Why aren't dobs good for imaging?

[NM51]

I wanted to know why Dobs are good for astrophotography. I have heard that they aren't but I don't for sure why?

[DaveW]

All kinds of reasons, though some don't apply to all dobs.

1) No tracking. Even on auto tracking dobs, tracking often isn't good enough to do more than webcam AP.

2) Alt-Az mount. Even with perfect tracking, long exposures would still suffer from field rotation because they aren't polar aligned.

3) Not enough back-focus. Many dobs (like my Z10) don't have enough backfocus to use a dSLR.

You can overcome these issues, and I'm sure there's more, but in general they just aren't good AP platforms. You get a lot of aperture with a dob, but for AP that isn't always needed. Better (and more expensive) to get a good quality GEM and a small telescope.

[sxinias]

Dave pretty much said it all.

[Sim]

Remember Dobsonian refers to the mount not the scope

[nrivard]

I have a similar back-focus problem on my Orion XT10. Recently bought an t-ring adapter and tried my DSLR on it, only to find out I can't reach focus. Focal point always is in front of the sensor. A friend lent me his 1.6 X barlow (2 in.) and that kind of fixed the issue.

The other problems DaveW mentionned are harder to fix, but for the moon and other bright objects, it's not really an issue since exposure time will be short. If your intend was to go deep, an equatorial mount (like a poncet) is the only solution I know.

[jenniferchristine]

Dobs are not good for photography because they aren't cameras?
Most of the reasons above are pretty valid.
However, they are ok for planetary with a webcam as the exposures are quite short and the results are stacked.
For lunar they are in fact quite effective with a single shot and Afocal.

[BobDob]

IMO, Dobs, using the DobDriver II or the ServoCat go-to/tracking systems, are fine for AP or Vidcams. For about the same money, you can go with the high end platforms for good results. And, there are fork mounted or GEMs for smaller apertures which give good results. But, these gather fewer protons per second, hence require longer exposures and more stacking of images.

Those who like Chevys like Chevys. Those who like Fords like Fords. Both are transportation. Pick one and enjoy your choice. They'll get you there.



Bob

[OleCuss]

Just to beat on a dead horse. . .

If you can solve the back-focus problem and already have a nice, big, fast Dob you can fix many of the problems by making something like a Poncet platform. It gives you limited equatorial capability and you get to use that big, beautiful glass for some pretty darned good photography.

So let's say I wanted a big, fast reflector for photographic purposes? I've already got a pretty fast 12 inch Dob. I can make a Poncet and for an hour at a time I can have equatorial functionality.

FWIW

[nrivard]

I agree with OleCuss, only wish I had one...

Another problem you may face is coma. I admit this is not something all dobs will have, but these days most dob are pretty fast (F5 or better) and because of that might show coma on your pictures. The point is the mirror's focal plane is curve while your camera's sensor is flat.

[KeithBC]

Another problem you may face is coma. I admit this is not something all dobs will have, but these days most dob are pretty fast (F5 or better) and because of that might show coma on your pictures. The point is the mirror's focal plane is curve while your camera's sensor is flat.

Coma can be fixed, but it will cost you.