Computerised Dobs.

[paul schofield]

Just a quick question regarding Computerised Dobs.

I am well aware of Dobsonians not being the best tools for astro photography, well for DSO's anyway. My question is, are Goto or Synscan dobs a different story ? As from research into these models and noticing they will track objects for you, would they then be suitable for A.P. ? Maybe a silly question but have always wondered about this. Some claim to be great A.P tools, but thought I would ask on here, (horses mouth n all that). Thanks for any replies.

[knobby]

From what little I know Paul, Dobs are alt az ie: up a bit right a bit etc. To accurately track you need an EQ ie: smooth tracking in an arc.

Alt az will follow the motion fairly accurately but can't follow the rotation... Aaah, someone will explain it much better than me in a minute !

[Mike73]

I've no idea about the Synscan dobs but David from EQ Platforms gets good results from his platforms. http://www.equatoria...r-early-results

I have one but prefer pencils to pixels so I've never tried AP on it.

[atmenterprises]

A dob mount would work for moon photography since the exposures would be short. For long exposure photography I doubt it. Since motorized dibs would work on two planes - altitude and azimuth - that means to track an object, the tube would move a little and the base would move a little kind of like a stair step pattern. This would cause field rotation and just wouldn't work for any long exposure stuff.

I own a dob and I do astrophotos with it, but only of the moon and (just now starting) planetary photography. Since those are short-exposure subjects I can get away with it. Clusters, nebula and galaxies won't be photographic targets for me.

However, you could buy a sturdy equatorial mount and move your Newt off the dob and on to it to allow long-exposure photography in the future. That's what I plan to do.

Hope that helps. I'm a few weeks down the astrophotography route with my dob so I'll help you with what I know.

[VigdisVZ]

My beginners belief is that the tracking in goto dobs (and in the fork mounts without the wedges) is to rough for AP.

Otherwise we'd see many more astrophotographers recommending them and using them.

[LukeSkywatcher]

Ive seen some amazing deep sky images done using a Dob and manual tracking (yes............manual tracking), but in general, no scope on a Alt-Az mount will be good enough for astro-imaging. Whoever took those images while manually tracking with a Dob in my opinion is either CRAZY or a GENIUS (not much in the difference).

Given that Dobs are not very well suited to imaging (except perhaps for short exposures of solar system objects), i cant see any reason to buy a more expensive Go-To,Synscan model. You would be better off buying a standard model and accepting the fact that you are limited to short exposures.

There is one way in which you may be able to image DSO's with a Dob that does not require you to be crazy or genius and do manual tracking, and that would be to take short exposures (the more the merrier) of DSO's and stack them with software such as DSS (Deep Sky Stacker). You really would be limited to exposures of about 4 seconds each..................but if you take a few hundred of them at 4 seconds each and stack them.......................you will end up with a combined image of a few minutes worth of data and the results can be amazing.

Its a bit like imaging DSO's using a camera on a fixed tripod.

It can be done.

The quality of image is never going to be the same as if you had imaged with an EQ mount, but as long as you have fun doing it and are happy with the results..........thats what it is all about.

I should add that using a Dob and a webcam for imaging solar system objects is a good way also of doing things.

Here is a thread running right now here about imaging Orion using only a DSLR camera on a fixed tripod and using and stacking multiple 4 second images.

http://stargazerslou...ith-4-sec-subs/

The results are quite amazing. I imagine that the results using a Dob would be similar.

[Richardisgreat]

From what little I know Paul, Dobs are alt az ie: up a bit right a bit etc. To accurately track you need an EQ ie: smooth tracking in an arc.

Alt az will follow the motion fairly accurately but can't follow the rotation... Aaah, someone will explain it much better than me in a minute !

Yep thats about right, Knobby explained it perfectly I have a 200p goto dob and like knobby said it is alt az so therefore does not factor in the field rotation. Putting it simply it suffers from field rotation so any long exposures would have star trails. I have imaged the moon and had good results and also had an attempt at imaging Saturn, I have now bought a clamp type thingy to attach my digital camera ( i dont own a dslr) to the eyepiece so i should be able to get some decent video or short exposures and stack them in registax.

Interesting what Luke Skywatcher said, i might have to try taking a few hundred short exposure photos of a bright dso and see what the results are like.

[LukeSkywatcher]

Interesting what Luke Skywatcher said, i might have to try taking a few hundred short exposure photos of a bright dso and see what the results are like.

It works for a DSLR on a fixed tripod, so i cant see how it would be any different for a dob...........its essentially the same setup.

Here is my first ever stacked image of a DSO taken with my 450D on a fixed tripod (no telescope involved). If i remember correctly..............it is a stacked image using 12 exposures of about 4-6 seconds each.

The fuzzy bright blob in the middle of the image is the Andromeda galaxy.

Imagine what results i would get if i took MORE short exposurers and maybe a bit more magnification.

[Richardisgreat]

i read somewhere on this forum that short exposure photos on dsos wouldnt pick up alot of detail. Something about if you had say a hundred, 4 second exposures, you would still only have 4 seconds of detail but just nice and clear due to the amount of stacked images.

but

If you had say an hour of exposure then you would have picked up far more light from fainter objects in the picture and be able to see more detail.

Just something i read on sgl ages ago, i have absolutly no experience at all of long exposure photography so im only recalling what i think i read somewhere

how accurate was this information?

