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Old 28-01-2019, 08:23 PM
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The_bluester (Paul)
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External SCT focusser and corrector/reducer

Having dipped my toe in to imaging with the C925, I have naturally arrived at a bit of a quandry. From what little I have been able to do so far the scope seems to perform fairly well. My biggest issue to date is sorting out spacing between the .63 reducer I have been using and the camera, at the moment the spacing is too great and stars at the edge of the field are not great, though they are better than they were previously as the original spacers I had set the distance even greater and severe coma was visible at the edge of the field.

Now, this is an SCT, and that means mirror flop so I was considering adding an external focusser to the train to at least prevent that from happening during focus changes, not much helping it under gravity as the pointing changes through the night, it is vanilla C925 so it does not have mirror locks. The problem is that if I fit an external focusser by just screwing it to the rear of the corrector, I may not be able to get the spacing right any more, and even if I can, changing focus will change the spacing and so the reduction.

Is this a non issue (Provided I am able to get the spacing right to begin with) in that focus movements should be so small as not to materially change the reduction offered by the reducer/corrector?

Last edited by The_bluester; 29-01-2019 at 05:39 AM.
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Old 28-01-2019, 09:37 PM
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Merlin66 (Ken)
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Paul,
For the standard x0.63 reducer the spacing is 105mm.
May not allow a focuser to be fitted....
Run the focuser through its full travel a few times to re-spread the grease on the baffle - I found this worked well.
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Old 29-01-2019, 05:46 AM
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The_bluester (Paul)
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I run the focusser through it's travel fairly often and it works about as smoothly as you could expect, but mirror shift is still a problem if you have to change direction of travel, enough that if you are guiding you generally have to restart that as the guide star will most likely move outside the search box on the focus direction change.


I have seen mention of a Moonlite external crayford type that has provision to fit a reducer inside the focusser so it moves with the drawtube, retaining the correct spacing as you change focus. But I an wary about spending money on this scope that could easily add up to the cost of a reasonable fast astrograph newt.


I am having a hard time researching at the moment, NBN recently did "maintenance" on our fixed wireless tower, it is not an improvement. We have gone from having maybe 40% of the advertised speeds in peak time (That being when you know, people are at home and awake to use it) to about 5% and we are back in the dark ages of failed page loads and timeouts.
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Old 29-01-2019, 06:01 AM
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The_bluester (Paul)
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Managed a look at the moonlites this morning while the net performance was OK and there is more of a compromise than I want in them. The ones with a thread end on the draw tube instead of compression rings don’t come in the right thread size to attach to my scope. So it would already be looking at thread adapters to fit them to the back of the tube. And presumably to carry the reducer internally as well.
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Old 16-02-2019, 11:16 AM
Tony_ (Tony)
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To avoid mirror flop - make sure the final focus is "uphill" - this is counterclockwise with a celestron sct.
That way the mirror is already at its bottom position and won't "flop" downwards. Since I have been doing this I have never had an issue with mirror flop.
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Old 16-02-2019, 11:24 AM
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Merlin66 (Ken)
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Paul,
Interesting...
I agree with Tony Re final focusing....
In my case with the C11 and JMI focuser, using a "reflective slit guider" at f10
prime focus and PHD2, the focus change direction slop is minimal and PHD doesn't loose guiding....
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Old 16-02-2019, 12:22 PM
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I have always come to focus "Uphill" having picked that trick up shortly after I got this scope (I have had it the best part of ten years, time flies!) The Feathertouch reduced the issue a lot but it is still there.

The problem will be if I automate focus it is probably going to want to rock through focus both ways to pick the best focus bu FWHM or HFD methods, and refocussing as temperature falls I think will end up moving the mirror down as the OTA contracts.

I have done a little more hunting and it looks like the bigger Moonlite is close to the best option despite requiring an adapter to be made to fit it to 2" threads. The adjustability in how the reducer is fitted to it (That is missing from the 2" version) might allow for me to fit it along with my TOAG and still have a decent chance of getting the spacing right. But it it a pretty sizeable gamble about getting it all to hang together and achieve focus. By the time I built that (If it works) I would be a third of the way cost wise to a pretty sweet imaging Newt.
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Old 16-02-2019, 07:32 PM
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The_bluester (Paul)
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I have emailed Moonlite to find a bit more info about one of their focusers. The larger format SCT one (2.5") does not have an off the shelf flange for the 2" SCT thread, but it has an adjustable positioner for fitting a reducer inside it's drawtube that might give me range of adjustment to get it all together with my existing TOAG and camera spacers. I am currently using less than 105mm reducer to sensor distance, it gives more like 0.68 reduction but also suffers less coma and vignetting at the edge of the field.


The 2" version had the right flange available for my OTA, but the reducer threads on to the scope end of the drawtube, by the time I thread the TOAG on the end and then the camera with the spacers required to get guide and imaging cameras in focus together it will be more like 150mm spacing. More reduction, significantly bigger FOV but probably spectacular vignetting and coma.
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