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Focal length and guiding

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Still trying to get my head round guiding.

I will (hopefully) soon be able to get my hands on an eq mount which is going to help me out leaps and bounds with my imaging (well fingers crossed!)

As far as guiding goes I have looked at various guide scope options and also OAG. As things stands I think I like the idea of the OAG route but digging deeper into the pocket to make it workable I think and since the mount is going to take up a lot of budget this is going to be a bit of a humbug.

There are some neat second hand scopes out there though and some lovely packages available with al the kit I would need to get started straight off the bat, however what I cant work out is this:

how will the focal length of a potential guide scope affect my ability to pull longer exposure times out the bag?

Im jumping the gun a lot I know but I like to plan ahead :)

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A lot will depend on the scope you are imaging with. If you keep below 1000mm focal length for your imaging scope, you should be able to get reasonably long subs with a separate guidescope. There are always some limiting factors.

If you want very long exposures, say for narrowband imaging, with 20 or 30 minute subs, then off axis guiding would be a better option, although getting used to OAG's can be difficult

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Thanks guys, after some sleep now I can probably explain my question better.

@Tim thanks mate, I will likely continue imaging on Megrez 90 for a long while FL is 621mm and like you say considering all options OAG does seem better but I think I am going to be counting the pennies hence I am thinking guide scoping for the short term.

@Bizibuilder, thanks I think you both confirmed a good focal length in my main scope. Really appreciate the info.

I think what I really meant to ask is, if my imaging scope has a focal ration of f6.9 and I choose a guide scope of say f8 will this cause my issues with guiding accurately?

My logic is this (this may be messed up logic mind!) if the main scope is capturing photons quicker than the guide scope and therefore the guide scope is measuring corrections slower than the imaging scope is acquiring the data (this sounding complicated yet?) then the captured data is not being corrected quick enough for the correction to be applied in time and so the error is still incurred but corrected after the necessary point in time?

Does this even make sense or am I barking up the wrong tree?

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You actually need to collect your "guide" images "slowly" or all you are doing is chasing the seeing. One guide correcton every 2 seconds or so is more likely to give good guiding than one every half second.

I think you may be confusing guiding with image aquisition. As long as the guide system collects an image of your guide star then it will guide on it. Your imaging scope is the one "collecting photons" to build up your faint DSO image.

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I have no trouble guiding half hour subs with a guidescope on the 980mm FL TEC, I must say, and find it far easier than an OAG, though at long FLs these do become necessary and certainly on reflectors where the mirror can move.

I like guidescopes...


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Anweniel, you were right to ask about focal length of guidescopes. I think the generally accepted rule is that your guidescope should have a focal length no less than 1/5th that of your imaging scope. So with a focal length of 621mm, you could get away with quite a small guidescope such as a finder-gudier.

However, a low f-ratio on your guidescope is also advantageous (as long as the FL is still long enough) because it increases the number of potential guide stars.

I recently purchased on of these little gems: Orion Mini 50mm Guide Scope - SCS Astro

Edit: or is is 1/4th... ??

Edited by Shibby
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I was thinking about this the other day... the difficulty is, that the sensor sizes are different... I'm thinking you need a guidescope that appears to be longer than the imaging scope... (that's based on the FOV though)... so it's easier if you define something like the crop factor... now...

Using my setup as an example (it's pretty close in fl terms) and operating on 35mm equiv focal lengths...

C80ED (600mm fl), 450d (1.6x crop factor)

Equiv = 960mm

ST80 (400mm fl), QHY5v (approx 8.5x crop factor)

Equiv = 3400mm

So, if my thinking is correct, I can guide up to a 2125mm fl scope with my ST80 and 5v, when using my 450d as the imaging camera.

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John, I think it's the pixel size that's important, not the crop factor. For example, my webcam has roughly the same pixel size as my 450d, so the 1/4 rule applies. (Assuming 1/4 was right in the first place!!)

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Ah ... ok thanks Lewis, that makes sense, so I guess that's a factor too ??

Hmm... so if the pixel sizes are the same, does that mean the equiv fl details work out ?? or am I missing something ?

Actually, 400 is 1/5 of the 2000... although my calculations put it as slightly longer...

Edited by jgs001
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There used to be a rule for no less than half the focal length for the guide scope versus the imaging scope but that was due to the need in the old days for manually moving the whole setup looking through the guide scope. These days it's computerized sub pixel guiding and it's even recommended to be slightly out of focus for the guide scope.

I have not heard about any particular rule on this these days, I think that a guide graph will quickly tell the tale of how it's going.

To me it seems that seeing, wind, mount oddities, guiding software anomalies and differential flexure on some systems kick in well before the focal length of the guide scope makes itself known on the subs...

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