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Dobsonian rotation

SlyReaper

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SlyReaper

SlyReaper

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Finally got my hands on a computerised dob and it occurs to me, since it uses an alt-azimuth mount, the image will rotate slowly as I'm tracking.  How big a problem is that for imaging?  I'd like to try my hand at imaging a galaxy or three (as soon as the weather stops trolling me), so how long can I set my exposures before the edges start to blur from the rotation? 

Unrelated, but is it normal for a dob to slowly droop over time?  I tried pointing it at a nearby radio tower just to set up the finder scope, and I found the front of the scope ever so slowly dropped down.  Is that normal?  I'd have expected it to be balanced with the heavy mirror at the back counterbalancing the extended front side.  Is this going to be a problem whenever I try looking at low down things?

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Uplooker

Uplooker

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Even a computerised Dob is not suitable for imaging because it is an alt-azimuth mount. You should be able to get certainly 30 sec at a real push 60 sec before star trailing get too bad.

Not sure which dob you are using  -Skywatcher, revelation, Orion Optics???? Depending which one, it may have a clutch that allows you adjust the "ease" of movement. Ready made Dobs apparently do not come finely built / adjusted for balance.

ian

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RT65CB-SWL

RT65CB-SWL

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+1 for Uplooker's comments.

For your imaging equipment, what will/or were you planning to use? You may 'get away' with a modded webcam and a filter in the nosepiece as they are lightweight.

With regards to the "...droop over time", maybe ok at +50º altitude to zenith. It maybe worth considering a sliding weight arrangement on the mirror end to compensate, (consider the resistance load on the motor(s) as it may cause it/them to burnout). Another thing is combatting de-rotation in azimuth too.

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brantuk

brantuk

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You can usually tighten or loosen the alt axis - there's either a knob you can turn or a nut you can tighten. You'll need to consult the manual to find out where it is - or state the make of your scope and someone here who owns a similar model will be able to help.

If there's no means of tightening the axis then you'll need to add some sort of counter balance - it's a common problem for most dob owners - especially if large heavy eyepieces are used - and there's plenty of solutions which you'll find in the diy sections (look for "dob mods"). Hth :)

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SlyReaper

SlyReaper

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Okay 30 second exposures will have to do.  Doesn't seem so bad, I'm not after the most impressive images ever, just cool images I can share.  Planetary images, I'll do with exposures measured in the milliseconds.  Not sure if registax or AS2 can do it, but there must be software out there somewhere that can derotate each frame before stacking them (hell, I'll do it manually, because with 30 second exposures, I won't have to do a huge number of frames).  And if all else fails, I'll build a ramp of 39° to park my telescope on.  Seems a bit extreme though, and not sure how the motors would cope with the unusual forces. 

The telescope is a 10 inch Skywatcher: http://www.firstlightoptics.com/dobsonians/skywatcher-skyliner-250px-flextube-goto.html

Camera is a ZWO ASI: http://www.zwoptical.com/eng/Cameras/ASI120/index.asp  It weighs about 100g, so I'm not concerned about its weight.

If it comes to it, I still have a 5 incher on a motorised EQ2 mount which I can use the camera on, and just use the dob for observing.  But I'd really like to combine the light gathering power of the dob with the new camera if at all possible. 

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ollypenrice

ollypenrice

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The effects of field rotation vary with regard to the position of the object in the sky. There is no one maximum time limit for alt az imaging. At the poles, alt-az is equatorial...

Don't put your Dob on a wedge to set it to the angle of your latitude. In Bristol it will fall off its trolley! Besides, this is not necessary. Check out equatorial platforms, which are very clever and provide an equatorial tracking platform without extreme angles (but for more limited times.)

For all that, a Dob is a visual instrument and is best viewed as such, though I'm not for a moment saying you shouldn't have a go. Pick something bright. Ring Nebula?

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2266922474&k=Sc3kgzc

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dph1nm

dph1nm

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I think the real issue here is not rotation, but how well the alt-az mount will track, as I think this will kill you long before rotation becomes a problem. If you can get decent 30 sec exposures then with a 10inch scope you can happily image most Messier DSOs and a good number of fainter NGC objects. But I would be (pleasantly) surprised if you can reliably  track to the ~2 arcsecs accuracy you will need. Certainly worth giving it a go though!

NigelM

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Landy Dave

Landy Dave

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I should also add if you image the moon by creating a mosaic then you have to work fast with a pattern that will cause the least rotation between images otherwise your first and last images do not line up due to the rotation between taking the images

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brantuk

brantuk

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That's a new one on me.  Why would the tracking be inaccurate?  I get that it has to track on two axes, but why is that hard?  It's just maths, which computers are good at.

The problem comes by virtue of the fact an alt/az mount has to track in two axes. It'll be accurate enough for observing and short exposures of planets - but for long exposure dso work you really need tracking in one axis only (ie RA), and guiding - it takes a whole new level of accuracy to keep photons from light years away arriving on a chip with pixel point accuracy as the Earth spins. You need to track the natural motion of the night sky.

Commercially produced alt/az mounts just aren't up to it - for dso imaging you need a precisely polar aligned EQ mount that follows the natural path of an object across the sky with the Earth's movement. However - with a wedge that allows a dob to be polar aligned, and a motorised dob, you may be able to get slightly longer subs than 30 secs.

Have a look here to see "barn door" wedges: http://www.equatorialplatforms.com/

And here's some results: http://www.obsessiontelescopes.com/galleries/20/index.php :)

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dph1nm

dph1nm

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I don't think there is any inherent mechanical problem in having two axes - I just suspect that the motors/gears etc used on tracking dobs are probably not of the highest quality.  But I don't have a tracking Dob, so I would be interested to hear the experience of someone who has.  It also depends what pixel scale you are imaging at - so if you are happy to display your images with e.g. 8" per pixel (which works fine for larger objects)  then you might get away with it.

NigelM

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ollypenrice

ollypenrice

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That's a new one on me.  Why would the tracking be inaccurate?  I get that it has to track on two axes, but why is that hard?  It's just maths, which computers are good at.

Computers are indeed good at sums and the computer knows exactly where to point the scope. The problem arises once you leave virtual reality in the computer and ask the machine (the mount) to do precisely as it's told. For this it needs feedback, either from very high resolution encoders on the axes or, more usually in EQ imaging, from a guide scope/camera. This is because the machine will not do precisely as it's told! It has to be free to move, which tends to mean it's also free to wobble around somewhat - and it will. Imaging at long focal lengths is akin to trying to pick up a pea with three metre long chopsticks. It is harder than using short ones...

Olly

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