istDS, lenses and Astronomy

I've been after a 500mm mirro telephoto lens for my istDS. I was going to get the Tamron, but I had trouble getting a 'K' adapter. I saw todat that Jessops are having a clearance sale and are selling a Centon 500mm f/8 mirror lens, which has a 'T2' mount for a bit over £49. That's half price!

I've just bought one and a 'K' to 'T2' adapter.
I've only taken a couple of shots, but the lens appears to work perfectly with the camera in Aperature priority mode.

On another point, one of my other hobbies is astronomy. Astro-photography needs long exposures, both when using didgital and film.

I know that Canon make a 'programmable timer' that allows multiple and long 'B' exposures.

Is there anyting similar for the Pentax digital cameras ? (I know the pin-out of the 2.5mm Jack, I am considering making some sort of timer - unless anyone has already done it.)

What's the longest 'B' exposure that can be made before the picture is overrun with elctronic noise ?

I think that the noise problem could be overcome by taking a long exposure picture, then a picture of the same exposure lenght with the lens covered (a picture of nothing).

The first picture will contain the required picture + noise.
The second picture will contain just noise.

If the second is subtracted from the first, then the result will be the wanted picture.

It might mean colour separating the pictures into R-G-B before subtracting, then recombining the R-G-B.

Any thoughts, or help ?

I don't have much knowledge to help you on this, but two things might be helpful.

The *istDS has long exposure noise reduction that works pretty much how you describe, basically deducting the noise image from the actual image.

There is a firmware upgrade on the Pentax website that is intended for astro-photography and apparently reduces the noise levels in the green channel. This is probably essential for your requirements.

John,

Thanks for replying

Yes, I've already seen and installed the updated firmware from the Pentax website. I'm still not sure how long an exposure I can make though. (And how to keep the shutter closed for a long time [it will drain the batteries])

The only sites where I've found astronomical photographs are taken using multiple exposures of 30s, or so, then added together.

I'll publish pictures on my website when I've taken some!

Regards,

Steve.

Hello Steve,

I had intended to try some star trail shots but never got the weather when I had the time but what I have found out may be of help to you.

When the Ds is taking a long exposure (B) the current drain is over 500mA and varying constantly around that value. Then after you close the shutter, the camera takes the dark frame for exactly the same length of time and same current drain. Because of the current drain, an external battery or power supply is a must. The back of the camera gets slightly warm after about 5 minutes. My one crack at the star trail shot failed because the battery went flat half way through the dark frame part. I had done a 40min exposure. In retrospect maybe that was a bit long for a first try but I wanted the starts to move!!

If I had known what the current drain was before I tried, I would not have done it because of camera heating. It would be interesting to know if there is a limit to the exposure time due to camera heat dissipation and obviously, this heat is going to have a big affect on picture noise.

I have done some 5 minute test exposures with the lens cap on and the noise is very low. This is easily managed on a good set of 2400 NiMH cells.

You have the pin out for the remote socket so I would suggest any timer that provides “dry” closing relay contacts would be suitable or just a toggle switch which is what I used. Time accuracy is not that critical when you are into many minutes exposure time.

One other point that may be of use to you – the focus connection on the remote socket also “wakes” up the metering circuit after it has timed out, so a push button on this is useful even if you don’t use the auto focus. The current drain when the metering has timed out is very low. I did not measure it but I would guess less than 1 micro amp.

I hope you let us know how you get on.
Good luck.
Dave.