SCT progress: collimation, visual, reducer

After a discouraging evening the other night, trying to do too many things at the same time, last night was more organized and more successful.

I took the SCT back to pure visual configuration and did a careful collimation using an out-of-focus star. More tuning later, using software help, but for now the collimation is pretty good.

Next, I tried a more methodical set of experiments with focal reducers. I couldn’t get the OpTec 0.5x reducer to work in any configuration that occured to me – only in a couple could I bring it to focus, and quality wasn’t good. I’ll try it more later but, for last night, I switched to the Celestron 0.63x reducer. It needs about 100mm distance to CCD so I had to mount the reducer outside the Crayford focuser, which meant a couple of threaded adaptors. That worked, and I was able to take test images and achieve focus. The unusual placement of the reducer means I’m not getting 0.63x reduction or f/6 results, but I haven’t done the math yet to figure out what reduction is happening. (Update: doing a plate solve on an image, this setup is yielding an effective reduction of 0.68x, for a resolution of 0.83 arc seconds per pixel.)

M51 Unguided

M51 Unguided

I took an unguided test image of M51 to get a sense of what this resolution will do. It’s quite a pleasing image scale – once guiding is working it should be good. I was surprised at the rather steep image vignetting that was happening in this setup – substantial flat correction was needed.

I could not get my flip-mirror working in the imaging chain with the focal reducer – not enough in-focus, so I have set that aside for now and may find a use for it, or a way to configure it, later. Unfortunately this left me with no guider capability as I was hoping to do off-axis guiding instead of a guide scope. I have two other off-axis guiders. The one I could find doesn’t work in this configuration since its low-profile case bumps the motor housing on the filter wheel. The other one – an Orion OAG – should work OK, but I didn’t have it handy so didn’t try it.

So, progress is showing. Next clear night I’ll put the OAG in the imaging train, tune up the mount balance, and start recalibrating the pointing model. The cable runs will also need re-work, since the distances to the focuser and filter wheel have changed substantially. I think I’ll do a new Periodic Error Correction calibration too, since it’s been a long time, and a big temperature change, since the last time that was done. About 2 or 3 clear nights from now we should be back on the air for imaging at approximately 1400mm f/6, a very different experience than the wide-field refractor has been.

As a final fall-back, I have a smaller (8″) OTA on hand, and may switch over to that for a bit less focal length and a bit more focus distance if I can’t get the 9.25 working well.

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