Autoguiding with a Alt-Az mount?

[Matty P (Matt)]

This may be a stupid question but there is a possibility it might work.

There is a autoguiding port on my Alt-Az mount and was wondering if I use it will it improve the tracking of the mount?

I have everything setup for autoguiding and it is probably worth a try.

Is there any reason that I should not Autoguide with my Alt-Az mount?

Thanks.

[Dennis]

Hi Matty

If you imagine your mount guiding on a star in the centre of the field of view, then the mount should faithfully follow that star and produce a nice, round stellar disc.

However, stars around this central star will show levels of trailling the further they are away from that central star; the whole field will appear to revolve around the central star. This applies to long duration images by the way.

For short duration images, where the exposure is insufficient to record star trails, you will have a series of overlapping frames that gradually rotate around the centre star.

Just imagine starting an exposure with your scope pointing at the SCP. If you take a series of photos, you will note that the Southern Cross may be on your left (East) when you start and with time it will cross the Meridian and then begin to drop into the West to your right.

So, on your starting frame you will have your guidestar in the middle with Crux to the left. Then, you will have the guidestar in the middle with Crux above it and finally, the guide star in the middle and Crux to the right. This is field rotation.

Cheers

Dennis

[Matty P (Matt)]

Thanks for your reply Dennis.

My intentions were to improve tracking for short duration imaging (30 seconds max).

I know that field rotation is inevitable with an Alt-Az mount. I was hoping that autoguiding would improve tracking so I could eliminate tracking errors in my images. Only 2 or 3 images out of 10 are suitable for stacking and I would like to get a better success rate.

I found that anything over 30 seconds a small amount of field rotation will start to appear. Gradually over time field rotation will come more and more apparent in the images but the stacking software will remove it.

Will using autoguiding eliminate tracking errors and give me a better success rate to getting round stars in my images?

[robgreaves]

The stacking software will only remove field rotation if it has the capability of making streaks back into stellar points again... Field rotation, over long exposures, is not simply a shift of a stars' position in the frame, but it will physically 'smear' and trail the stars at the edges of the field.

Meade still do (I think!) a field derotator, which does work. Not cheap though... probably easier to spend $100 bucks or so on a second hand wedge, which will solve most of the issues.

Interestingly, due to the mechanical aspects, a lot of large observatory class telescopes are still on alt-az mounts, and use the field derotator method.

Regards,
Rob.

[Babalyon 5]

Well, really, your scope may stay centered on your guide star, but your image will always rotate around the centre. If you wanna guide and do longer exposures, you need to go equatrial. You could build yourself a very cheap and serviceable barn door type of wedge, then you need to master polar alignment and drifting for accuracy.

[Matty P (Matt)]

Thanks for your replies,

At the moment a EQ mount is out of the question. I've read about a field derotator but they are a bit expensive and a wedge would probably be a better option.

On the other hand, I am bit concerned that my mount wont be able to handle a wedge.

My 8" SCT OTA is on a single fork arm Alt-Az mount. I've read and know that a single fork arm is possibly not stable enough to support the OTA, camera, guidescope in unusual positions when put on a wedge. I've also heard that the mount can start to act erratically but don’t know that for sure.

Celestron sell a wedge for my scope and it would probably be a great way to get into Polar alignment etc.

Has anyone used a scope mounted on a wedge with a single fork arm?

[alela (Alejandro)]

Hi Matty. I just bought a Celestron 8SE and I am curious to know if you succeeded with the wedge for polar aligning the scope. I am also in the south hemisphere and I read the specs of the 8SE and tracking is only for EQ North, and not EQ South. That prevented me to buy the wedge. What's your experience so far.

Regards,

Alejandro

[gary]

Originally Posted by Matty P

Is there any reason that I should not Autoguide with my Alt-Az mount?

Hi Matty,

Despite the limitations of field rotation, there are plenty of enthusiasts out
there achieving stunning results with autoguiding on Alt/Az mounts using
short exposures and software derotation.

For example, check out these images by Johannes Brachtendorf
using an 18" Obsession, Argo Navis, ServoCAT, SBIG ST 9E and software
derotation. As Johannes says, "no platform, no derotator, no adaptive optics".
http://homepages.uni-tuebingen.de/jo...r/Galaxies.htm
http://homepages.uni-tuebingen.de/jo...er/Nebulae.htm

Also be sure to read Johannes notes on the subject -
http://homepages.uni-tuebingen.de/jo...dorf/index.htm

So you won't be breaking any new ground in giving it a try. Now and then
this topic comes up and some will say deep sky imaging is not practical
on an Alt/Az mount without a field derotator or platform. Take a look
at Johannes images and join in the chorus of "Myth busted!".

Enjoy!

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au