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I’m trying to use a 1980s era C14 which has been fitted with a Moonlite focuser and could not get my CCD to focus (or even show any out of focus stars).
When I measured where the Moon image is briought to focus, I found that it was some 3-4 inches beyond the furthest outward position of the focuser – way more than the backfocus of my CCD.
While I am content to add some extension tubes, I wondered if this was because the mirror had been placed in its furthest forward position prior to being locked in place. If that was the case I might be able to wind it back a bit and the focus moved to within the range of the electronic focuser.
I just wanted to check before messing around. On a 1980s era C14, if I turn the focuser in a clockwise direction, does that move the mirror toward the secondary mirror/corrector plate or away from it?
Secondly, has anyone an idea how far the mirror can be moved to achieve focus?
All responses very welcome…
One of the advantages of an SCT is that you can move the focus in and out by a lot through small shifts of the primary mirror. You should be able to solve your problem by releasing the mirror lock and turning the focus knob. Given that this is an old C14 any mirror lock will have been added afterwards so you will need to work out how to release it.
Anticlockwise rotation of the focus knob (as seen from the back) moves the focal point away from the telescope (at least in all the SCTs I have) but you can check how the focal plane is moving by looking at your projected moon image. It is a characteristic of this method of focussing that the effective focal length changes a bit as you shift the primary back and forth. Don’t worry about the mirror falling off the central baffle. The focus knob will stop moving at the end of its travel.
Once you’ve got a decent back focus distance you can lock the mirror again and use your Moonlite focusser for fine adjustments.
Thanks for replying Nick. Thanks for the info on the direction of rotation.
There is a focus knob replacement + two clampable rods at 120 deg intervals from the knob round the main axis. I wound the focus knob fully clockwise and got a distance from scope body to an image of the Moon on a piece of card of approximately 310mm. Then I wound it fully anticlockwise and the focal point went out to more than 600mm – the dome wall got in the way of precise determination.
At best then the focal point is 310mm beyond the back of rhe scope body – which is approximately twice what I have heard reported on Cloudy Nights before. I’m not using a diagonal, so it looks like a long way – does that sound right to you?
I’m going to take the Moonlite to pieces in the light tomorrow as I’m beginning to wonder if theres a Barlow lens or something in there rather than a focal reducer. Good for planets…
Pic of the scope focuser when it was indoors attached.
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That seems very wrong. I have used a low-profile focusser on my 11-inch conventional SCT with the camera sensor only around 80mm from the rear of the scope. In fact, I’ve used planetary cameras directly on the back of them without an external focusser, probably less than 40mm from the back.
Maybe the 14-inch is different but I doubt it. Where did this one come from?
Yeah, I’m confused/concerned, but having never owned an SCT before, I assumed that I was being dumb and missing something obvious.
The OTA previously belonged to Ron Arbour. He said he had obtained after it was stored in a garage that flooded – I think that is what was said – it was a long time ago and in passing, as we were actually talking about some automated image alignment software I had written for him.
He clearly did a nice job as the mirror is pristine and the inside of the tube looks good too. The focuser works fine as well. I don’t recall seeing another C14 there, but there might have been.
Will dismantle further this afternoon and am really hoping I find a lens in there…
Hi Grant,
As Nick says, the beauty of the SCT is that you can make huge changes in focus with the knob that moves the primary mirror. You need to set the Moonlite so it is in the middle of it’s travel, then go to a nice bright star and focus with the main mirror knob (you might need to loosen the mirror lock knobs if you have them). Then you can re-lock the primary (if applicable) then you can use the Moonlite from then on. Note that the star might be hugely out of focus which van be confusing. If in doubt, I use the Moon, or distant objects in the day.
Cheers
Ian.Hi Ian,
Yeah, thats roughly what I hoped to do, but I found that when I hung the CCD on the focuser, even when the focuser is set at its furthest from the OTA, the Moon comes to focus way beyond the CCD – hence the need for lots of adaptor tubes. 🙂
Inevitably, I didn’t find this out until I had run the focuser across its full 3″ range in 1/8″ jumps looking for star images at each position. I was so convinced it should work that I did it twice with different exposures. Saw a couple of bright stars travel through – while filling 25% of the screen – none anywhere near in focus.
So, that was 2 clear nights wasted as at -4C I wasn’t sitting out there for more than 2 hours at a time.
More after I have a look inside the Moonlite…
EDIT: No glassware found in the Moonlite. Now wondering why the focus knob was modified…
Grant
It sounds as if something is very broken in that C14 if you can’t move the focus to within 30cm of the back plate. Ron was good at taking things apart and generally good at putting them back together again. Maybe he got something wrong with this one.
I looked at the focusing knob on my C14 and, also, pictures of older C14s and noticed that only mine has a thin long bolt coming out of the top of them and, also, a wider addition. The layer of cork between the addition and what may be the original is suggestive – for locking?
I’m wondering if the bolt stops the focus knob being wound fully clockwise. Will have a look tomorrow – I need to rebalance the scope anyway and the weather should be good.
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This reply was modified 16 hours, 32 minutes ago by .
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This reply was modified 16 hours, 32 minutes ago by .
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