From: Milton Aupperle <milton@outcastsoft.com>
Date: November 14, 2009 12:52:16 PM MST
To: Astro_IIDC@yahoogroups.com
Subject: Re: [Astro_IIDC] Re: ST3 Pro Exposure calculator...
Hi Perry;
I can not see how manually guiding would be better than an Auto Software method, except in a few isolated cases. I don't have the patience to sit there and watch on screen or at the eyepiece a star jiggle back and forth to try and correct it, maybe you do or other do. I normally shoot for hours at a time and expect my software and hardware to just work. Usually it does - not always due to a variety of factors (largely mechanical, electrical or turbulence) that muck things up, but generally pretty good.
The limiting points for manual.
1) The prism on the OAG delivers only a fraction of the light you get normally, achieving sharp focus is usually a problem and your using stars from the edge of the field which have distortions including coma. So your going to have a very dim distorted guide star to deal with visually. Increasing exposure time, increasing the gains or binning the camera can solve the brightness issue pretty easily.
2) At say 800 mm with a camera, your talking about 1 arc second per pixel and you can easily detect that a star isn't round if it's more than 20% off of this. Your going to need very high (likely 5x) magnification to detect small changes so a short focal length / small exit pupil eyepiece for manual guiding. For onscreen manually guiding using Astro IIDC with it's scrolling windows, you can zoom in on the image up to 8 times and watch it twitch across the screen. For Auto guiding, a good centroid algorithm can detect fractions of a pixel change and respond to them - your eyes likely won't detect such subtly changes nor will it be able to quantify that we need a 172 ms Ra left motion and 32 ms Up Dec motion (Astro IIDC 4 can send 2 correction controls to correct motion per measurement) to move the star back into position.
3) Boredom factor. People are usually not good at sitting and staring at tiny little lights for 30 minutes and respond to there every twitch, computers are. I've done it before using the Cross hairs on Astro IIDC and it's exceedingly tedious and eventually you make a mistake (go left instead of right etc.). That's one of the main reasons I don't do PEC on my mount, I tend to mess it up. If my mount would accept PEC from the auto guide port, then I'd be more likely to do it but not manually.
4) You also have to translate the motion the star has moved into the correct RA/Dec axis motion to correct it, which isn't easy to always get 100%. The OAG view position will change each time you adjust the prism angle or the rotation for the prism too, especially if you add a diagonal to make it easier for the observer eyepiece. This aspect alone can cause many misfires (star goes left, do I click left or right button and how much up / down etc.) when at the Eyepiece. With Astro IIDC if you rotate the guide camera so that the RA axis is parallel the X axis, you can then use the Flip Horizontally and Flip vertically to make the star motion mirror how you move the controller and that make sit easier. With Auto Guiding (in Astro IIDC ), it automatically figures this out for you when it calibrates and handles it.
5) Tracking on multiple stars is possible with some guiding software (Astro IIDC 4 can do that using the average centroids for multiple stars), which helps average out local aberrations from boiling turbulence. You can't do that with a single reticule visually or onscreen.
As to possible good points:
1) If you have lots of satellites or planes or disappearing - reappearing hot pixels, your brain will filter them out. That's tough to do in software, although Astro IIDC does that while tracking long as the intrusive element isn't within 64 pixels of a single guide star.
2) Manual tracking for fast moving objects that are not following the RA/DEC axis and can not be calibrated using normal tracking. It's possible to do it, but it requires a very different methodology and in fact would work best using amount that is Alt Azimuth aligned (i.e.. a "North is Vertically Up" as in a fork mount with no wedge) with no tracking enabled.
3) Bad sporadic turbulence (guide stars disappear) with disappearing - reappearing hot pixels that your brain can deal with easily, which I went through 2 nights ago and Astro IIDC can't handle. But in reality, I should not be out imaging anyhow as the images will be soft from the turbulence anyhow and I should be retiring cameras that have lost of defects too.
Hope Some of this helps..
Milton Aupperle
On 14-Nov-09, at 7:53 AM, H P wrote:
Hey to all,
I read this collection of posts with great interest.
As a result, I want to pose a question. I know the answer is fraught with the effect of many variables, including from the equipment used down to the skill and eyesight of the observer!
But please, don't dismiss it because of that. My intentions are quite honorable!! I have no reason to start a conflict here!
Milton, your tolerance would be most appreciated, since this question perhaps should be in the autoguiding group or the macastronomer group.
Just how well can one manually guide with an OAG, as compared to autoguiding with same,:
a) strictly manually, or
b) manually with software?
Let's limit exposures to 30 minutes or less and have FOVs similar of those in scopes with less than 7-800mm FL - no cassegrains, please.
I've almost stopped reading the posts on the Stark Labs group because of all the problems presented by the writers. Gawd, I've gotten so I don't want to open the daily digest. It's become a forum of gloom and despair not to mention a significant increase in the minutiae of technical details. As a neophyte to guiding, I'm not interested in those.
Aren't most of those from PHD users using PCs and not Macs??? Well, that's question number two. I've tried several times to get those posters to put either "PC" or "Mac" in their subject line but to no avail.
Well, I'm also venting a little here. Please excuse that. However, your input to the query would be most appreciated.
And thanks,
Perry