How's my tracking?

[StuartT]

This evening I took a couple of star field exposures at the prime focus of my 20cm SCT. It's only on an alt-az mount with tracking, but I was surprised I was seeing trails at fairly short exposures (30s and 60s). Does this mean my tracking is rubbish?

 

 

[vlaiv]

In principle - yes.

However, don't despair, even mounts that are designed for astrophotography (some decent EQ mounts) - could easily end up with such exposures.

That is the reason people guide.

With 2000mm FL - any modern DSLR with relatively large pixels is going to be oversampling - or in another words - "very zoomed in".

I measured your star trails on second image and they are about 200px long. With modern DSLR with say 4.3µm pixel size, you'll be imaging at 0.44"/px.  That means that your star trails are 88 arc seconds long.

Mounts like EQ5 or HEQ5 and even EQ6 which are considered good for AP - can easily have 45" peak to peak periodic error. If periodic error is fast moving (usually it takes 5 minutes for mount to drift 45 arc seconds) - you can get star trails of 30-40 arc seconds long in matter of minute or two.

For visual - you would not notice the difference, even at planetary scales. Imagine you are observing Jupiter - 88 arc seconds means that it will drift back and forth about two diameters over minute.  That is small portion of field of view in the eyepiece and you would not even notice it.

For astrophotography - yes, it is a problem - but most people have this problem and if they want to do long exposure - they guide. You can't do that on AltAz mount so if you want to do imaging - you'll have to limit your exposures to maybe 5-10 seconds to avoid excessive trailing - and even then, be prepared to loose some of the subs as not every frame will be equally affected by this (as you can see from your example).

[StuartT]

Thanks for this explanation vlaiv! I really appreciate it. So far I have been mostly taking images of the moon (where obviously this is not a problem). But I decided I'd see what happened if I pointed the scope at some stars instead.

I am considering buying an EQ mount so I can then consider guiding (if I am clever enough to figure out how to do it!)