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Old 24-01-2010, 04:25 PM
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koputai (Jason)
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Imaging vs Guiding Focal Length

Folks,

What should the relationship between the focal lengths of your imaging scope and your guidescope be? If imaging with an ED80 (600mm), would 150mm be ok for the guider? What about the converse, if imaging with a 150mm lens, does the 600mm of the ED80 make for good guiding?

Thanks,
Jason.
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Old 24-01-2010, 04:32 PM
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multiweb (Marc)
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Originally Posted by koputai View Post
Folks,

What should the relationship between the focal lengths of your imaging scope and your guidescope be? If imaging with an ED80 (600mm), would 150mm be ok for the guider? What about the converse, if imaging with a 150mm lens, does the 600mm of the ED80 make for good guiding?

Thanks,
Jason.
This is what I've observed. In my experience (I guide with PHD) it doesn't matter. I guided a 5"newt at F/5 with and ED80. Similar image scale. I guided a C11 at F/10 with an OAG and with a finder 8x50mm. PHD can detect the motion of a fraction of a pixel in your guider. Changing the guider image scale didn't make the slightest difference IMO and having more pixels in your stars on the guider is not going to make you mount react faster or go from A to B any better.
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Old 24-01-2010, 04:42 PM
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yes, guiding FL matters less these days, more of a problem is the weight and flex of a large guide scope. I guided a 3m FL SCT with an ST80 on a G11 and it was fine. It wasnt with an ED80 just because of its weight and flexure. Guiding with the big OTA is fine, just bin the guide cam at 3. Cleaner exposures too, bonus.

As Marc says, you could use anything to guide an ED80, even a finder.
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Old 24-01-2010, 09:11 PM
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Marc hit the nail on the head here... I've used OAG's, similar focal length scopes, fairly varied focal lengths (C11 @ 2800mm with a 400mm F/L guider) and even as Marc said, a finder scope..

All of these things worked just as well as the next, however I've had best results from an OAG or self guided cameras. These really are the best way to remove differential flexture from the equation entirely.. The downside is you can't really use an OAG in with a DSLR lens... However I'd use a finderscope for DSLR lens imaging..
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Old 28-01-2010, 08:22 AM
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I do not think the relative fls matter too much either but you should consider the image scale of each camera. While it is true that PHD (and others) will perfom sub-pixel guiding if you are guiding at 10"/pixel or worse you will have trouble if you are imaging at 1" or better. Bear in mind you want your corrections to be +/- 2 pixels or less on the imaging sensor for visually acceptable results.
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Old 28-01-2010, 09:38 AM
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Thanks guys, good info there. I can probably give it a go with what I've got.

Cheers,
Jason.
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Old 30-01-2010, 09:17 AM
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Jason, while the above I agree with there is some evidence from my own experience that suggests guiding with a focal length under half of your primary can lead to pronounced flexure issues. PHD and other programs do have the ability to guide as sub pixel but the longer your focal length against the guide scope the worse the flexure becomes. Combating flexure in the end is the real problem. Alex is probably right about OAG's and self guided cameras, although my experience so far is quite frustrating.
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Old 30-01-2010, 09:40 AM
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Jason, while the above I agree with there is some evidence from my own experience that suggests guiding with a focal length under half of your primary can lead to pronounced flexure issues.
I understand OAG is the way to go when flexure is involved (ie. large SCT) but how does keeping the guider and imager FL closer resolves this or makes it better? Whether you guide an 11" SCT imaging let's say at f/6.5 with an 8x50 guider or en ED80 is not going to make much difference in your final pic? Flexure is a separate issue with focuser sag in refractor or mirror flop in SCT or other bracing moving over time in respect to the guider, or even the guider too heavy moving in respect to the main scope.
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Old 30-01-2010, 11:56 AM
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I am not sure of the peer reviewed science ! . . .

The focal length is really only half of the equation - on its own it doesnt give an answer, its the image scale that matters. So the size of the CCD pixels for Imaging and Guide cameras is equally as important as the focal lengths.

It is the image scale of the guide system compared to the image scale of the imaging system are what matters, considered against the guiding software's ability to resolve the centroid of the guide star.

What you are wanting to ensure is that the minimum resolution and correction by the guide system equates to 1 or less pixels on the image system (in a perfect world ! - but 2 or more is probably acceptable in the real world) - that way you dont get blurred stars (increased FWHM) of the image as a result of guiding errors - in this case we are concerned about an under correction.

Since most guiding software can resolve the centroid of a guide star to around 1/10 of a pixel, but certainly well under 1 pixel in good conditions.

It would seem logical to me that the image scale of the guide system could be significantly higher than that of the imaging system.
What amount I really dont know - but I would hazard a guess that it could be 5x bigger and still operate within the other errors of the average imaging system, maybe even as much as 10 times bigger.
Averaged out over time that error could conceivabley be even greater.

However if the guider image is highly undersampled (where a star is less than 1 pixel) this would impose an upper limit on the image scale - say maybe 3.0 to 6.0 arc secs/pixel
Depends a little bit on the star chosen - if its mag 3 you would be OK if its mag 8 you might struggle at the upper limits.

The fact that most guide cameras tend to have smaller pixels compared to the average imaging camera is in our favour in terms of the focal lengths of the guide scope being lower than the focal length of the imaging scope.

Not exactly a straight answer.
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Old 30-01-2010, 12:58 PM
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You are correct, and there really is no straight answer as everyone's setup is different...

I do like to keep my guiding and imaging at similar resolution.. that way if I monitor my guiding graph, and it says that the RMS movement of the star is .2 of a pixel, I know in my image it is less that .2 of a pixel, and there for, its going to make a nice image... Currently using an OAG I've got 5.2um pixels on the guider, and 5.4um pixels on the imaging camera... The guiding is always tighter than 0.3pix correction distances, and I've not lost an exposure due to guiding errors in months..
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Old 30-01-2010, 07:11 PM
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Marc, Alex's answer is mainly the nuts and bolts of what I was going to say or to that effect. To clarify, from my own experience if the star moves just a little at one magnification then the imaging scope needs to move the same distance. If for instance the pixels of each sensor are close then the scale is different and even if PHD can guide to sub pixel this error can be more evident if the focal length is double the size or more. That is because the error is magnified in the larger imaging scale.

Added to this is flexure. Any error is magnified even more with the elements of flexure involved. So instead of the error being double it then becomes triple or more. If you guide with a guide scope you will have some element of flexure.

Guiding can never keep up with that. So that is why I prefer to keep the focal length at least half the distance of the primary.

Mathematically, what Rally says is correct but in practice this is not what really happens.

Hope that makes sense.
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