Old 29-12-2018, 11:31 PM
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SuperG is offline
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Best Focal Length ratio for a guide scope

I have two main scopes. The first is an ed80 with a zwo 60280 guidescope. I normally get good imaging results with little effort for DSO.

The second is a c6 SCT that I normally use for planetary. But I have tried to use it for DSO a couple of times now for increased resolving power with an f6.3 FR and an Orion 50mm mini guider (FL= 160mm). But the results are horrible. I have heard of flexsure but no one has ever explained that either.

My real question is what is the optimal FL ratio for a guidescope: imaging scope. I have heard the minimum is 1:5. Please provide comments.
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Old 30-12-2018, 07:00 AM
kens (Ken)
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FL ratio is just one factor. A factor of 1:5 or 1:4 is indeed a practical minimum assuming the same pixel size on both imaging and guide cameras.
In your case flexure, or more correctly, differential flexure is a potential issue. That's when the guide camera moves relative to the imaging camera so the guiding corrections keep the guide star centred in the guide camera but on the imaging camera it is moving.
At long focal lengths this becomes very noticeable as even small differences show up. The SCT has a particular problem in that it focusses by moving the primary mirror so the mirror is not fixed relative to the tube and can therefore move relative to the guide scope.
For this reason, for long focal lengths and especially for SCTs the recommendation is to use an OAG so the guide camera and imaging camera are looking at the same thing.
That of course introduces a whole lot of other issues but it does help.
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Old 31-12-2018, 12:13 PM
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Astrofriend (Lars)
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I don't have very long focallength, 910 mm refractor as main telescope. To that I use an telephoto lens with 400 mm fl as guide telescope. The main camera is a Canon 6D fullframe. The guide camera is a qhy5, just a bit smaller pixels compare to main camera. Works well and I have not noted any flex between the tubes.

See here how it's setup:

Earlier I had off-axis guiding, if get in trouble with the new setup I build a new off-axis system to the new 3" field flattener. But it will cost a lot of money so good if I don't have to.

One advantage with a separate guide telescope is that I can have a wider field and then easier to find guide stars.

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Old 16-01-2019, 12:34 PM
Capella_Ben (Ben)
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+1 for OAG.

I have only ever used an off axis guider and have had great results with them. I almost never have problems finding a guide star (even on my RC8).
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Old 18-01-2019, 10:00 AM
dan_iana (Daniel and Iana)
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I've had pretty reasonable success guiding my c8 edge hd (with 0.7x reducer) with a 50mm Orion guide scope (focal length 162mm) and asi290mc (doubles as my planteray).
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