get that it won't change significantly at a single focal length setting but what I meant was that physically the "400mm" lens at infinity focus might not actually be 400mm.
Yes. I think we were both making the same point but not quite understanding eachother.
03-23-2022, 09:20 AM   #56
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I have tried (in bad conditions from home with wind and in the middle of Toulouse, a large city with tons of light!) Astrotracer type 3 on my K3-III: it works very well!
I have a question: according to the manual it seems that the pre-shot must not frame any ground, only the sky?
So for example can I take a picture of the Milky way plus a tree etc. with a 16mm? (in other words will the tree perturbate the pre-shot?)
03-23-2022, 11:08 AM   #57
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QuoteOriginally posted by LaurentD Quote
So for example can I take a picture of the Milky way plus a tree etc. with a 16mm? (in other words will the tree perturbate the pre-shot?)
That I don't know but I would think that type 3 astrotracer would struggle with a lens like that as even type 1 would mess things up in the corners. As far as the tree in the shot it may throw it off as any trails that enter or exit the tree would be truncated leading to it calculating the wrong amount of tracking needed. Even there I am just guessing as I haven't tried that as with short lenses like that I just shoot untracked and let sequator do the stacking and freeze the foreground and background.
03-23-2022, 04:39 PM   #58
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QuoteOriginally posted by MossyRocks Quote
That I don't know but I would think that type 3 astrotracer would struggle with a lens like that as even type 1 would mess things up in the corners. As far as the tree in the shot it may throw it off as any trails that enter or exit the tree would be truncated leading to it calculating the wrong amount of tracking needed. Even there I am just guessing as I haven't tried that as with short lenses like that I just shoot untracked and let sequator do the stacking and freeze the foreground and background.
I'm curious if Astrotracer 3 can detect rotation, or it always assumes a straight line of motion. If it always assumes a straight line, presumably any wide shot looking towards one of the celestial poles would fail.

I took the image below with Astrotracer type 1 (on K5iis) at 17mm. It seems to have worked well for a 120s exposure in this case. If I am seeing the southern cross and pointers correctly, this photo is looking toward the south celestial pole.


Milky Way and Magellanic Clouds
by RobGeraghty, on Flickr

Anyway, maybe a stack of 20s exposures without astrotracer would be better for wide angle as you say?
03-24-2022, 02:19 AM   #59
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QuoteOriginally posted by RobG Quote
I'm curious if Astrotracer 3 can detect rotation
Of course it does!

Otherwise even with a 16mm after 1 minute the stars would have a vertical trailing.
I have done many Milky Way time lapses with my previous body (KP) from the Pyrenees and the stars move not only horizontally.
Near the same location (Toulouse) I tried Astrotracer Type 3 and after 1 minute the stars are dead sharp.

---------- Post added 03-24-22 at 02:23 AM ----------

QuoteOriginally posted by RobG Quote
Anyway, maybe a stack of 20s exposures without astrotracer would be better for wide angle as you say?
I have purchased the KP and now the K3-III for their astrotracer feature, for Mliky way photography.
I use mainly the famous Sigma 18-35mm f/1.8.
The images with astrotracer at 30s or 60s are dead sharp (except on the 15% right side). Much better than stacking images without astrotracer.

---------- Post added 03-24-22 at 02:26 AM ----------

QuoteOriginally posted by MossyRocks Quote
That I don't know but I would think that type 3 astrotracer would struggle with a lens like that as even type 1 would mess things up in the corners. As far as the tree in the shot it may throw it off as any trails that enter or exit the tree would be truncated leading to it calculating the wrong amount of tracking needed.
Even there I am just guessing as I haven't tried that as with short lenses like that I just shoot untracked and let sequator do the stacking and freeze the foreground and background.
I have tried yesterday, the Astrotracer Type 3 works even with the earth in the frame (16mm, a hill in 15% of the image)
03-26-2022, 08:21 PM   #60
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QuoteOriginally posted by RobG Quote
I'm curious if Astrotracer 3 can detect rotation, or it always assumes a straight line of motion. If it always assumes a straight line, presumably any wide shot looking towards one of the celestial poles would fail.
Yes it does. I pointed it in the general area of polaris, near but not at it as there is a lot of crap blocking the view there from the park behind my house. It didn't have problems tracking with a 50mm there where there is a lot of rotation still.
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