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Old 15-01-2013, 12:42 PM
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Shiraz (Ray)
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Is better tracking required for long focal lengths?

Hi - need help.

often see the assertion that long fl scopes need better tracking for imaging than shorter ones - basic question is why?

Tracking accuracy is an angular term, so if the scope angular resolution is limited by the atmosphere (which is generally a fair assumption for typical conditions and scope sizes), a given tracking error will produce the same degree of angular distortion in star shape regardless of scope fl. As a numerical example, if the resolution of two different scopes is limited by the seeing so that they both produce 3 arc sec star blobs and the tracking introduces a wobble of 2 arc sec in RA, then both scopes will produce stars that are 5x3 arc sec in shape. The stars might have completely different sizes in the final images, depending on the fl and sampling, but they will still have the same underlying 5x3 aspect ratio. As far as I can see, unless the seeing is so good that at least one scope is not seeing-limited, fl does not come into it at all.

However, the idea that focal length determines required tracking performance is widespread and is held by experienced users, so clearly I am missing something - very grateful for an explanation of what it might be.

regards Ray
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Old 15-01-2013, 01:23 PM
WingnutR32 (Sam)
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From a very basic person here, my understanding is that short focal length yields wider fields and therefore the "same" tracking error that you would get in your mount whether you use a long or short focal length, just doesn't visually show up as much on the short focal length.

Longer focal ratios would 'zoom' in more on the field of view and allow you to see more of the error.
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Old 15-01-2013, 01:58 PM
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As they say in "I, robot" - your logic is undeniable...
The longer focal length and associated plate scale just make the "error" more obvious - sort of "in your face".
With shorter focal lengths it gets "lost" and not so obvious.
(The results actually depend on how the tracking error accumulates on the image- it may loop back and forth or sit more accurately during one part of the worm cycle the quick "jump" to an extreme. These changes in object/ star recorded intensity can dramatically change the look of the image....)
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Old 15-01-2013, 02:03 PM
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Poor tracking is going to be more obvious in images with smaller image scales, so in that sense I think it is true that long focal lengths require better tracking. I agree that this doesn't imply that you need to guide at a longer focal length. Here's an article that claims that a very short focal length guide scope is sufficient unless you have outstanding seeing:
http://www.wilmslowastro.com/tips/autoguiding.htm

The other aspect to consider is that there are other causes of bad tracking like differential flexure between a scope and guidescope or movement of a primary mirror. A common solution to these problems is off-axis guiding. The benefit from OAG is not that a longer focal length is being used - it is that the guider is seeing exactly the same field as the imaging camera, subject to the same mechanical variations.

Cheers,
Rick.
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Old 15-01-2013, 03:37 PM
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Sam, Ken Rick - thanks guys. So it's a perception issue, not physics.

again taking a numerical example to summarise: assume a 1000mm fl scope is nicely matched to a camera with 5 micron pixels. if the scope is changed to 2000mm fl without changing the camera, the stars will be oversampled and the (dimmer star) tracking distortion will be more noticeable to the eye than with the 1000mm fl scope. Alternatively, a 500mm fl scope will produce an undersampled image and the dimmer stars will appear as irregular blocky points almost regardless of their underlying shape. (the current Polarie craze shows that idea taken to the extreme).

However, if I chose a camera with 10micron pixels for the 2000mm scope and one with 2.5 micron pixels for the 500mm scope, the star images for these setups would be identical to the 1000/5 setup if the tracking is the same - star shape would be independent of focal length.

So the assertion that a longer focal length scope requires better tracking should have the very important caveat "If you use the same camera".

You could just as accurately say "a longer focal length scope does not require better tracking if the angular pixel scale of the camera is maintained"

thanks, I can understand that better now. regards Ray

Last edited by Shiraz; 17-01-2013 at 06:09 PM.
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Old 15-01-2013, 04:26 PM
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Quote:
Originally Posted by Shiraz View Post
So the assertion that a longer focal length scope requires better tracking should have the very important caveat "If you use the same camera"
Yes, I think it's image scale that's important rather than focal length.
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Old 15-01-2013, 07:09 PM
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Everything is magnified including the tracking errors.

Greg.
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Old 16-01-2013, 10:08 AM
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Hi,

Is there a sweet spot or optimum range for image scales in a guiding system that provide the best autoguiding?
eg. many use the finder guider where at a 200mm FL and a camera like the qhy5, the image scale is 5.4 odd arcsec/pixel.
would it be beneficial to use a longer FL guidescope with the same camera and if so are there any guidelines or numbers that help determine the optimum guidescope FL?
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Old 16-01-2013, 11:11 AM
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Quote:
Originally Posted by alistairsam View Post
Hi,

Is there a sweet spot or optimum range for image scales in a guiding system that provide the best autoguiding?
eg. many use the finder guider where at a 200mm FL and a camera like the qhy5, the image scale is 5.4 odd arcsec/pixel.
would it be beneficial to use a longer FL guidescope with the same camera and if so are there any guidelines or numbers that help determine the optimum guidescope FL?
There's some discussion of this and a calculator in the article I referenced: http://www.wilmslowastro.com/tips/autoguiding.htm

Cheers,
Rick.
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Old 17-01-2013, 04:44 PM
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Quote:
Originally Posted by gregbradley View Post
Everything is magnified including the tracking errors.

Greg.
thanks Greg. I understood that OK, but did not quite see how magnifying the distorted stars would make for a worse outcome - they would still be the same shape. As pointed out in other posts, its a perception issue - the stars are no different in shape for given tracking error, regardless of fl, but if they are bigger you notice distortion more.

