Added f/6.3 reducer and focus is WAY off

[sharptrack2]

OK,

Celestron Celestar 8 SCT with Moonlight CS focuser (w/ DC servo). I haven't searched the forum yet so please forgive me if this has come up before.

Recently picked up a Celestron f/6.3 reducer on sale and I'm reasonably sure that I put it in correctly, but the scope will not focus. I've moved the mirror as far forward as it will go and the focuser still runs out of travel. It still wants to go "in".

According to the documentation, there should have only been 3-5 turns on the mirror to get back to focus. I've also tried installing the reducer inside the focuser, as the flange I have does accommodate it, but severely limits focus tube travel and still will not focus.

So am I missing something? Is the focuser the wrong model for doing this? Moonlight does have a specific setup recommended but it seems that I should be able to get there with what I have.

[glend]

Well Kevin i have no experience with that reducer, but the one i use requires spacing to the camera. My reducer needs to have 55mm of spacer ring area to the camera sensor. A quick internet search shows that reducer does need correct spacing to the camera,. Are you using a t-adaptor behind it? Most reducer/correctirs or flatteners require spacing of some sort.

[sharptrack2]

Haven't connected camera yet, was just checking visual to start with. The magnitude of de-focus seems to indicate that it will still not reach focus. All indications are that I need less distance to the focus point.

I've sent an email to Ron at Moonlight as well since he would be familiar with the telescope and focusing solution.

[glend]

Sensor or eye, correct spacing is still required. Just search for Celestron f6.3 reducer, plenty to read about use with a C8. It will not work with an Edge model.

[ChrisV]

Have you got the focuser with the large flange that holds the reducer ? I had one of these on a C8 (last year) and it was fine when i viewed with a 1/3" camera.

Spacing from reducer to sensor was ~80mm to get it down to f6.3. If you can reduce that spacing you'll get less focal reduction but won't need as much forward focus.

Can you get it to focus without the moonlite ?

[Merlin66]

The original x0.63 reducer required a spacing of close to 110mm between the rear of the knurled ring to the CCD chip.
This was achieved when a visual adaptor and a DSLR was fitted to the rear.

[sharptrack2]

Admittedly, I didn't spend much time investigating the issue.

Feedback from Ron at Moonlight suggests that I have the wrong perception of how I might go about using the lens. I had it in my mind that I would install it and leave it, which is actually not recommended.

I still had my 2" diagonal in the path which is probably why the system kept wanting to be forward focused. Having done a bit more research into how the optical path is actually changing, I now understand better what is happening. ;)

If I want to do any imaging, which was the intent to begin with, I can either make do with the flange I have on the focuser and have very limited travel, using the original mirror adjust for coarse focus, or purchase the proper flange and have more options available. For visual I can simply go back to the original configuration and maybe put a stepper motor on the mirror adjust to get remote control.

Thinking through it all, I should go ahead and purchase the flange, install the reducer in one of the two Moonlight focusers I have, and treat them as one complete assembly dedicated to a specific purpose... :question:

[ChrisV]

I used it with the big flange for video astro -then focusing the moonlite didn't affect the FR-camera distance and the amount of focal reduction.

Forget the diagonal. On top of the moonlite the FR-camera distance will probably be too long.

I really liked the moonlite but sold it in the end as i couldn't get it to work with a meade f3.3. Couldn't get it to focus. Just used a little celestron motor focuser on the C8's focuser knob. Was good enough for a hack like me.

[sharptrack2]

I do have the option of keeping the reducer out of the focal train, which accomplishes the same as what you did, without investing in another flange. :thumbsup:

I think my biggest problem was that I kept the diagonal in the optical path, expecting some clearing tonight and will be trying it again without the diagonal and just an EP. Just want to see the effect without connecting up the camera and computer. :P

Getting another flange and setting up each moonlight focuser for a specific purpose is sounding quite attractive. One will be imaging only and the other visual only. I should be able to calibrate the mirror travel, in # of turns of the adjustment for each setup, and each Moonlight should be able to be locked down and be in focus (or at least close) with minimal effort. :D

[Camelopardalis]

As Ken said, with the reducer in the optical path the back focus is reduced before you consider how much the Moonlite is consuming. Then you're trying to use a 2" diagonal which is pushing the eyepiece out even further...try it with a 1.25" diagonal and see how you go.

