Guide Scope - is a longer focal length better??

[AntHart]

Hey

A though I had...probably obvious to the More experienced but being a beginner I’d rather ask than make mistakes...

if I have a guide scope with for example a 200mm focal length is it better than a 150mm one as it ‘enlarges’ the guide star more and hence is an easier target to track ??

in my mind if something is larger it’s easier to see, if it’s easier to see it’s easier to follow...which in my noob terms is what a guide scope and camera does...mixed in with a little bit of algorithmic magic  

 

if I’m wrong...please let me know why so I can learn

is a guide scopes focal length dependant on what your imaging...kind of the same way a small vs large refractor is? or is there an ‘ideal/one for all’ guide scope size...and recommend kit Be great to hear some advice.

 

i have a Skywatcher EQ6 pro , Canon 6d DSLR and soon to be WO Z61 or Z73....literally just starting out!

 

thanks for any help

Ant

[vlaiv]

It is - up to a point.

It's not focal length of guide scope that is important - it is sampling rate of guide system - so focal length + pixel size of camera used.

Here is a bit of math to explain what you should be paying attention to.

Centroid algorithm is said to have precision between 1/16 and 1/20 of a single pixel. This means that if you for example have 4"/px sampling rate - your precision in determining star position will be limited to about 0.2" - 0.25".

If you have a mount that can guide to 0.2"-0.3" RMS - this is clearly not enough because guide system will issue correction larger than that because of error in star position.

What is appropriate guide focal length then? You will hear different recommendations depending on who you ask, and here is my reply:

Either start with your imaging resolution of your guide performance. If you start with imaging resolution - use half of that as you need at most that much guide RMS error. If you take RMS error - then just use that value. You need your guide precision to be at least 3-4 less than that.

Let's say that you are imaging at 1.5"/px, and you guide at 0.8" RMS. You want your guide precision to be something like 0.2" (1/4 of rms). Let's say that your guide camera has 3.75um pixels and you need your guiding resolution to be at least x16 times larger than measured error which is 0.2 so your guide resolution is going to be 3.2"/px.

From 3.2"/px and 3.75um pixel size we calculate required focal length to be about 240mm

[Skipper Billy]

There is a very good guide outfit / imaging outfit calculator here that will effortlessly do the maths for you and maybe make life easier.

http://astronomy.tools/calculators/guidescope_suitability

 

 

[Erling G-P]

Not always, or it depends on the guider is probably the right answer.

I too am starting out, and after a year of unguided imaging, I'm trying to get a guiding solution working.  Initial intentions were to use a 2nd hand Synguider 2, but I've found the tiny LCD display difficult to use, and so far my attempts at guiding with it has not met with success.  After reading the review of the Staraid Revolution in the latest issue of Astronomy Now, I decided to get one, as it sounded just like the simple solution I was looking for.  It is designed to work optimally with focal lengths of 100-150mm however, so my 10x60 guidescope from TS, at 248mm was not ideal.  As a solution, I've gotten a 0.5x reducer for it, turning it into a 124mm F/2, which I hope will work well - weather (of course) hasn't allowed me to test it yet.

Regards,

Erling G-P