Dob Photography - Moon

[Solanum]

From past threads I know I'm not the only one with a dSLR and a Dob and a desire to put the two together. We're pretty much limited to the moon and even then the speed of the scopes (f 4.8 in my case) means that we can't get focus without using a barlow, which prevents whole disc shots.

:(

Well, with only one possible subject it's still worth a go, and I'm sure it's good practice, so here's my first proper attempt, using a simple t piece, 1.25" adaptor and 2x barlow. Air had a lot of wobble in it, so they could be crisper (and I don't rate my barlow either).

If anyone has ideas to improve the shots, ideas for a dSLR + 10" Dob combination, or ways to get focus without the barlow I'm more than happy to hear them!

Also, which crater is the really bright one near the middle of the limb in pic 3?

[jjjnettie]

I use my camera afocal on my Dob to get moon shots. Butting the lens of the camera up to a 30mm eyepiece.
Mine isn't a dslr so I haven't a clue about your focus problems.
I'm sure someone here will be able to give you a solution.

[iceman]

Hi Everard, great first attempts!

The crater near the limb in your third shot is Aristarchus. It's a beauty.

Regarding focusing your DSLR, you either need to:
a) Get a very low profile focuser
b) Move your mirror up the tube

I didn't want to do either of these to my 12" because I use it for high resolution imaging, and didn't want to lose focal length.

So that leaves you with 2 choices:
1) Go afocal. Use a 30mm eyepiece and a lens that lets you squish up against it
2) Get a smaller scope for full disc moon shots.

I used to do #1, now I do #2. I got an ED80 for the full disc shots.

[Solanum]

I recently replaced my focuser with a 10:1 and debated the low profile for this reason, but was told that the GSO/Bintel one was unstable with a dSLR attached, so decided there wasn't much point!

As for moving the mirror, that would make the scope even faster wouldn't it? Thus coma problems with certain eyepieces would be even more of a problem? Also, I thought the mirror was ground to match a certain focal length, presumably that creates problems too?



I find that with afocal, using both the dSLR and my wife's compact digital camera I get dark patches in the image and vignetting is a problem too. Perhaps the dark patches are due to some sort of vignetting?


I have been wondering about getting a small refractor on a driven mount for imaging. Don't have the cash at the moment though.... :(

[Lee]

The profile of the mirror is what gives a scope its focal length/f-ratio, hence moving it won't affect either - it may give you vignetting though.
I chopped the base off my Newt, and shifted the mirror 35mm up I think I remember - prime focus with DSLR no problem now. I just use an extender for visual with EP's.....

Thread showing "the chop" (http://www.iceinspace.com.au/forum/showthread.php?t=13631&highlight=cutting)

[jjjnettie]

That dark patch is from your secondary mirror. While the moon is in half phase you can dodge around it.
When the moon is approaching full it becomes more of a problem. If you keep the apeture of the camera wide and the exposure short, it shouldn't show up on the photo, even though you see it through the viewfinder. Just check the photo after you've taken it, and play around with the settings until it disappears.
You'll work it out I'm sure and then you'll be amazed with what you can capture afocal.:thumbsup:

[Solanum]

Ah, so you just have to be careful that the secondary still reflects the entire light cone, presumably that limits how far you can shift things. Is there an easy calculation to get the focal plane from the f ratio and physical distance between secondary & primary?

[rmcpb]

Everard,

Have a play with NEWT (http://www.dalekeller.net/atm/newtonians/newtsoft/newtsoft.htm). Its an old bit of software but really good for setting up a newtonian scope. Within reason you will be OK moving the primary up the tube as GS scopes tend to have oversize secondaries for some reason.

Cheers

[Hagar]

I also had the same focus problems with a 12" GSO and ended up removing 38mm of tube from the bottom. This fixed the focus problem and didn't change the focal length at all. I purchased a 50mm extender for use with EP's.
Check out Paul Mayo's web site (http://www.skylab.com.au/pmsa/)for some really extrordinary photography with a modified DOB modified as above.
Good luck and make the mod.

Doug

[Alchemy]

i loped of 50mm of my 12inch gso, the 2ndary is oversized so still reflects all the light and i just picked up a 50mm extension from bintel for visual use

You may not believe this but when i did the cut upon putting everything back together it was in collimation. People seem afraid to modify their gear, sometimes with good cause , sometimes not

[Solanum]

Thanks for all the info. I'll think long and hard about imaging, but it does sound as if shortening the tube a little doesn't have to severe a drawback for visual observation.

Incidentally, NEWT runs fine under Wine for any other Linux users out there.