| Is there enough improvement to justify? | |
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| Stay with guide scope | |
| Go for On Axis Guider | |
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I'm also using the 174MM with an OAG on a 9.25 SCT and always have at least a few stars to choose from/multi-star guide with, and that's with the ZWO OAG which has a smaller prism than the Celestron. FWIW. Cheers, Scott |
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| Then it's probably fair to say then that the camera sensor size offers the greatest improvement in number of stars in view. I think if you want to use an 8" plus SCT you need something like the 174 MM (mini), it is expensive but in the end will provide better results. I like to be able to carefully setup the OAG to be positioned in parallel with the long edge of the sensor. This is the position where you have the least chance of vignetting issues. Once set you need to have at least one suitable star in view and preferably more. With the QHY5 that I used initially I had situations where I had to rotate the OAG to find a star which also meant I had to reframe my image to keep the edge of the prism parallel with the long edge of the sensor. I really don't want to be doing this when I should be getting images. |
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I have a Stellarvue SVX080T-3SV on Skywatcher HEQ6-R controlled by ASIair pro. Initial ZWO guidescope was mounted on a plastic shoe mount... I reckon that the guidescope mounting would be where your issue was coming from, rather than the use of a guidescope versus OAG, or any improvement from multi star guiding. My experience of multi star guiding in PHD2 is that it gives an incremental improvement, not an order of magnitude. I also have an SVX80T-3SV and have guided it with two different OAG's, a guidescope and the inbuilt iGuider on my CEM70G mount. The first OAG was not rigid enough for the task and resulted in tilt problems, so I mounted the SV's predecessor (A Skywatcher Evostar 72mm) in very solid rings op top of the Stellarvue and guided with that while I was waiting for my new OAG to arrive (Given my plan was always to sell the Evostar once I bought the SVX) There was no effective difference between the Evostar as a guidescope, or the good OAG. Likewise the iGuider while the guide stats it produced were not as good as the OAG, produced images with no measurable difference at the image scale of the SVX80T. I still needed a guide solution for both scopes (Other than the iGuider) as the SVX80 now sits on my Orion Atlas (AZEQ6 clone) and the CEM70G carries a 10"F4 Newtonian, the image scale and potential for differential flexure with that scope really making an OAG the only practical way to go. Likewise, I used to image with an SCT and the long focal length plus potential for mirror movement as well as the usual sources of flexure made an OAG essential |
