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I am wondering if there are some basic rules about the resolution and focal length of the guide scope relative to the imaging scope. In the old days of manual guiding, you always wanted to guide at a much longer EFL compared to the imaging scope but it seems that when using autoguiding with PHD2, it doesn't seem like it matters with guiding being done at a lower resolution and EFL. Is there something that PHD2 does that allows it to state guiding errors that are beyond the resolution of the guide scope? How is this done?
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I think that the general recommendation is that the guide scope focal length can’t be less than a tenth of the main telescope focal length
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Back in the days of film and manual guiding (which I did a fair bit of) the question wasn't the EFL of the guide scope but the magnification at which you guided.
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Hi Tony, the rule-of-thumb I remember for astrophotography is: guidescope_fl >= main_fl/3. But nowadays, with PHDs multi-star guiding, lower guidescope fls are possible. Currently I am using an RC10 telescope with 1600mm fl and a 200mm guidescope. This works well until seeing is below about 1.3arcsec. Then I begin to see that guiding in RA is slightly worse than in DEC: stars become elongated. PHD gives you the RMS guiding accuracy in RA and DEC. For my 200mm guiding scope on a CEM70 mount values are just now 0.33 and 0.25arcsec. This are just statistical values. Of course they can be lower then the guidescope resolution, because many images are accumulated to determine the RMS values. Limits for the RMS values achievable arise in my experience by:
The guide scope resolution most of the time does not play a direct role here, in the sense that you can guide more accurate: For my guide scope the resolution is 0.58arcsec. Instead, there is a general relation between focal length of a telescope, the pixel size of the sensor, and the seeing conditions: you neither want to have excessive oversampling nor undersampling. Here is a good web site about this topic: https://astronomy.tools/calculators/ccd_suitability Hope this helps Götz |
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That does help my thinking on this, I'll check out the link. Thanks!
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Many people get really hung up on guiding figures and with no need too, you hear people say “my mount is great it guides t 0.3”/pixel” well who cares when they are imaging at 1.5”/pixel and there seeing at best is about the same, then there is no need to have your guiding any better than this, as it’s seeing or scope limited anyway…so in this instance guiding figures of 1.0 RMS is more than good enough, in fact it’s great…. To answer the original question simply, just keep your guiding image scale no more than 4 times your imaging pixel scale and don’t get to hung up on the RMS figures, as most of the time they will be way better than your seeing anyway….👍🏻 |
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It's not so much a matter of focal length as pixel arc/sec/pix ratio of the guide camera to imaging camera. I read years ago that anything below 1 to 10 was fine. The more current thinking ia a ratio of 1 to 5. I like to be at 1 to 3. The lower number being the imaging camera. Therefor a guide star can move 3 pixels on the guide chip to 1 pixel on the imaging chip. Lynn K. |
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I did some experiments with my 10" f/12 mak imaging at 3000mm, with piggyback guide scopes. The main scope was in rings and the guide scopes also were mounted in rings on Losmandy dovetails so no flexure. None, zip, nada. Guide camera was an ASI 533MC Pro (yes, overkill I know, but hey, it is a great guide camera). Mount is a SW CQ350, controlled by ASIAir Plus. With a small guider (typical achromat, 50mm aperture 250mm focal length) ASIAir reported it as guiding around 0.15 arcsec but the real guide accuracy as revealed by the FWHM of stars was consistently around 1.5-2, sometimes 3 arcsec. Quite fat. Not useful for a scope that can resolve 0.4 arcsec and less. All it means is that the guidescope is so small it isn't resolving the errors or the seeing. With larger guidescope - a TS Photoline 70mm quad APO with a focal length of 475mm - visually this a really, really nice little scope - guiding was consistently around 1 arcsec in average seeing. Now… plug in the ASI2600MC DUO on the main scope - guiding at 3000mm… it's a whole different ballgame. Most nights it chases the seeing around 0.5 arcsec, but if the seeing settles I have seen sustained long-term guiding at 0.2 arcsec. This is a whole different ballgame when you have a scope with the resolution to take advantage of that. So I would have to say the old rule-of-thimb that you need a guidescope not less than ⅓ the focal length of the primary scope is quite likely to be valid - assuming your primary scope has good optics. Most SCT's however show "fat stars" anyway, so the little guide scopes from ZWO will work well enough on SCT's upto a C11. |
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AstroShed: The only problem with this line of thinking is that it isn’t just the RMS value, it is also the outlier short lived excursions that don’t really change the RMS value much but will certainly contribute to star eccentricity. in general, a higher RMS value will also mean larger excursions well above the RMS value and it is these that destroy your star shape. A guiding RMS of 1" does not mean a circle of diameter 1" with nothing outside it. It means a set of points which, on average, deviate from the mean by about 1". Note that the same can be said of seeing variation, but seeing variations are random and average out over time scales much shorter than our exposures. However, that is not true of guiding variations, and this is why poor guiding leads to poor star shapes. |
