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Forum: Pentax DSLR Discussion
01-22-2011, 02:28 PM
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Thanks, Angus! I am encouraged, since the stacked image does indeed show a fair amount of red. Were ALL the exposures 0.3 seconds as you data showed? How many such subframes did you stack?
My 2.5 second exposure suggestion was based on a rule from Michael Covington's "Astrophotography for the Amateur." The original rule as published (for 35mm frames) was Max Exposure = 1200mm-sec, so a 300mm lens would allow 4 seconds to meet "no obvious trailing of stars" parameter. (More is possible if you are far from the celestial equator but that is irrelevant for Orion.) For APS-size frames, this would be reduced by the lens factor, so you want to keep the exposure below 800mm-sec. 2.5 sec x 300mm = 750mm-sec, so you should be OK. Now, since you were in a light polluted area, with a fast (f/2.8) lens, I can imagine that there was simply too much light to avoid washing out the entire frame with longer exposures than you used, especially at highest ISO. (Brad Miller and I use ISO 12,800 pretty much exclusively in our Kx's. But around home I MUST use light pollution rejection filters...)
I will look forward to seeing what you get from a dark location! Let us see what the most extreme frames look like, if you would. $1500 is a big enough purchase for me that I want to be VERY sure I will like the outcome.... and you're our only hope, Obiwan Kenobi!
Bob
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Forum: Pentax DSLR Discussion
01-15-2011, 01:15 PM
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Angus wrote:
some feedback on K-5
I have the K-5 after doing some astroshots with K10D and astrotrac before. The difference is amazing. Unfortunately my attachment budget is pretty much used up, but I managed to shoot the plejades from a light-polluted balcony using my 300/2.8 lens and ISO 12800, 25600 and 51200 and got away without guiding with already some of the haze showing (in the stacked pic) - I can't wait to get out into darker areas with my astrotrac.
Here's a straight JPEG ooc at 12800 with 1/2 sec and nothing done to it.
Angus
Thanks for that post... Here are my requests:
1. You should be able to get away with about 2.5 seconds of exposure at 300mm without trailing (i.e. fixed tripod, not using the AstroTrac.) Can you show us what that exposure duration would give you at the different ISO's?
2. What I REALLY want to see is the same series done on The Orion Nebula (M-42,) to see how the red sensitivity is on the K-5! The red sensitivity of the stock Kx is WAY better than anything else I have seen. (All the Canons must have the UV/IR filter on the sensor replaced with one that has decent pass at 653 nm, whereas the Kx is very good stock.)
I have done a similar fixed tripod exercise with the Kx, using a William Optics 90mm ED refractor telecompressed to 300mm, shooting at ISO 12,800. After stacking about 60 such images (IIRC) using Deepsky Stacker, you get a surprisingly good image of the Nebula, including the outer loop.
I would happily settle for single images, because I am DYING to know if the K5 red sensitivity is anything like the Kx.
Thanks!
Bob
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Forum: Pentax DSLR Discussion
04-21-2010, 03:19 PM
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Seeme like this might be a dynamite thing to try with the Pentax Kx's High Dynamic Range feature!
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Forum: Pentax DSLR Discussion
04-21-2010, 06:51 AM
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Jupiter-shine! I believe I have never see that done before. Well, I just got to the Florida Keys with my Kx, and Jupiter is now just rising at dawn, so I will have to give that a shot! Thanks for the inspiration!
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Forum: Troubleshooting and Beginner Help
03-10-2010, 07:19 PM
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I tried HDR the other night for astrophotography with my Pentax Kx, shooting the Orion Nebula with a Celestron Comet Catcher telescope (500mm FL, f/3.64.) As near as I can tell, the "normal" HDR setting (versus strong) seems to give you three exposures at +/- 3 stops. So a 3 second exposure gave me one 3 second exposure plus a short one of about 1/3 second plus a 24 second esposure. The thing that surprised me is that there was no Dark Frame Subtraction done, even for the 24 second shot (at ISO 12,800!) A stack of 30 such HDR exposures were then stacked with Deepsky Stacker to give a decent final image. I did have more detail, and less burned out area, than I would have gotten with thirty 30-second exposures... but I got less of the nebulosity as well.
I wonder what the STRONG setting does??? +/- FIVE stops, maybe??
Does anyone KNOW what the range of exposures are for the two settings?
Bob
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Forum: Pentax DSLR Discussion
02-28-2010, 10:29 AM
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I bought the PClix to use with my Kx for astrophotography. It basically works fine. It can be used wireless, like a TV remote, but for long term astrophotography, it's useful to use their optical fiber connector so that the IR signal is always right at the sensor. I am not thrilled with the connection ("use a piece of electrical tape to hold the optical fiber over the sensor.") This is fine for a while, at least in dry conditions, but if you get a lot of dew, the optical fiber has been known to come loose from the tape. I think I will try to fab a slip with Kydex.
In fairness to the people at PClix, I haven't given this feedback to them yet.
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Forum: Pentax DSLR Discussion
02-27-2010, 11:30 AM
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I just joined, so I have to figure out how to post some astrophotos I did recently with my new Kx. I am thrilled with this camera:
1. Box stock, it has acceptable red sensitivity. (I got good response on the Rosette Nebula, Orion Nebula, Horsehead and Flame Nebulae.) I will load a single M-42 shot to give you an idea.
2. Noise (after mandatory DFS) at ISO 12,800 really isn't so bad... and that speed lets you actually SEE that you have at least registered something on very dim objects (e.g. IC2177 Seagull Nebula.) OK, it takes a minute to get a 30 second photo.... but at that speed, my old Canon Rebel at ISO1600 would theoretically take 4 minutes to get the same density.
3. Light and Compact.
I was FURIOUS that there was no hard-contact remote switch jack. But I was able to get decent shots just using the self timer on 30 second shots, and I have picked up a PClix automatic exposure timer that uses IR for the remote so that I can fully automate my shooting sessions.
I also was furious that they eliminated the external power jack. I haven't found a source for the Canon DRebel - style external power adapter yet... but that WILL let me get the battery heat source out of the camera. That would let me get more pix before the temperature gets high enough that the camera actually enforces the "required DFS." (If the camera is cold and you are shooting short exposures e.g. fixed tripod, DFS won't actually be done until you shoot enough to warm the camera up.) I was VERY pleased that one set of rechargable batteries lasted at least half the night with essentially constant 1/minute shooting!
OK, that is enough for a teaser... now let me see what I can do about posting images....
Bob
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