Hi John,
I had been using a SV50mm guidescope with QHY5L-II mono autoguider very rigidity mounted using the Stellarvue non-adjustable clamshell fasted to the top dovetail of the imaging optic.? I recently switched to all ZWO to use the ASIAIR-PRO so I have no data from this equipment yet.
But a few years ago I was out in the eastern Sierra testing a classic Losmandy straight drivetrain 2-piece worm block G11 with a Gemini 1 I ran a couple of quick guided PE tests with my usual 50mm guidescope piggybacked and at prime focus of a M10" f/6.3 SCT OTA. The guiding results were a surprise to me because they were virtually inidentical. Both runs were well under <0.5" PE RMS, more typically centered on 0.4" RMS. Based on these results I see no reason use a large scope for guiding and the only reason to use an OAG as far as I can see is mirror shift.?
When I later returned for similar testing with the same mount now fully updated by Losmandy with tucked motors and spring loaded worms etc. the results were not surprising and again virtually identical. This tells me there is something else in the system that is limiting improvement of guided PE of this system. But with this level of autoguided PE I'm not going to lose any sleep over it.?
I did buy a newer smaller pixel ASI290MM-mini to see if the claims from ZWO about the smaller pixels producing improved autoguiding have any basis in fact. And to double check the math I also bought an ASI120MM-mini to use as a baseline because the QHY5L-II mono used a similar spec chip.
I think the old guidelines for guided scope to imaging scope have become obsolete due to the huge improvements in sensor QE and the jumps that PHD2 have made. This is evident even from my several years old data.?
John KmetzDec 14? ?
Hi Terry,
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A C925 Edge HD is what I have and usually I mount an Orion CT-80 on top as the guide scope. Please see pics. The CT-80 (FL-400mm) is not great for photography but is more than adequate for guiding. With a 0.7X FR, the C925 goes from 2350 to 1645 mm FL, so a 400mm guide scope fits into that thumb rule of guide to main scope FL ratio of 20-25%. It would probably be OK without the focal reducer too.
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But on top of the C925 I have a Losmandy DC 9.25 dovetail made specifically for that scope. And I also use the DVR 108/66 scope rings. This ring set has two to hold the guide scope, and another smaller one to hold onto the extensions for the guide camera, so you don't have to worry about flexure of the focuser rack causing guiding error when the mount changes position over the course of an evening. This was a nice idea the Losmandy came up with.
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Hope this info gives you some ideas.
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Regards,
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John
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Chip Louie Chief Daydreamer Imagination Hardware?
? ?Astropheric Weather Forecast - South Pasadena, CA?
Chip Louie