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Image Rotation Exposure Limits

BlueAstra

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BlueAstra

BlueAstra

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I have an Alt-Az Goto mount. It will track objects but they do eventually rotate in the field of view. What would be a typical exposure time limit on a tracked object to prevent star trails from rotation?

Assuming I then have a sequence of frames of sharp star fields, the sharp star field may have rotated from one frame to the next. Will registax take out this rotation, or does it stack only by shifting laterally?

Thanks for any answers.

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Beyond_Vision

Beyond_Vision

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It will depend on the location of the target and the focal length of the telescope plus CCD/DSLR chip size. With a focal length of 2500mm (1575mm with focal reducer) for my LX90, somewhere around 20 to 40 secs with a meade DSI camera.

I would also recommend a free stacking program called Deep Sky Stacker. This will take care of any rotation between frames. The end image will have to be cropped as the stacked image will cover a smaller area due to the frame rotating.

Hopefully someone may have a link to a calculator that can tell you the optimum time for exposure given the target position and focal length and CCD/DSLR chip size

Regards

Kevin

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Dave Smith

Dave Smith

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I'm not absolutely positive but I don't think Registax deals with field rotation. As has already been stated DeepSkyStacker does and it's also free. DSS aligns on stars which is appropriate for longer exposures.

Dave

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MartinB

MartinB

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If you want some idea of what you can do with an alt-az set up have a look at some of Roger Warner's pics here http://www.astropixels.co.uk/Roger_Warner.htm#RogerWarner

Rog used an 8" Nexstar GPS often with a fastar set up. Both Rog and myself found we could push it to around 45 secs using an Atik 2HS which is basically a modded webcam with small pixels. A larger chip will probably show more rotation.

There is a neat trick to reduce star trails from field rotation in photoshop. You create a duplicate layer, select darken as the blend mode. Then use the rotate tool under the edit menu making sure the rotation point is at the centre of the image. You only need to rotate a tiny amount. Once it's tidied up, flatten the image. This can help but I don't think Rog needed to do this for any of the images shown here.

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jgs001

jgs001

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I've shot at 2 minutes exposures in Alt Az, using a very light weight scope, an ST80. The limiting factor in terms of the sub length, assuming shooting in the same part of the sky is weight, balance and the mount motors. If you target objects that are low in the east or west, then you can go for longer before star trails start to appear. If you go for higher in the sky or further north/south, the duration comes down. This doesn't have to be a problem in some cases. I took a series of subs of M52 in Cassie a while back, so sub length was limited to 40 seconds, and after stacking some 150 subs over 2 nights, I was able to get quite an amount of the bubble nebula showing up (ok it was very small...)

If you have field rotation in a sub, then neither stacking program will deal with it, and in fact they may well reject the sub. However, rotation effects between shots will be dealt with nicely.

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BlueAstra

BlueAstra

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Thanks for all the useful replies. I am very new to all this so pardon my ignorance! I believe Registax aligns frames and then allows you to process the resulting aligned 'summed' image to sharpen it up. Great for planets, moon, wide field objects. However, it cannot correct rotated 'good' frames.

DST will correct rotated 'good' frames. Does it also have the processing ability to sharpen up the images, or do you have to use some other software for this, if required?

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rikyuu

rikyuu

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hi,

I tried Registax 5 to stack 5x30s subs from an ALT AZ mount, but missed seeing the option to de-rotate the images (until I've just seen the post above), even with multiple alignment features, it was unable to account for field rotation. Will have to investigate the derotation check box now that I know.

I hadn't used DSS before, but tried it out and it worked great. It automatically detected stars for alignment and auto rotated. Very good results with this.

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