[sharptrack2]

Thanks Dunk,

I've done some testing and found that no matter where the reducer is inserted, at the visual back or on the focal tube, focus can only be reached by moving the mirror as far forward as possible and mounting the camera or EP as close as possible to the reducer.

There is a disconnect between what Celestron says about the behavior of the reducer in the telescope and what actually happens. When the reducer is mounted in the focal tube of the Moonlight and a camera installed with the shortest adapter I have, which inserts completely to the camera body, I can just reach focus with the mirror at its uppermost travel (maximum counterclockwise turns on SCT focus). The distance to the sensor would be very close to 110 mm as Merlin suggests, but the mirror location is at odds with what Celestron says "should" happen.

I've decided to do some more research and testing to understand exactly what is happening. Good learning experience, I'll have to actually study the mechanics of setting up an imaging rig, not just read the words. :)

[glend]

Celestron maybe telling you what you should see using a stock focuser. Moonlights may have different length draw tubes and setback (profile) distances racked in and out compared to the original focuser. Do you have the Regular or Tall Profile focuser? Was the scope ever used for imaging by the previous owner with the Moonlight on it?

[Camelopardalis]

Yes, external focusers usually steal some back focus.

My Celestron f/6.3 reducer claims back focus to be 105mm. This is less than the optical path of most/all 2" diagonals.

[multiweb]

I've used a reducer with my C11 once and I believe I had it screwed directly into the primary cell, then the focuser came after. I don't recall any issues with backfocus. Then again it was a while ago. Will have to check. SCTs are usually very generous with back focus. I'm surprised you've hit the limit.

PS: Have you tried putting the reducer at the backfocus then grab an eye piece, look into it and go back and forth until you get focus? This will tell you where you need to be with spacers. No need to move the primary as you will find focus by moving out further. Once you've got it ball park then get closer to the back cell then find a good spot for the primary. Small travel of the primary translates to huge changes in back focus so you should be able to find it IMHO.

[Merlin66]

I regularly have used the original Celestron x0.63 reducer on the LX200 SCT's (8", 10" and 12") and both the C9.25 and the C11....with a camera or spectroscope.
I also have a spreadsheet I prepared a few years ago to calculate the changes due to the main mirror shift etc. Note this only applies to the pre-AFC SCT designs and also note the current MEADE reducer has a much shorter focal length and back focus.

[sharptrack2]

Ok...

One at a time...



Glen,

Yes, as far as I know, stock Moonlight CS focuser for SCT, with DC servo motor. Measured drawtube at 78mm long with 25mm travel, housing with flange mount is 64mm and current flange is 18mm. It is a short version, I didn't think I would ever image through the SCT so opted for the visual configuration. To my knowledge, Alex (mental4astro), did not image through this scope.




"some" being the operative word... not "all of it"! :screwy:




Marc,

I installed the reducer on the backfocus but then attached the Moonlight and used a 15mm EP. The focuser had to be racked all the way in and the mirror moved as far forward as it would go and I was still a few mm away from focus. Put the reducer inside the focuser as it should be mounted according to Moonlight and I could just get focus. I'll have to try with just an EP and nothing else and see what I see...;):lol:





Ken,

This is the confusing part, so many people have used this reducer in so many configurations, I don't understand why it doesn't work in mine. I must be missing something very basic and making a fool of myself. This is a late 1990's, early 2000's black tube Celestar 8, originally on a wedge mount. I can't imagine the design being too much different from the Meade's of that era. Being smallish might be something to consider, I note that the Meade's have a larger backfocus attachment point and therefore sticks out a bit more, but in my case that would be worse according to what I'm observing. :shrug:



I'm ordering the tall flange just to have it as part of my focuser collection. If I end up selling one it will be easier to find a buyer. I don't think I'll do much more with trying to configure the SCT for imaging, I've got a Bresser 127 f/5 arriving in the next few days. I hope that will become my astro-camera setup.

But that doesn't mean I'm giving up! :rofl: I still want to find out why this is being such a PITA!:P;):thumbsup:

[sharptrack2]

Oh, and :thanx: for the spreadsheet Ken! I'll be referring to that as I work this out, great reference to have.

[Merlin66]

Kevin,
Try the following set-ups to verify there are no other issues....
1. Remove the rear focuser, add a eyepiece adaptor (and a 40mm spacer if you have one) - see if distant objects come into focus.
1. Remove the rear focuser, add the reducer, add a eyepiece adaptor (and a 40mm spacer if you have one) - see if distant objects come into focus.
Let me know.

