Celestar 8 focus - options

[sharptrack2 (Kevin)]

G'day all,

I have observed some behavior in the focus of my 8" SCT that I would like to get some feedback on.

I was observing Jupiter last week, wanting to try and compare eye pieces (see if one had better contrast than another). My views have been fuzzy since day one and I would like to isolate EP's, versus collimation, versus equipment fundamental capability. Having looked around the web at photos and descriptions people have posted, I've decided that something is not quite right. But that topic is not what I'm interested in at the moment.

While doing the testing between EP's, I found the the focuser seems to have a "dead" spot, where nothing happens for almost a half turn. Research gave up the design of the focusing mechanism and I have developed a hypothesis. Given that for most of its life, the scope would have been used for observations, and low to mid range magnification EP's would have been used, is it plausible that the lead screw is worn very badly at a certain spot simply because the focus always ends up in the same 2-3 thread area?

I don't notice a "step" or "focus shift" when I run the focus from one extreme to another. It seems to only be noticeable when I attempt to reverse focus, right at the optimum focus point. Before everyone starts with the obvious, I don't think that it is simply wear in the barrel that the lead screw threads into. This behavior seems to only occur at sky focus and not at any other spot in the total travel length. I have confirmed in daylight, with closer objects (2-3 kms away), that the focus mechanism is smooth in both directions through focus.

This all brings me my question, is it common to install a micro-focuser, or 10:1 focuser on this type of telescope. A micro-focuser would probably not be the ultimate way to go, but could buy me some time to save for a motorised focuser.

Am I correct in concluding that the addition of a focus mechanism at the visual back would allow me to leave the mirror adjustment at an arbitrary point, close to focus and still achieve good focus across a range of EP's?

Thanks in advance!

[iborg (Philip)]

Hi

I think I have observed the same thing. I think it is due to the focusing mechanism moving from 'pushing' the mirror to 'pulling' it. There would be a lag as the mirror is not 'hard locked' to the thread.

I tend to go past good focus come back, then go forward again.

It does make sense to me that a fine focus at the visual back would largely elimanate this shift.

Philip

[sharptrack2 (Kevin)]

Thanks Phillip,

Comforting to hear that other SCT's seem to have the same behavior.

I would agree with you if I could replicate the behavior at all points on the focuser travel. There are two points of contact where that reasoning holds true, the barrel that the lead screw turns through and the joint at the mirror carriage. If the joint or barrel was the weak point, the behavior would be observed at all points of focus.

I'm going to have a go at trying to test multiple points in a 10 turn range around the prime focus point. Need to fabricate a spacer to extend the EP out another 10mm or so. Not sure yet how I will go about shortening the focal plane, but I may not need to. We'll see what the results are of my first test.

Just have to wait out this weather front passing through .

[louie_the_fly (Stew)]

I put a 10:1 focuser on my Celestar 8 and was happy with the overall results. The standard focusing mechanism is a bit iffy to begin with I think. Seems to be quite a bit of backlash in it. It would be good to be able to lock the mirror once it's in focus. I doubt that the thread is worn as it really doesn't do much work, as such.

[sharptrack2 (Kevin)]

Thanks Stewart,

That's encouraging news. It seems pretty straight forward. It looks like JMI makes a compatible motorized focuser that fits into the visual back, which looks like a good investment. My only concern is that it might reduce my apparent FoV, even more than it already is, if not implemented correctly, which I have to assume means I will need the mirror carriage to remain movable.

The penny pinching effort is now underway... no more Friday lunch outings for a couple of months.