Guide Cameras

I’ll make a start…

Most of the time I take short exposures that don’t require guiding, but when imaging in colour or Ha guiding is unavoidable.

It’s not really that much extra setup on the night once you have the gear sort out.

I use a ZWO 60280 guide scope and an ASI120MM mono guide camera, with PHD2 guiding software.

The guide scope is 60mm aperture and 280mm focal length. Focal ratio f/4.6. It is a standard achromatic doublet, with a helical focuser.

ZWO 60280 Finder and Guide Scope

It’s a pretty cost effective guide scope with plenty of light grasp, especially when combined with the mono camera. Being a fast achromat with a mono camera, the stars do have a halo of unfocused IR & UV light, but this does not effect guiding. A filter would help remove this, but there’s no real advantage that I can see. The slightly unfocused extra light may even help.

The guide scope comes with mounting hardware, including a finder scope dovetail bar and a vixen dovetail bar, plus adjustable rings for alignment. It does not include a dovetail base bracket, but your scope may already have one for the finder. It can also be mounted more solidly without the dovetail.

I added a second dovetail bracket at the mirror end of my 6″ Newtonian tube, on the opposite side to the eyepiece to help balance the weight of the camera with the guide scope. It’s actually 135 degrees from the eyepiece so that rotating the tube in the rings changes the balance between the camera and guide scope to get the center of gravity centered side to side. Balancing an equatorial mount side to side means that changes in declination do not effect the balance in right ascension, and ensures that the dec axis is balanced regardless of where it is pointing.

A dovetail bracket is not the most solid mount for a guide scope but it seems to work fine for me, with this weight of guide scope and camera.

The ASI120MM has a pixel size of 3.75um, and there is no RGB matrix or antialiasing filter to reduce resolution or cause issues with different brightness in adjacent pixels of different colour. Since a mono camera does not filter out any light, it’s more sensitive too.

At 280mm focal length, each pixel is 2.8 arc seconds, about the same as the diffraction limit of a 60mm aperture. However this does not limit the accuracy of the guide camera. The light from the star spreads out over several pixels and PHD2 takes an average to find the centre of the star with sub-pixel accuracy. A slightly fuzzy star actually helps as it gives more pixels to average. However defocusing makes the stars dimmer, so it still needs to be reasonably well focused. Having plenty of aperture and a mono camera helps with brightness. Seeing is usually the limiting factor for guiding accuracy.

The field of view with this combination is 0.73 x 0.98 degrees. There is almost always a star in the guide scope field of view that it easily bright enough to guide with.

Focusing

The ZWO guide scope and the ASI camera are intended to work together so there is no issue with back focus as you might have with other setups, ie the camera can be positioned so that it reaches focus. The helical focuser is not ideal because it rotates the camera, but I got it roughly right then loosened the grub screws and nudged the camera a bit to get it straight and in reasonable focus. Once done it does not need to be adjusted and it can be securely locked in position. Perfect focus is not required.

The barrel style cameras like the ASI mini or QHY5L-II allow the sensor to be pushed further down the focuser tube for more focus range. Extension tubes can be added to focus further back.

The USB2 ASI120MM (which I use) is no longer available. It has been replaced by the slightly more expensive USB3 version the ASI120MM-S, or the barrel style USB2 ASI120-MM-mini at the same price as the old USB2 ASI120MM.

Exposure

I typically use 0.5 or 1 second exposures. My mount (i-Optron 25P) does not get much error over this time. This exposure seems to be enough with my setup to get a good guide star most of the time.

Make a dark library in PHD2. It automatically takes darks for a range of exposure times.

Have fun!

Rob