Tracking problems - Star trails! Grr

[Intergage]

Hey all,

I'm having some problems with tracking on my new Skywatcher synscan 8" dob. I can only get ~5 second exposures unless I want very noticeable star trails. Even 5 seconds can produce visible trails.

Is it to do with having no EQ mount as it's a dob?
The object is high in the sky when I'm taking these pictures so if it was closer to the horizon would it be less noticeable when taking longer exposures?

I have tried using all the different types of tracking it gives me: Lunar, Solar and Sidetrails(?) all are pretty much the same.

[Logieberra]

It may just boil down to the scope you own. As you've rightly pointed out, equatorial mounts are suited to long exposure photography...

[skysurfer]

Well, even when it is a fixed Dob you can take images.
The rule of thumb is 12*(sensor diagonal in mm)/(focal length) to get pinpoint stars without tracking.
That means 320/F for an APS-C sensor.
For a 200mm Dob (usually 1200mm long) you can take hundreds of frames of 1/4 seconds and stack them. So you can attach your SLR to the Dob.
Take a 'burst' of 30 images of 1/4 second each (using continuous shooting mode) and then manually adjust the Dob and then again ...
This should work.

[rat156]

Even doing this you will get field rotation, it is an unavoidable consequence of using an Alt/Az type mount.

The star trails you are seeing are probably a combination of tracking errors and field rotation. You have to remember that this scope was not designed for long exposure deep sky imaging, it's a visual scope for the most part.

Cheers
Stuart

[knightrider]

I've successfully taken up to 30sec with my skywatcher DOB up near zenith. About 30sec and I'll be getting star trails begin around the outside of the frame.

[knightrider]

This one for example, un-processed still is a 40sec exposure@ISO3200, you can see the trailing beginning.

157985

Sometimes it takes a few shots to get a good one. You also need to consider if it's other movement causing the 'star trails'.

The actual tracking increments of the scope can cause trailing of the stars too sometimes. Some nights you can even see it through the eyepiece the object jitting as the motors step to track the object.

And you also want to remove anything from the camera that will cause vibration like the neck strap blowing in the breeze. Don't use your finger to start the exposure on the camera, rather use the inbuilt timer or do it via cable/IR remote. Consider using the mirror lock up function on your camera.

And depending on the position in the sky also will determine how long the exposure will be before it starts trailing (well at least that's what I've found, I haven't got a theory why yet). Some can only take 10-15sec max others up to 30-40sec like the image above.

I found my exposures needed to be shorter as I approached the south celestial pole, and I could increase them further away. The Orion photo above was taken pointing near on straight up, and it handled a lot longer than I expected. Whereas when I tried to capture the Tarantula I was flat out getting 10sec before stars dragged.

[White Rabbit]

You could make a wedge for your latitude and tilt the mount to the required angle and only track in az and point it towards the scp. Effectively making it an equatorial mount, it shouldn't be that hard. If you make it so the it's height is adjustable you could drift align it. If your tracking in zap is any good there is no reason why you couldn't get your exposure up to 60 secs or more.

[Shano592]

I think you may have meant sidereal? Not sidetrails.

From the Swinburne Uni site: "Relative to the stars, the sidereal period (http://astronomy.swin.edu.au/cosmos/S/Sidereal+Period) is the time it takes for a planet (http://astronomy.swin.edu.au/cosmos/P/Planet) to complete one orbit (http://astronomy.swin.edu.au/cosmos/O/Orbit)."

In our case 365.25636 days.

And from Google:

1 sidereal day =
23.9344696 hours