Orion Low Profile Focuser installation Issue XT8i

I recently bought an Orion Dual Speed Low Profile focuser for my XT8i dob. It is the discontinued one (product #13030), however, per Orion's website, it is a drop-in replacement on all 8"/10"/12" Orion dobs. I tried installing it on my XT8i but the holes do not line up. I can get the top two or bottom two but not all 4. With some difficulty I was able to get the top right and bottom left screws screwed in at the same time. I tightened them down, nice and tight, but cannot get the remaining screws in their respective holes with this improvised setup.

My questions: Is there any risk to the OTA with the focuser only being secured by 2 screws? Is it placing undue stress on the tube? Is it possible that the screws get stripped over time after loading up the focuser with heavy EP after heavy EP? Basically--is this a bad idea? The focuser seems solid enough on the tube and there is not even a hint of play though just secured with the two screws.

I would use the 2-screw solution for a *short* while.... test it under the stars one night to ensure squareness, smoothness of operation and load-carrying ability. If it all meets your expectations, drill 2 new holes in the tube so you can use all 4 screws.

I know this an issue with the focuser itself and has nothing to with the installation problem I menionted, but I had the scope out in the backyard to test out the new low profile focuser and couldn't reach focus with my Pan 35 or my Pentax XL 21. IIRC, Pan needed more out-focus and the Pentax more in-focus. Bummer. I tried both with extension tubes and then the situations were reversed.

At least the 13 Ethos reached focus and I just kept that in there for the rest of the short outing. I think I might have to revert to the stock 2" focuser.

This isn't the first time I've heard of low profile focuser problems with dobs not coming into focus. Beginning to wonder if they shouldn't tell people about it.

What's the benefit of a low profile focuser? I too have an XT8i so I'm curious.

Guess the fact that its lower. Me, Give me all the focus length I can get.

I entered this hobby too recently to know for sure but from pictures I've seen I've concluded that the XT8 and 8" f/4.9 OTAs were originally equipped with R/P focusers. Presumeably this places the focal plane a long way outside the tube, and out of reach for a low profile focuser mounted directly to the OTA.

Such was the case with my Orion 8" f/4.9 when I replaced the stock crayford focuser [which has an integral drawtube extension (25 mm) as part of the 2" eyepiece holder] with a MoonLite CR2. Both the 1" and ½" spacers from the installation kit were necessary to raise the MoonLite high enough for the drawtube travel to bracket the focused positions of my eyepieces.

A nice side effect of this was that the drawtube no longer extended into the light path, or even inside the tube for that matter.

I haven't checked in a long time but when last I did the now stock single rate crayford was available as a replacement for the original R/P, and the extended eyepiece holder I mentioned above was available as a separate item. I have a vague recollection of a note regarding the circumstances that necessitated the use of that eyepiece holder.

What's the benefit of a low profile focuser? I too have an XT8i so I'm curious.

Hi:

I will try to give the short story on Low Profile Focusers.

If one is designing a Newtonian, the benefit of a low profile focuser is that the focal plane (the place where an eyepiece is in focus) can be closer to the secondary mirror which allows the use of a smaller secondary mirror, this results in a small but worthwhile improvement in contrast.

If one is replacing a focuser, the position of the focal plane does not move unless the primary mirror is moved so that it is further from the secondary. If one is not willing to go to the trouble of lengthening the tube and moving the mirror, then normally one would use a focuser of the same height as the original as there is no advantage to the low profile focuser. In this case, the fact that the 35mm Panoptic no longer focuses is the result of using a focuser that was too low.

Of course it the original focuser is too tall, then a lower profile focuser could fix that but I don't think that was the case here.

Also, generally low profile focusers have shorter drawtube and therefore a shorter range of travel.

Jon

What's the benefit of a low profile focuser? I too have an XT8i so I'm curious.

The Orion Low Profile focuser is very well built and an impressive piece of equipment compared to the stock offering, IMO. The main benefits of the low profile focuser for me were the 11:1 dual speed focusing ability and the compression rings to secure both 2" and 1.25" eyepieces.

If it were me I'd return that focuser for a GSO 2-speed if you're on a budget; IMO a much better focuser than the Orion. And if you order from Scopestuff, they'll even fit it to a standard Orion flange. If you're not on a budget, then get a Moonlite. Either way I'd send the Orion back.

Orion does mention focuser travel problems in the comments when you click on the single-speed Crayford focuser. I couldn't get my 35 Pan to reach focus without "cheating" it, leaving it about 3/8" above the end of the tube. I bought the $13 extension and solved the problem. All of my EPs (Pans and T6s) now reach focus. The killer to me in your situation is not being able to go in far enough for your Pentax. Had this Pentax dilemma not occurred, I'd have said to buy the extension ring and drill two new holes (or enlarge all 4 original holes if they were all close). The alignment is the most important thing. Now I think you have to go with the more expensive focusers after getting assurances from their tech people that their product will work for you.

Orion does mention focuser travel problems in the comments when you click on the single-speed Crayford focuser. I couldn't get my 35 Pan to reach focus without "cheating" it, leaving it about 3/8" above the end of the tube. I bought the $13 extension and solved the problem. All of my EPs (Pans and T6s) now reach focus. The killer to me in your situation is not being able to go in far enough for your Pentax. Had this Pentax dilemma not occurred, I'd have said to buy the extension ring and drill two new holes (or enlarge all 4 original holes if they were all close). The alignment is the most important thing. Now I think you have to go with the more expensive focusers after getting assurances from their tech people that their product will work for you.

