Dobsonians for astrophotography?

[pipnina]

I've noticed that some dobsonian-mounted reflecors have GoTo capability. (Such as http://www.wexphotographic.com/buy-sky-watcher-skyliner-200p-flextube-synscan-go-to-parabolic-dobsonian-telescope/p1524331?cm_mmc=googlebase-extension-_-telescopes-_-dobsonian-telescopes-_-sky-watcher-skyliner-200p-flextube-synscan-go-to-parabolic-dobsonian-telescope_1524331&utm_source=googlebase-extension&cm_mmc=google+-+warehouse+cameras+%26+lenses-_-shopping+-+other+products-_-&mkwid=430wvtp4&pcrid=54652739529&gclid=cj0keqia99qnbrdnryckt4clzzabeiqavqpalgl7kaup7doogpcxlgukzrn197ru0kfd2snyz59kufiaarw28p8haq) I presume this means they can track objects in the sky like a motorised equatorial.

How do these work? You point them at polaris, press a button and it's aligned?

(Also, is it a bad idea to move a GoTo manually?)

[Owmuchonomy]

Yes they exist. I own one. The mount is an Alt Az type but not equatorially adjusted. So whilst tracking occurs it is not aligned with the Earth' axis and thus the field of view gradually rotates. So other than for very short exposures they are not suitable for AP. Initial alignment is remarkably easy and only comprises a couple more steps than you suggest. It's quite ok to move a GoTo scope manually albeit with a bit more effort. Mine has encoders so the software can maintain alignment even if you manually move the scope.

[pipnina]

Yes they exist. I own one. The mount is an Alt Az type but not equatorially adjusted. So whilst tracking occurs it is not aligned with the Earth' axis and thus the field of view gradually rotates. So other than for very short exposures they are not suitable for AP. Initial alignment is remarkably easy and only comprises a couple more steps than you suggest.

Ah, that's a shame. As I undertand dobsonian is the only way to mount especially large telescopes, if they could work with AP that would make them much more appealing as using an equatorial mount for observing requires you to be double-jointed! (And you constantly have to re-balance and rotate the tube if you want to look at a different part of the sky, agh!)

Although, I presume a dob could work for AP if the tube could rotate with the earth's axis, but that would ruin the dob's use for observing. Maybe if the dob's base and the tube were assembled on-site... I may have some thinking to do.

[DaveS]

You *can* buy field rotators to de-rotate the field but they are hideously expensive, and one more thing to go wrong. Probably only useful in megabucks 'scopes (Try six-figures).

I've never had any problem with equatorialy mounted 'scopes, probably because that's what I started with, 40+ years ago ( )

Edit:

Something like this http://www.iankingimaging.com/show_product.php?id=1554 it's about the cheapest one I can find. Others go upwards of £1k

[ollypenrice]

If all you want to do to define a Dob is describe an alt-azimuth Newtonian then they certainly can take good astrophotos. If you have a spare six million euros you could buy something like this which lives about 20Km from me. http://a405.idata.over-blog.com/4/15/06/42/DSCN7169.JPG  It just has a field de-rotator before the focuser and uses direct drive technology to run unguided. Modern professional scopes have not been equatorially mounted for some time.

There is another way to make your motorized Dob equatorial. Take it to the north or south pole....

lly

[Astrobits]

Put it on an Equatorial platform = No field rotation.

Nigel

[jgs001]

With an AltAz mount you can get exposures of between 30 seconds and 2 minutes, depending on where in the sky you're shooting. It's not an ideal option, and certainly not going to produce the results of a decent EQ mount, however, go for it.

N/S and through the zenith is the shortest exposure times, whereas low in the east/west is up to 2 minutes, without trailing from field rotation effects.

The biggest restriction is actually more like to be the tracking drives. I found my AltAz mount drives suffered from too many defects on the longer exposures, and that keeping the exposures to under 1 minute was the best option.

However, even with 40 second exposures, you can achieve results, including capturing the bubble nebula with an unmodded dSLR (ok, I had 132 lights in the stack, it took a very very long time to process). 

You're only real option would be to test it out and see what you can achieve. Just don't expect to be able to get the real faint detail, as that requires much longer exposures, and you will not be able to achieve those. 

[dph1nm]

A big aperture Dob with a camera with decent sized pixels (i.e. 1" or so) shouldn't  need more than 30sec or so subs to image anything - so it ought to be perfectly possible if the alt-az tracking is reasonably good.  I suspect you don't see many images like this because (a) people are told it isn't possible so don't try, ( these mounts don't track very well, and ©  cameras with big pixels are expensive!

NigelM

[ronin]

Pipnina, if you intend to go down the AP line then really get the proper equipment in the first instance.

It will save a lot of time, trouble and ultimately expense, and it will deliver the type of images you expect.

