Jupiter 9/18/2011

[darbyvet (Carl Darby)]

Finally a break in the clouds and rain.Here are some pics takne through my Celestron nexstar 8SE and astrnomik filters and a 2.5 telelvue powermate.The first image is taken with a DFK21 camera, the second with a DMK21 and the other 2 with a DMK 21 with the new 618 chip.Seeing q variable

[Daveskywill (David)]

I've heard a man (Billy Kelley) from Arizona who was into telescope making, near Lowell, and knew Bob Ballard (about wreck of the Titanic), (made 100 mirrors) and has passed away since: said that the limiting aperture for best viewing is 8". I guess it can be proven. And with larger sizes it allows in more light, but also the light will be more disturbed (from heat for example). So he said the Lowell Observatory has even stopped down their 24" Clark to like 8" for this reason. I guess it applies until you add adaptive optics or get it out into space.

David

[darbyvet (Carl Darby)]

David,
I do more imaging than visual observing, but I think it is much more complex an issue than aperture size.
For imaging you want to get as short an exposure as possible to "freeze" the seeing.If you have a camera with a high frame rate (60fps) you can capture many frames that are in focus and not affected by atmospheric disturbance (seeing).
Now the larger the aperture you have the more light you gather and the shorter the exposure you need.Most of the worlds best planetary imagers use a 14 inch SCT (see Damian Peachs jaw dropping images at www.damianpeach.com).

The trade off with larger apertures is that the optics take longer to cool down and you can get tube currents that affect the image quality. I use an 8 and 11 inch SCT for imaging.My 11 inch gets put outside at least 4 hours before imaging to cool down and sometimes it still has tube currents for the first hour or so of imaging.

I really think the major issue is seeing.If the seeing is good you can get good pics.If the seeing is bad you cant. I think in this case larger aperture scopes are affected more by seeing than smaller aperture so maybe that is why they use an 8 inch aperture at Lowell.If you had good seeing you would want to use the largest aperture possible to get more light and detail.

You can see the effects of seeing by looking at my pics.The pics in this post were taken with good seeing (3/5).I also posted a pics on 9/21/2011.These pics were taken with very good seeing (4/5).You can see that with the same camera and telescope the detail in the image taken on 9/21/2011 is much better.

[Poita (Peter)]

Thanks so much, it is great to see how the different cameras compare.
Which is your favourite image by the way, and what are your thoughts re the pros and cons of each of the cameras?

[darbyvet (Carl Darby)]

I think I like the DMK618 image the best.

The DFK is a great color camera.It is easy to get good images and you dont need to worry about filters.It is not as sensitive as the mono cameras and you get less detail.It can be hard to get the color balance right.I turn debayering on in ICcapture to view the color image and then adjust the red and blue balance.You have to remember to turn off debayering before you record.

The DMK 21 has been my standard camera.Great sensitivity and better detail possible than with the DFK.Generally you can use shorter exposures than with the DFK and that means you can get better images if the seeing is less than perfect.

The DMK 21-618 is my new camera.It is much more sensitive than the DMK 21.I can shoot at 1/76s exposure even through the 8 inch aperture.The image I took on 9/21/2011 shows the kind of detail you can pick up with the 618-much more detail than with the DMK

[Poita (Peter)]

Thanks for the info, how much was the 618?

[darbyvet (Carl Darby)]

I believe it was about $450 US.