Quote:
Originally Posted by alistairsam View Post
Hi,

Is there a sweet spot or optimum range for image scales in a guiding system that provide the best autoguiding?
eg. many use the finder guider where at a 200mm FL and a camera like the qhy5, the image scale is 5.4 odd arcsec/pixel.
would it be beneficial to use a longer FL guidescope with the same camera and if so are there any guidelines or numbers that help determine the optimum guidescope FL?
Craig Stark (phd guiding) suggests that his software is capable of at least 1/5 pixel resolution and much finer for high SNR. On that basis, if the angular pixel scale of the guider is less than about 2.5x that of the imager, you should be OK. http://www.stark-labs.com/help/blog/...-ask-craig.php (about the fourth article).

I use a QHY5 at 200mm (about 5.5 arc sec/pixel) to guide a QHY8 on an 800mm fl scope (about 2 arc sec/pixel). This is a track/image scale ratio of about 2.7 and it works fairly well if the seeing is OK. If any use to you, attached is a 10% crop cut from a lagoon image at native resolution - the tracking is definitely not perfect, but it is as good as I would expect from a 12kg package on an EQ6 - sub pixel tracking works pretty well. Have tried up to a 900mm fl guidescope - no benefit over 200mm and hard to find guide stars.

You indirectly make the point that we should be talking in terms of pixel scale, not focal length, when dealing with these issues - strongly agree.

regards Ray
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Last edited by Shiraz; 18-01-2013 at 11:24 AM.
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Old 18-01-2013, 06:15 AM
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Quote:
Originally Posted by alistairsam View Post
Hi,

Is there a sweet spot or optimum range for image scales in a guiding system that provide the best autoguiding?
eg. many use the finder guider where at a 200mm FL and a camera like the qhy5, the image scale is 5.4 odd arcsec/pixel.
would it be beneficial to use a longer FL guidescope with the same camera and if so are there any guidelines or numbers that help determine the optimum guidescope FL?

Have a look on the SBIG site. There is an article discussing this regarding the lens attachment available for the STi guider.

Short answer is short focal lengths for guide scopes work well as it makes it easy to get a guide star and software uses sub pixel accuracy in guiding.

Although AstroPhysics guide scope is about 900mm as I recall.

Greg.
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Old 18-01-2013, 06:19 AM
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Craig Stark (phd guiding) suggests that his software is capable of at least 1/5 pixel resolution and much finer for high SNR. On that basis, if the angular pixel scale of the guider is less than about 2.5x that of the imager, you should be OK.
regards Ray[/QUOTE]


That seems like a good rule of thumb.

At the end of the day though guide scopes are a thing of the past and give variable results due to flexure.

I'd treat them more as a last resort if you can't fit an off axis guider. OAG's performance leaves the best guide scope for dead.

Greg.
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Old 19-01-2013, 10:57 AM
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Hi Greg,
I agree -
you cannot beat an OAG because it guides at the native focal length & there is no differential flexure.
I have learnt this from bitter experience.

Use a short focal length guide scope only as a last resort
or when you're still learning.
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Old 19-01-2013, 01:17 PM
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Hi,
I've been thinking of testing a finder guider with a 1.25inch barlow? anyone done this?
theoretically, it changes the pixel scale from 5.6arcsec/pix to 2.8arcsec/pixel. (50mm guidescope with a qhy5)
the main scope pixel scale is 1.6arcsec/pix with a qhy8 and a 10inch F4.
I know the whole point of this thread was whether longer focal length guidescopes help or not. also that phd can do 1/5 sub pixel guiding.

but in testing, I've found that although my phd graph looked quite good, stars were elongated in RA.
it could be my mount not coping with the payload but I was expecting to see that in the phd graph, can't understand why it won't show
https://www.dropbox.com/s/jagwn011z280pgx/phd-2.png

hence the thought of using a barlow. will that work?
Thanks
edit: also came across this forum section that's dedicated to unguided imaging. there are some 10min unguided shots in there with round stars.
http://www.progressiveastroimaging.c...faf0837a6cea63

Last edited by alistairsam; 19-01-2013 at 01:28 PM.
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Old 19-01-2013, 05:11 PM
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Quote:
Originally Posted by alistairsam View Post
Hi,

I know the whole point of this thread was whether longer focal length guidescopes help or not.
yes it has sort of gone that way, but my basic question was whether going to a longer imaging scope required better tracking per se - the answer was "not necessarily" - don't need better tracking if the camera pixel scale is preserved.

I found that my HEQ5 was right on its comfort limit with an f4 200 and sometimes irregular tracking would produce RA extension of stars. EQ6 is a lot more reliable at 10+kg. Could also be due to differential flex/mirror slip. Doubt that a longer guide fl would help - easy to test with a Barlow though. As others point out, OAG is a better guide solution, but it is not necessarily easy to arrange at f4.

regards Ray

Last edited by Shiraz; 20-01-2013 at 07:38 AM.
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Old 21-01-2013, 01:43 PM
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Quote:
Originally Posted by Shiraz View Post
Doubt that a longer guide fl would help - easy to test with a Barlow though.

regards Ray
Hi,
would the barlow need to be at a specific distance from the qhy5 sensor?
Thanks

Last edited by alistairsam; 21-01-2013 at 02:39 PM.
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Old 21-01-2013, 04:38 PM
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no that's not critical at all. you will get more magnification as you extend the camera out from the flange of the Barlow, so maybe don't try more than a 2x device. I was amazed how much harder it was to find stars when they were much dimmer and I was looking at 1/4 as much sky.
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