Alan, The longer exposure times are thanks to the ingenious Pentax O-GPS1. It slips in the cameras hot shoe and communicates with the SR system to actually move the sensor during the long exposure, to compensate for the apparent movement of the stars.
The camera has tilt sensors and the O-GPS provides a compass, so the camera knows what direction its lens in pointing in an X, Y, and Z axis to do the proper calculation. I've seen people doing 45 second exposures with a Bigma at 500mm (that's all I could manage with my 135mm) but I hope to figure out a more precise calibration technique that will get me longer results. In any event, I'm also planning on constructing a "barndoor" drive (Type 4) that will be powered by a stepper motor (and Arduino). That should allow me to do longer exposures without using the O-GPS1 (but will require me to align the barndoor hinges at the north star for proper tracking). More info, for those interested: http://davetrott.com/inventions/double-arm-barn-door-drive/ On Tue, Oct 15, 2013 at 11:40 PM, Alan C <c...@lantic.net> wrote: > I am very impressed with your efforts, Darren (and for you explanation of > how it was done too). However, I am surprised that you can expose for as > long as 45sec without any obvious trailing. In 1965 I took some shots of the > Ikeya-Seki comet with my S1a and a 135mm Tak. I used 1250ASA film and the > exposures were 10sec if I remember correctly. Even at 10sec some "movement" > is obvious. > > Alan > > -----Original Message----- From: Darren Addy > Sent: Wednesday, October 16, 2013 4:12 AM > To: Pentax-Discuss Mail List > Subject: Re: PESO: My first DeepSkyStacker attempt > > You are technically correct, Stan. But I was out there for approx. 75 > minutes to get my 6 minutes of integrated exposure time. After doing > the precise calibration, a lot of my first shots were complete misses > (Andromeda not even in the frame). And then when I finally located it > and started taking images, I was trying to find the maximum amount of > time I could expose and not get star trails. For me, in this part of > the sky, with this lens, on this night, that max was about 45 seconds. > I then rejected over half of the images that I shot, only stacking the > very best (which ended up to be 11). > > So, if one could go out, locate the object immediately, and get 100% > "keepers", the sky would only be rolling for 10-20 minutes or so. But > the reality is that you need to shoot more than you keep, and you need > to get the object in the frame first. I'm looking into better ways to > do that, but mostly they involve mounting a Telrad (or other laser > projection "finder") beside the camera and looking in the exact same > place. Then that plate, with camera and Telrad mounted, would have to > be put on the ballhead and oriented around. > > On Tue, Oct 15, 2013 at 8:46 PM, Stan Halpin > <s...@stans-photography.info> wrote: >> >> Huh. I had imagined that you would locate the target or target region, set >> the camera, fire a series of shots, and then stack away. From your comments >> it seems that you need to re-target the target for each shot. I know the >> stars are "moving" relative to us, but I hadn't thought they would move that >> significantly in 6-8 minutes . . . >> >> stan >> >> On Oct 15, 2013, at 9:24 PM, Darren Addy wrote: >> >>> Thanks to all who have looked (and/or commented). The image is a crop >>> of what I got from doing an "overlap" stack in DSS. It is difficult to >>> frame each sub-exposure exactly the same (particularly when simply >>> using a ballhead), but with DSS that doesn't matter because it will >>> stack only the common parts of each image. From that result I cropped >>> in even closer resulting in the "centered" composition that Stan >>> didn't care for. I could have left more stars on the right and bottom, >>> but I guess my thinking was more like what Paul expressed earlier. >>> Also, the nature of astrophotography, particular with the lens near >>> wide open, is that the edges will drop in quality due to coma, CA, >>> etc. I didn't really have M31 centered well in any of the shots. It >>> was mostly left-of-center or lower left. So again, lots of room for >>> improvement with future images - even of the same subject. It is a >>> learning experience and I have a LOT left to learn. >>> >>> I don't see the banner either, but I'm also a paid account (if that >>> makes a difference). Rather annoying to hear about, though. : \ >>> >>> By the way, I had a couple of astro imagers suggest a free PhotoShop >>> plugin called HLVG (HastaLaVistaGreen) which I think I will >>> find/download and try on this image. I thought I got the green out, >>> but apparently not enough. >>> >>> To Larry's earlier question... not sure about stacking software for >>> the Mac. I believe Steve Sharpe mentioned one up-thread. >>> DeepSkyStacker and Registax are the two biggies (both free, I believe) >>> on Windows. If anyone is interested, I can share a good link that >>> helped me with the histogram/curve part of DSS. >>> >>> On Tue, Oct 15, 2013 at 8:07 PM, Larry Colen <l...@red4est.com> wrote: >>>> >>>> On Tue, Oct 15, 2013 at 08:38:04PM -0400, Paul Stenquist wrote: >>>>> >>>>> As I said, a fabulous shot. But what's with Flickr putting a banner in >>>>> the corner of your frame, covering part of the image -- "Try our New Photo >>>>> Experience." And people complain about ads on photo.net?? Bizarre. >>>> >>>> >>>> I don't see that baner. But then, I have a paid account. >>>> >>>> -- >>>> Larry Colen l...@red4est.com http://red4est.com/lrc >>>> >>>> >>>> -- >>>> PDML Pentax-Discuss Mail List >>>> PDML@pdml.net >>>> http://pdml.net/mailman/listinfo/pdml_pdml.net >>>> to UNSUBSCRIBE from the PDML, please visit the link directly above and >>>> follow the directions. >>> >>> >>> >>> >>> -- >>> Nothing is sure but death and Pentaxes. >>> >>> -- >>> PDML Pentax-Discuss Mail List >>> PDML@pdml.net >>> http://pdml.net/mailman/listinfo/pdml_pdml.net >>> to UNSUBSCRIBE from the PDML, please visit the link directly above and >>> follow the directions. >> >> >> >> -- >> PDML Pentax-Discuss Mail List >> PDML@pdml.net >> http://pdml.net/mailman/listinfo/pdml_pdml.net >> to UNSUBSCRIBE from the PDML, please visit the link directly above and >> follow the directions. > > > > > -- > Nothing is sure but death and Pentaxes. > > -- > PDML Pentax-Discuss Mail List > PDML@pdml.net > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and > follow the directions. > > -- > PDML Pentax-Discuss Mail List > PDML@pdml.net > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and > follow the directions. -- Nothing is sure but death and Pentaxes. -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