[LukeSkywatcher]

i read somewhere on this forum that short exposure photos on dsos wouldnt pick up alot of detail. Something about if you had say a hundred, 4 second exposures, you would still only have 4 seconds of detail but just nice and clear due to the amount of stacked images.

but

If you had say an hour of exposure then you would have picked up far more light from fainter objects in the picture and be able to see more detail.

Just something i read on sgl ages ago, i have absolutly no experience at all of long exposure photography so im only recalling what i think i read somewhere

how accurate was this information?

A hundred exposures at 4 seconds long each when stacked = the same amount of time and light gathered from one single exposure equaling 400 seconds on a single long exposure.

Thats my understanding.

To pick up detail, you need more magnification. More magnification means more shorter exposures, so if you double the magnification, you need to take twice as many exposures at half the time lenght (2 seconds each).

[Richardisgreat]

ok sounds reasonable to me. I was led to believe that the sensor in the camera picked up more photons for longer exposures, where as short exposures picked up less even when stacked.

for example take imaging M27. If you took a 4 second exposure you would be lucky to see anything at all. So if you took hundreds of the same photo, i thought even stacked you would see a barley visable image.

But if you took an hour or more you should see a nice bright-ish Dumbell nebular.

Somebody correct me if im wrong please. It would be super cool if i am wrong because that would mean i could get some ok shots of some dsos.

[cosmojaydee]

Its all in the photons, the more short exposures you take , the more photons you grab so taking many short exposures can compensate for taking 1 long exposure

[Richardisgreat]

cool, thats made my day Just need these blasted clouds to clear now. Whos bought something expensive? come on own up

[LukeSkywatcher]

I was led to believe that the sensor in the camera picked up more photons for longer exposures, where as short exposures picked up less even when stacked.

Im no expert but i think both scenario gather the same amount of light.

for example take imaging M27. If you took a 4 second exposure you would be lucky to see anything at all. So if you took hundreds of the same photo, i thought even stacked you would see a barley visable image.

Nope..................each and every second of exposure adds data to the stacked image so you end up with a combined image that equates to the exposures taken and not just a single 4 second exposure.

But if you took an hour or more you should see a nice bright-ish Dumbell nebula.

The more exposures you take (the longer the exposure when stacked) will give you a better image.

Somebody correct me if im wrong please. It would be super cool if i am wrong because that would mean i could get some ok shots of some dsos.

I'm happy to say that you are wrong. 10,000 exposures of 4 seconds long each does NOT mean you end up with a single 4 seconds exposure.

I'll leave you to do the maths on how many minutes a combined (stacked) image of 10,000 @ 4 seconds each equates to.

[Richardisgreat]

thanks guys, sorry for hyjacking this post, i have learnt something here today. Thanks guys

[Andymac79]

You cant just take lots of very short exposure images, stack them and get the same result as a longer exposure, its the long exposure that gathers the light and brings out the fainter detail

[LukeSkywatcher]

Many short exposures = the same data (light gathering) as a single long exposure.

[LukeSkywatcher]

You cant just take lots of very short exposure images, stack them and get the same result as a longer exposure, its the long exposure that gathers the light and brings out the fainter detail

I agree. Many short exposures will NEVER give the same results as one single long exposure. It will do a fine job though.

But you are correct, there simply is no substitute for a long single exposure.

[Richardisgreat]

one last quick question, would it not be better to take say a couple of minutes of video .avi files and get registax to break it down into frames then stack the best few hundred frames, when imaging dsos, or is the video less sensitive to light than say taking lots of 4 second exposures? I know on manual mode taking stills you can set the iso as well as the exposure time, but on the video mode everything is set automatically on my digital camera. I guess a follow up question would be are digital cameras really not much good when imaging dsos, and to save my pennies and get a slr?

[LukeSkywatcher]

one last quick question, would it not be better to take say a couple of minutes of video .avi files and get registax to break it down into frames then stack the best few hundred frames, when imaging dsos, or is the video less sensitive to light than say taking lots of 4 second exposures? I know on manual mode taking stills you can set the iso as well as the exposure time, but on the video mode everything is set automatically on my digital camera. I guess a follow up question would be are digital cameras really not much good when imaging dsos, and to save my pennies and get a slr?

video/webcam images taken and then stacked in Registax only work with solar system objects such as the Moon and inner planets.

Webcams dont work for imaging DSO.

[Richardisgreat]

Ah thanks, why is this?

[Dellis]

Just a quick question regarding Computerised Dobs.

I am well aware of Dobsonians not being the best tools for astro photography, well for DSO's anyway. My question is, are Goto or Synscan dobs a different story ? As from research into these models and noticing they will track objects for you, would they then be suitable for A.P. ? Maybe a silly question but have always wondered about this. Some claim to be great A.P tools, but thought I would ask on here, (horses mouth n all that). Thanks for any replies.

A dob or any other sort of altazimuth mount will track but the field of view will rotate thus each image taken will be slightly different so stacking is not really practical, with an equatorial mount mount the fov does not rotate thus each image covers the same area of sky and the whole image can be stacked.

Dave

[LukeSkywatcher]

Ah thanks, why is this?

The sensor on webcams etc is not sensitive enough to collect photons from DSO's. If you want to image DSO with a "cam", then you really need to invest in a CCD camera.

[Richardisgreat]

Thanks paul, and well explained Dave

[paul schofield]

Thanks for explaining that to me chaps. Will have to read through replies again (went off track a bit) but think I understand now. I thought I had found a cheaper entry into Astro Photography (like millions of people had not thought this before) Learning every day me. Appreciate all advice on this, thanks again all. Clear skies.