For AP (DSO imaging) the requirtements are a stable equitorial mount, dual motors but goto is better, a short fast scope either reflector or refractor, camera that can take exposures of 60 seconds or more, few attachments/adaptors.

Planetary imaging is actually different - as in not the same equipment. So planetary imaging equipment does not carry over to DSO imaging equipment. Effectively do not buy one and expect to do both easily or at all.

[ollypenrice]

A big aperture Dob with a camera with decent sized pixels (i.e. 1" or so) shouldn't  need more than 30sec or so subs to image anything - so it ought to be perfectly possible if the alt-az tracking is reasonably good.  I suspect you don't see many images like this because (a) people are told it isn't possible so don't try, ( these mounts don't track very well, and ©  cameras with big pixels are expensive!

NigelM

You're going to image the IFN or the tidal tail in the Leo Triplet in 30 second subs? No theory allowed here. Just give us practical proof. In my opinion it can't be done. These were hard enough in 30 minute subs.

Olly

[jgs001]

An AltAz mount is not the ideal tool for imaging, however, if that's what's currently available, it will be able to achieve results, and some excellent ones at that.

For someone starting out, we need to provide encouragement, not argue over whether a large Dob would be capable of getting the incredibly faint detail. This is not the place to get into an argument over what is and isn't achievable. 

Pipnina, having reread your post, it's not clear if you have already purchased the dobsonian scope. If not, then is your main objective visual with the occasional image, or mainly imaging with the occasional visual ? If mainly visual, then if you haven't purchased anything, the Dob is going to be an excellent choice. If mainly imaging, then I'd suggest a rethink, as the Dob is not ideal. 

[ollypenrice]

An AltAz mount is not the ideal tool for imaging, however, if that's what's currently available, it will be able to achieve results, and some excellent ones at that.

For someone starting out, we need to provide encouragement, not argue over whether a large Dob would be capable of getting the incredibly faint detail. This is not the place to get into an argument over what is and isn't achievable. 

Pipnina, having reread your post, it's not clear if you have already purchased the dobsonian scope. If not, then is your main objective visual with the occasional image, or mainly imaging with the occasional visual ? If mainly visual, then if you haven't purchased anything, the Dob is going to be an excellent choice. If mainly imaging, then I'd suggest a rethink, as the Dob is not ideal. 

OK, my wrist slapped, but the OP said, ' I presume this means they can track objects in the sky like a motorised equatorial.' The only accurate response to this is to say, 'No, that is not correct. They can track an object but not like an equatorial because of field rotation.' Having gone that far it's worth pointing out that motorized alt az mounts (such as Dobs) can be used for short exposures but that short exposures are rather limiting.

I think this is a perfectly reasonable and accurate answer but at no point does it suggest that if you have a motorized Dob you shouldn't bother. However, if choosing a scope with AP in mind it implies (as in my view it should) that a motorized Dob is a bad choice.

Olly

[barkis]

Don't be put off trying to Image with your Dob. Pipnina. Of course the Dob. is not an imaging configuration, for the reasons Owemuchmoney stated, but results are possible,

and the main thing is you are not aiming to satisfy anyone on SGL with the results, but you yourself.

As jgs001 said, do some trial work, but stay away from low latitudes.

Help in achieving decent images is available from Dob. owners on the forum, and I'm sure they will be pleased to assist.

Best Wishes.

Ron.

[dph1nm]

You're going to image the IFN or the tidal tail in the Leo Triplet in 30 second subs?

Well, I am sure it could be done (if you could get a wide enough field with a large enough aperture), but I really just meant that you should be able to image most Messier/NGC objects (OK, maybe not the wide field stuff)  quite happily, if you are prepared to stack enough subs, on, say, a 16 inch Dob. But I have no practical experience of how well these tracking Dob mounts work (nor have I seen anything on-line about this) - I know my SLT alt-az isn't great, but you can do imaging with it at 'a few' arcsecs per pixel.

NigelM

[Leveye]

Keep in mind that Dobsonian only refers to the base it's on. It's a Newtonian scope design. Take it off that base and mount it to a sturdy tracking GEM or such and you have a powerful imaging Newtonian light bucket. I love my Newt for imaging or visual. You may in some cases have to move the primary mirror up some to Image. Keep that in mind as well.

[niallk]

Interesting to watch out for SW EQ-Dob! No field rotation... Haven't heard any update in a while... A little like ap on a platform - not going to rival a proper ap setup, but probably lots of fun!

https://m.facebook.com/SkyWatcherTelescope/photos/pcb.823794347672808/823793394339570/?type=1&source=49

[quaoar]

You could try this mob http://www.astro-video.com/ They are however painfully slow to deliver ( I've been waiting for a Mark IV for a year now, just a few more weeks apparently lol ). They have a Yahoo group that you can log onto and see images and discussions.