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07-18-2020, 11:52 AM   #1
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Astrotracker advice linking focal length to time, Neowise Comet

Hi Folks.

I just started to use the Astrotracker function on my K1 with a Pentax 80-200 f2.8 FA lens for the comet. I never really used this function much before, certainly not with a longer lens, so I was wondering, especially on the 200mm end, how long of an exposure can I get away with before I start to see subject movement? I did some at 30 seconds and that seems a bit too long. We will have cloudy skies it looks like tonight so I won't be able to run any test shots.

For that matter, what ISO and shutter speed combos with your lenses have you been shooting the comet? My skies aren't super dark so I don't want to use really high ISOs.





07-18-2020, 12:33 PM   #2
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Staking a claim in the thread not because I know but because I'm also going in an hour and could use some extra info

I tried my new-to-me Sigma 400/5.6 a couple nights ago to test the waters with the Astrotracer and got some decent results at ~20 seconds after running the "precise calibration". Have you done that one? Maybe it will help (So calibrate GPS plus calibrate Astrotracer itself).
07-18-2020, 12:57 PM   #3
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QuoteOriginally posted by Serkevan Quote
Staking a claim in the thread not because I know but because I'm also going in an hour and could use some extra info

I tried my new-to-me Sigma 400/5.6 a couple nights ago to test the waters with the Astrotracer and got some decent results at ~20 seconds after running the "precise calibration". Have you done that one? Maybe it will help (So calibrate GPS plus calibrate Astrotracer itself).
Yeah, I used the "precise calibration" but it still showed some star movement at 30 seconds at 200mm.
07-18-2020, 02:42 PM - 2 Likes   #4
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Single exposure, 30sec @ f/5.6 with my 18-55mm at 55mm, ISO 12800, with my K-70 & O-GPS1 using the 'Astrophoto' default User Mode.


The dark background was tweaked slightly in DCU. Other than that, just as it came from the camera and resized for posting





07-18-2020, 03:07 PM   #5
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Your second picture looks out of focus, with some coma on the right hand side. Manual focus with magnified live view on a bright star is the way to go. Don't trust just setting your lens to "inifinity" (the end of focus travel).

I got 11 seconds with essentially no trailing with my 300mm lens: First try of Astrotracer - Venus in the Pleiades - Not too bad! - PentaxForums.com

I'll post a few comet pictures shortly, but I was using ISOs from 12800 to 25600 last night with my K-1 and DA* 60-250mm zoom at f/4.0 for up to 15 seconds (without astrotracker, so a bit of trailing, but not bad). I am impressed with how well those high ISOs work. Try it - if your sky is too bright, lower the ISO or shorten your exposures, and go for stacking.
07-19-2020, 07:32 AM - 2 Likes   #6
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With the Astrotracer I've gotten around 15 second exposures without trailing while using a 200 mm lens. (on a K5ii).
This is (cropped) Jupiter at 200 mm. 6 second exposure. 800 ISO. Zooming in you can see four of Jupiter's moons.


This is Neowise at 200mm. 2.5 second exposure at 3200 ISO. (with some cropping.)


This one is with the Pentax A50 f1.4. 8 second exposure f1.8 at 800 ISO.


I've been pleased with the results of the Pentax FA Macro 100mm f3.5. 5 second exposure f8 ISO 1600.

I haven't needed to use the Astrotracer for the comet.
07-19-2020, 10:28 AM   #7
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I was out just last night with my KP and the 55-300mm PLM. Although I could have, I didn't employ the Astrotracer, figuring I could get some decent exposures without. After about a dozen shots, the longest I had the shutter open without easily seen star movement was 4 seconds. Anything longer and movement could be seen when enlarging the image.

In theory, the "500" rule ( 500/(Crop factor X Focal length) = Shutter Speed ) would limit exposure at 200mm to 1.7 seconds with a 1.5 crop factor... I saw that could be fudged somewhat.

Here's the comet at 260mm on my KP at 4 seconds, f/5.6, ISO 3200:



Cheers, Allan
07-23-2020, 10:23 AM   #8
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Astrotracer gave me inconsistent results, with a real oddity. In several 20-30 sec. exposures, star movement showed on the left side of the frame, but not on the right. Right was northwards for this evening session, because I was facing northwest. The PF article on Astrotracer noted that results were best aimed north towards Polaris, and got worse as you aimed east or west. I'd upload an example, but that feature doesn't seem to be working for me now.

07-23-2020, 03:42 PM - 1 Like   #9
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QuoteOriginally posted by Wheatridger Quote
Astrotracer gave me inconsistent results, with a real oddity. In several 20-30 sec. exposures, star movement showed on the left side of the frame, but not on the right. Right was northwards for this evening session, because I was facing northwest. The PF article on Astrotracer noted that results were best aimed north towards Polaris, and got worse as you aimed east or west. I'd upload an example, but that feature doesn't seem to be working for me now.


My experience suggests results like this can occur if the camera/GPS is calibrated or operated when it's too near to a magnetic disturbance, be that a car, a balcony railing or a metal garden table or similar. When calibrated and operated at least 10ft clear of any adjacent potential problems everything usually goes very smoothly. However, an ultra-wide lens can exacerbate the issue.
07-23-2020, 04:40 PM   #10
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I probably was within 10 feet of my car. Thanks!


Now if you can tell me why my upload stalled? I've reduced the size to meet the site's JPG requirements, and my account is showing plenty of green bar, which I assume means space available. The Manage Attachments window just sits on Uploading File(s)- Please Wait.
07-24-2020, 12:30 AM   #11
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QuoteOriginally posted by Wheatridger Quote
I probably was within 10 feet of my car. Thanks!
Now if you can tell me why my upload stalled? I've reduced the size to meet the site's JPG requirements, and my account is showing plenty of green bar, which I assume means space available. The Manage Attachments window just sits on Uploading File(s)- Please Wait.


Again, limited personal experience, but browser compatibility can affect functions like this, especially if someone has been "fiddling" with ('updating') the web-page code
Internet Explorer is becoming less and less compatible as time goes by ... I revert to Firefox or Edge where necessary, but I'm too used to IE's interface to change over completely until absolutely necessary No current experience with operating systems other than Windows, so other factors may be at play here.

Last edited by kypfer; 07-24-2020 at 12:31 AM. Reason: punctuation
07-25-2020, 04:34 PM   #12
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I was shooting the comet with exposures around 30 seconds last night with the Pentax D FA 70-200 at 200 MM. There were minimal star trails that I probably could have eliminated at around 20 seconds exposure. To me, this is pretty impressive performance for the Astrotracer in my K-1. This was my first ever use of the Astrotracer and i was surprised by how easy it was to calibrate, even with a big lens attached. I certainly hope to improve my technique before the comet returns in 6800 years.

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