Guidescope theory - True or False

[1ponders] (Paul) · #1 03-06-2005, 06:05 PM

Here's a question for the guidescopers out there. Its one that's been nagging at the back of my mind (quiet Tony) for a while now. I maybe burnt at the stake for considering this heresy but here it is.

According to theory a guidescope should be at least the same focal length as the imaging scope, preferably 1.5x or longer. Now I understand the theory behind this. If your guidescope has a shorter FL than your imaging scope then your imaging scope may get trailing before you notice the movement in your guidescope, particularly if there is a large differences in FLs

That may well have been true when manually guiding using a reticle, but does the same hold true now when autoguiding using a webcam is so popular. I've noticed while using K3CCDTOOLS V2 that the guiding is done at a pixel level with the stars at almost their pin point level of focus (particularly when you drop the gain and brighness down), far more accurately than visually, no matter how high your magnification/long your focal length or accurate your reticle.

So given these considerations of smallness of target, responsiveness of guiding software and accuracy of the new breeds of motorized mounts is it still neccessary to have a guidescope FL equal or longer than the imaging FL. BTW I'm not considering camera focal length type lenghts (200-400mm) but short tube refractors of 450 - 600mm FL on, say, 2000mm imaging focal lengths.

Cheers

Robby · #2 03-06-2005, 09:22 PM

Hi Paul,
I find I can autoguide at 400mm while imaging at 1500mm without noticable star trails... That, in theory is a big no no! But it works fine for me. Have even been able to image at 2350mm with the same setup. See M83 pic from Astrophoto weekend.
Rules are meant to be broken!!

acropolite (Phil) · #3 03-06-2005, 10:27 PM

Paul, If you haven't already visited, go to Pete's astronomy page. He uses an LX90 8 inch with a Sky-Watcher 804AZ3 Achromatic Refractor and ToUcam for a guidescope, pretty well the same equipment you are using. He also uses Guidedog and an EOS 300D. http://www.users.bigpond.com/lansma/Default.htm

[1ponders] (Paul) · #4 04-06-2005, 10:47 AM

Thanks Phil. Yes I've seen Pete's page, he does some great imaging. This question is more along the lines of getting some discussion going on what's possible now using digital guiding rather than reticle/visual guiding, and how short a FL is too short for autoguiding. As you can see Robby's really pushing it

and I'm starting to wonder if it would be possible with a 200mm camera lens for a 2000mm imaging scope.

What is the limit and still maintain trail free images?

darthjohnnyb · #5 15-06-2005, 09:53 PM

For some figures for this one (if anyone is still interested)....

An 80mm f6 guidescope with a phillips webcam, such as a toucam, will have a theoretical resolution of 2.4 arcseconds per pixel. Most software uses stars calculated out as centroids and can detect movement of the centre of this centroid. Eg K3CCD will allow subpixel guiding, down to about 1/4 of a pixel. This then gives a correction of better than 1 arcsecond.

A 200mm f3.5 lens with the same webcam would give a theoretical resolution per pixel of 5.7 arcseconds.....with subpixel guiding at 1/4 of a pixel the minimum error detectable would be about 1.4arcseconds. However the resolution of a 2000mm f6 is 0.53 arcseconds per pixel (assuming same pixel size as the webcam).....Given that typical seeing conditions would rarely allow such fine resolution you may well get away with the 200mm lens.

Its hard to be that precise when eyeballing it!

Cheers,
John B

[1ponders] (Paul) · #6 15-06-2005, 10:21 PM

I'm certainly interested John. It got me thinking along new lines of autoguiding. I'd certainly be interested in the methods for working out these details.

Thanks

mch62 (Mark) · #7 16-06-2005, 08:45 AM

Stack a couple of 2x teleconverters on it.
You can pick cheapies up from Cash converters.
Did this my self on my old 6"refractor & telephoto lens.

gbeal · #8 16-06-2005, 09:32 AM

Paul,
good thread, and I am in a similar boat to Rob, similar guidescope/camera/rings/software, with almost similar imaging scopes.
I had no problem with the 400mm guidescope and the 2160mm or 1650mm focal lengths of the Mewlon. Ditto with the 1250mm of the 10" newt. Likewise with the ED80/840, or any of the camera lenses I have tried.
Gut feeling is if you already have the 200mm lens, try it. If you get trailing/guiding errors then maybe rethink it, but if not just do it. If I had a problem with the guiding setup I currently have it would be the CA and blobby stars I get on the monitor. How it guides with these beats me.
Gary

darthjohnnyb · #9 16-06-2005, 02:39 PM

Ijust used the freeware version of the CCD calculator from new astronomy.

http://www.newastro.com/newastro/dow...cdCalcFree.htm

You can put in the values for whatever equipment you have/want and let it do the calculations. If your camera isn't listed you may have to enter some data Eg chip dimensions and max resolution to get the pixel sizes, but its not to difficult. The freeware version has 3 images you can use to compare imaging areas of various setups.

It certainly makes it easy to quickly compare several different setups/combos.

Cheers,
John