Guiding The Telescope

This page is actively under construction!
It will contain quite some information so building this page will take time. I will most likely re-write the text shortly.

This is a bit complicated stuff which is why I cut the text up in many sections in order to give you time to consume and comprehend the matter. Illustrations will follow later.

There are three causes that make a star move in in your field of view after having your mount correctly aligned with the celestial pole:

1. Polar alignment is never 100% so there will always be a small amount of the Earths rotation left.
2. Every mount that carries a telescope has flaws. It is a mechanical device with tolerances in its parts. Those tolerances add up resulting in an inaccuracy to some extend.
3. The atmosphere is never stable, the more wind the less stability. It is like looking into water and see your view being distorted by waves and streams. This is simply called 'The Seeing'.

To correct the first two you need a guider. A guider is a camera that monitors the sky and that corrects the mount when needed. This camera can be integrated in your main camera or it can be the alternative for an eyepiece in a small telescope parallel to your main telescope. The third one can be corrected in large telescopes in observatories but to my knowledge that technique is not (yet?) available for home based movable telescopes. That being said you can avoid that the guider corrects on star movement by seeing because as opposed to the first two the isn't moving in the sky, it's just a short temporary distortion in the path of the light.

There are a few very good reasons for a separate guide camera on its own scope:

1. If you have more than 1 camera for different sorts of astrophotography you either loose the guide camera when you switch or you have to buy every camera in dual version which makes it quite a bit more expensive. And not nearly every camera exists in dual version.
2. A separate camera can be used for other things.
3. In the next section I will explain why you may want an integrated guide camera in the main one as they are sort of connected to the combi of scope and camera.

So how does guiding work?

1. The longer the focal length of the scope the smaller the angle of its view.
2. A cameras sensor chip consists of millions of tiny sensors which have a certain size. They all take a tiny bit of the viewing angle of the telescope. So: there is a certain percentage of the scope's angle that each sensor cell (pixel) covers. The tinier the pixel the smaller the part of the scope's angle is covered. This is called: 'arc"/pixel'.
   So there are two variables in this equation: the angle of the (guide)scope and the size of the pixel.
3. The correction movement of the spot on the sky to keep matching the pixel is called the RMS which means 'Root Mean Square' which equals RA^2 + DEC^2 (both the axis's to be corrected are perpendicular). Good old Pythagoras🤓. So as long as the RMS is smaller than or equals the angle ratio in arc"/pixel you have a steady lock and whatever is there in the sky wil stay in place on the pixel creating a sharp image. Ratio explained in the next paragraph.

In other words: so if you want a sharp image you have to take care that the part of the sky that the pixel sees stays where it is and when there is any movement of the scope that may cause that that seen part of the sky leaves the pixel the scope is taken back to the right spot so that it stays within the pixel where it should be.
But matching the main scope with the guide scope isn't possible as they have their fixed parameters. So: divide the arc"/pixel of the guide scope/camera by the arc"/pixel of the main scope/camera and as long as you are within the 1:5 ratio or less you should be ok. It's a rule of thumb, no guarantee.
Calculate arc"/pixel in Astronomy Tools.

So if the mount is to slow the guide camera sends a command to the mount to speed it up and vice versa.
But what if the combination of the pixel of the guide camera and the sky that it covers is not the same as is the case with the combi of the main camera and main scope? Then the correction delivers a certain amount of blur, it either takes more pixels than needed or details get lost as they all occupy the same pixel. This is a bit like under and over sampling. (explained elsewhere later).

One way to overcome this problem is to build a guide camera chip into the main camera. As long as the pixel sizes of both chips are the same or close to each other the problem is solved. Direct advantage is that they stay together where ever they go or whatever the focal length of the scope becomes while reducers or barlows are applied.

An external combination of guide camera and guide scope immediately creates the problem that guide scopes have a much shorter focal length and thus a very much wider field of view or angle. To compensate for that in order to get somewhat equal to the main camera and scope the separate guide camera needs even tinier pixels. Bare in mind that specifically with large telescopes like SCT's and Newtonians (the dual mirror types) it will be near impossible to match the correction by the RMS-values that guide camera is acting on and the ones the main camera needs. In other words: the corrections are measured, calculated and issued for a different pixel size than needed.

How to calibrate your guiding.

First: calibrate your guiding I will write out some tips here but watch this video of Peter Zelinka first:
How to Improve Guiding Performance - ASIAir Tutorial. This is aimed at the proces in the ASI Air models but apart from the ASI Air as an example the parameters go for all systems
When I start the guiding camera it automatically enters the calibration routine. Set the gain to a value that allows for an exposure time of 3 seconds to start with. As part of the 'seeing' the atmosphere is always in motion. Turbulence will wobble your seeing just like the air does very well visible above a hot surface. This turbulence caused wobbling very shortly displaces a star. If your exposure time is that short that it captures a star at the moment of displacement the guide camera will issue a correction to the mount for a movement that really wasn't there. So a longer exposure time will greatly reduce these faulty measures as in that time frame the star will 'return' to its real position.

Some links:
Article on Stargazers Lounge

Wido's Astroforum

Simple guiding tutorial