DSI LPI and autoguiding

Hi

Im abit unsure about some stuff.

Autoguiding is for making the telescope track a star 100% and correct for the not 100% tracking from the telescopes own computer?

You need to autoguide to take long exposure shots, else it will be blurred out?

Do you need one camera for autoguiding and one for taking the shots?

Can both LPI and DSI be used for autoguiding?

What would u recomend getting for my ext 70?

Hi Eirik,

You are essentially correct. Autoguiding corrects the errors in the telescopes own mount. This means that stars do not 'trail' in long exposure photographs. Unfortunately this requires the telescope to have a port (plug socket) which accepts input from an autoguider. This autoguider is usually another camera aside from the main one which you are imaging through. Also normally you would need a second telescope attached to the main telescope which you would guide through.


It works like this. A camera is attached to the main telescope, another camera is looking through a smaller telescope (not the finderscope, this is too small). The guiding camera is looking at a star and if that star moves even a tiny bit, a correction is sent to the mount to keep it tracking 100% perfect.

I do not beleive the ETX has such an input port for an autoguider.

The good news is you may not need to autoguide. The focal length of the ETX is just 350mm this means that tracking errors are less noticable than in larger telescopes. You may well be able to take some shots up to say, 30 seconds or so without worrying about autoguiding.

Hope this is of some help

Erik - I'd echo Dave G's sentiments above, and add a couple

In an ideal world, all telescopes would sit on top of a mount that tracks the stars perfectly for hours on end. Unfortunately, even with an unlimited supply of dosh (well, unless you could launch a scope into space) all mounts have inherent tracking errors due to mechanical imperfections.

Added to this, tracking ability will depend on where the mount is pointed at in the sky. Objects move at different speeds, towards the horizon, you have to contend with larger amounts of air mass (and diffraction, seeing etc).

Then you have the way the scope is mounted onto the mount - the optical path of the scope and the polar alignment of the mount must be the same (orthogonality).

So, there's a whole bunch of reasons why tracking isn;t perfect. Of course, some mounts are better than others. But even the twin Keck's don't track perfectly...

For visual observing, tracking is not crucial. For astrophotography, it *may* be.

For short exposure shots (bright objects such as planets), tracking is not a big issue. Equally, for short focal length scopes and wide-field shots, tracking isn't a big deal.

However, for long focal lengths, narrow field shots and dim objects, youneed to collect as much light as possible (both to show the object and to get the best signal-to-noise ratio you can). Hence long exposures.

Long exposures will show up tracking errors. Egg-shaped stars, trailing, wobbles and so on. Autoguiding is necessary - it simply corrects mount errors on-the-fly.

There are two solutions - two imagers and two scopes (one taking the image, one supplying the corrections to the mount) or a dual chip imager. This has two chips in the same housing and negates errors due to flexure between the two scopes.

To answer your specific questions, I know some folks who use an LPI/DSI combo. The DSI images, the LPI guides. But as Dave says, you need a mount with an autoguider input which the ETX70 doesn't have.

But to re-iterate Dave's point, the ETX70 will probably take up to 30 second exposures with few problems.

Hope this helps...

Cheers

Dave

Eirik,
You certainly can do some deepsky work with the ETX70 withoug getting into autoguiding/PEC/computer-control and all that set up. Like Dave said above, the trick is to keep the exposures short.
What would greatly facilitate you is a camera like the canon 300D or 350, an expensive camera, but it has great sensitivity and wide field which is great on this scope.

Here is an image of the orion nebula I took with the ETX70 and the 300D a while back, all I did was attach the camera to the back of the scope and made sure the scope was balanced (no camera lens nor eyepiece were used).
One trick I learned with such a mount as the ETX70 is that its best to image an object when its near the meridan, as the declination drive is running slowest at that point, thus tracking errors from that drive are less likely to hit as the dec drive is almost stopped.

homepage.eircom.net/~sac/graphics/deepsky/m42_etx.jpg

I hope this helps.

Thx for all the answeres. I get it now

Thats a great shot And only 40 sec exposure

So what would best option for me be? DSI/LPI or a new camera?

Could the LPI make just as good shot as what u showed Dave?

As for shops, ive only found a danish site, and norway has limit else u have to pay tax, vat ect (and it adds up a bit) but far cheaper then the 300D

Vixen VX-2 anyone have any pros cons about that camera? How is it compared to (modified) 300d

Any good with my etx 70?