[glend]

Kevin, just reading through the Moonlight notes on the SCT focuser it says the Tall adaptor should be used for imaging, and that the reducer should be screwed into the tube (scope side). This seems to imply that the reducer (outward and inward range) might be further out using a Tall adaptor than the short one.

[sharptrack2]

Thanks Ken,

Will try to test that out before the weekend... might have a shot at it tomorrow evening after work. I don't have much in the way of spacers, but I'm sure I can make up something in a pinch.




Glen,

Yes, I noted that as well. I don't expect it to fix this issue but will make the focuser easier to sell later on.



A random thought crossed my mind, could it be possible that the lens is in backwards in the housing? Not sure how you would tell looking through it. :question:

[Merlin66]

You can check the reducer by focusing the sun on a piece of paper and measuring the focal length - it should be close to 240mm.

[sharptrack2]

Is it correct that with the most common design used, that focus would be achieved at that length in only one direction? In other words, the reducers are not typically simple positive lens and behave like a typical refractor objective lens? Referencing this explanation... https://starizona.com/acb/basics/equip_optics101_optical.aspx

[Merlin66]

Kevin,
Generally you are correct.
Just measure what you have before worrying.

[sharptrack2]

Reducer confirmed, approximately 240mm focal length. Didn't have the sun to work with but putting my eye right up to the glass and moving the lens up and down a ruler while focusing on an Anzac biscuit worked as well, and I got a treat afterwards. :rofl:

Scope check tomorrow definitely, don't have time tonight to try. I'm kicking myself for not getting the mount out right when I got home... looking like a reasonable evening to have done some more testing or observing.

One of my T adaptors is roughly 40mm long. I'll try to finagle something with that to test with an EP. My other T adaptor is about 60mm, I might be able to fit one of my Meade HD EPs in such a way as to have the first element near 40mm distance from the reducer.

[redbeard]

Hi,

I have a Meade 10" SCT and I also have no issues with focus using the 6.3 FR.

I use a Moonlite focuser with the large flange that screws directly to the back of the scope, it has the .95 draw tube and the focal reducer screws inside this and only screws on one way so you can't get that wrong.

I have heaps of travel in both ways in relation to focus from the telescope itself. Then I fine adjust with the Moonlite.

I have also used the small flange with a 2" diagonal and the focal reducer this time was put after the diagonal and before the camera. Again, no issues with focus.

One thing mentioned is the distance from the camera to the FR. On my scope, the distance is about 110mm but the only thing the distance is for, is to how close to 6.3 it is, It does not have an affect on focus from my experience in this situation.

Cheers

[sharptrack2]

Good evening,

Amazing what can be accomplished when you start with a plan and take your time. :whistle:

Obvious human error in this one. Don't know where I erred but it had to be me. :shrug::screwy:

The moon was my target and the test setup was the original Celestron visual back, with a spacer/adaptor that came with my Bintel 2" diagonal. Not really sure why it was there, I think it was to connect the diagonal directly to the focus back. Anyway, its about 30mm long, and a 32mm EP was used to check where the focus point of the mirror was. Which was about 10 turns clockwise from the stop (fully counter clockwise). See photo #1.

Inserted the reducer into the path, at the focus back. Focus reached at approximately 8 t (CW from CCW stop). See photo #2

Connected Moonlight focuser directly to focus back, focus tube racked all the way forward. Mirror focus was achieved at ~8 t CW. Racked the focus tube all the way back and the mirror movement was approximately 1 t CCW. See photo #3

Installed the reducer in the focus tube, focuser racked all the way out and short flange installed. Mirror focus achieved at ~6-7 t CW.

I did look through the scope without an EP and managed to get focus with my eye at roughly 5 cm away from the EP holder (visual back), but I didn't count the number of turns. However I can say anecdotally, it was about the same number of turns as with the EP +/- 1 t.

So all appears to be working correctly so far. I ran out of time so haven't captured the number of turns with focuser and diagonal. I'm expecting it to be considerably higher, between 15-20. Hopefully I'll get that done tomorrow evening after work. Then on to the camera and making measurements to see what I need to make it work, hopefully nothing as it should work. Still going to order the proper flange to use with the reducer, might make a difference although I don't readily see how it could.

:thanx: for everyone's input. I'll put this one to bed but will update when I get the flange and test with the diagonal.

[Merlin66]

All's well that ends well.
Glad you got it sorted out.