The more I think about it, I think I was mistaken when I said the Pentax did not have enough in-focus. It must have been lacking out-focus just like the Pan 35. I have a cheap 2" extension tube that I used with the Pan, but that gave me too much length and then I didn't have enough in-focus. For the Pentax XL 21, I used the Shorty Barlow with the bottom element removed, but then had the same situation as I did with the Pan 35. Frustrating, especially when this problem nearly squandered a nice, clear evening!

Orion does mention focuser travel problems in the comments when you click on the single-speed Crayford focuser. I couldn't get my 35 Pan to reach focus without "cheating" it, leaving it about 3/8" above the end of the tube. I bought the $13 extension and solved the problem. All of my EPs (Pans and T6s) now reach focus. The killer to me in your situation is not being able to go in far enough for your Pentax. Had this Pentax dilemma not occurred, I'd have said to buy the extension ring and drill two new holes (or enlarge all 4 original holes if they were all close). The alignment is the most important thing. Now I think you have to go with the more expensive focusers after getting assurances from their tech people that their product will work for you.

The more I think about it, I think I was mistaken when I said the Pentax did not have enough in-focus. It must have been lacking out-focus just like the Pan 35. I have a cheap 2" extension tube that I used with the Pan, but that gave me too much length and then I didn't have enough in-focus. For the Pentax XL 21, I used the Shorty Barlow with the bottom element removed, but then had the same situation as I did with the Pan 35. Frustrating, especially when this problem nearly squandered a nice, clear evening!

I have to do the same thing with my 13mm Nagler. I have the single speed crayford. I either have to leave about 1/2" of the barrel above the focuser, or use my 1.25" adapter, which comes into focus barely before it reaches its lowest point. Where did you get an extension tube?

Orion does mention focuser travel problems in the comments when you click on the single-speed Crayford focuser. I couldn't get my 35 Pan to reach focus without "cheating" it, leaving it about 3/8" above the end of the tube. I bought the $13 extension and solved the problem. All of my EPs (Pans and T6s) now reach focus. The killer to me in your situation is not being able to go in far enough for your Pentax. Had this Pentax dilemma not occurred, I'd have said to buy the extension ring and drill two new holes (or enlarge all 4 original holes if they were all close). The alignment is the most important thing. Now I think you have to go with the more expensive focusers after getting assurances from their tech people that their product will work for you.

The more I think about it, I think I was mistaken when I said the Pentax did not have enough in-focus. It must have been lacking out-focus just like the Pan 35. I have a cheap 2" extension tube that I used with the Pan, but that gave me too much length and then I didn't have enough in-focus. For the Pentax XL 21, I used the Shorty Barlow with the bottom element removed, but then had the same situation as I did with the Pan 35. Frustrating, especially when this problem nearly squandered a nice, clear evening!

That makes better sense! If the Pentax did not have enough in focus on the low profile focuser, it certainly wouldn't have had enough in focus on a taller focuser.

dan

I don't know who you're asking about which extension. I got mine from Orion. It appears on screen when you click for more info on the single-speed Crayford focuser.

An update:

Well... I will certainly have egg on my face after this revelation... I didn't realize that there was an extra screw/compression ring on this adapter. This one, when loosened, lets you extend the extension tube! This extension tube will surely fix the lack of out-focus travel problems I was having. See picture below of unextended/extended tube.

The problem remains that only 2 screws will only fit into the pre-drilled holes in the OTA. Now I know a couple of you mentioned drilling new holes/making the holes larger... I am all thumbs over here, but wouldn't making the holes larger cause the focuser to move around?? I need something solid to handle this kind of moment arm on the focuser (worst case scenario as my Pan 35 is my heaviest EP):

The problem with the extension is that unless you pull it all the way out it will more than likely be cocked. Unfortunately that leaves a sizable gap in the focus range -- there is a 20mm range in which focus can only be achieved with the extension at a point other than the ends of its travel. My favored eyepieces achieve focus in this range and thus are almost guaranteed to have axial alignment issues...

You know, it is very odd that only two bolts will go in as I own an XT8i and I installed this exact focuser on it a few years back. I'm sure I didn't need to drill any holes but I may have needed to insert the bolts prior to having the focuser inserted all the way into the OTA to make things easier to line up. Which two holes aren't lining up??

Also, you do have an intelliscope don't you? Just thought I should check because the installation instructions mention that XT8 Classic scopes prior to 2007 will require holes to be drilled.

You know, it is very odd that only two bolts will go in as I own an XT8i and I installed this exact focuser on it a few years back. I'm sure I didn't need to drill any holes but I may have needed to insert the bolts prior to having the focuser inserted all the way into the OTA to make things easier to line up. Which two holes aren't lining up??

Also, you do have an intelliscope don't you? Just thought I should check because the installation instructions mention that XT8 Classic scopes prior to 2007 will require holes to be drilled.

My Intelliscope I bought used, the original owner bought it in 2010 (he handed me over all his paperwork upon sale). It must be noted that this Orion Low Profile Focuser is the older version, Model 13030. I don't know how it differs from the current version (08881) at all, but since the Intelliscope hasn't changed in years there *should* be no problem. The simple drawing shows which holes I had to line up with much difficulty and I'm sure those holes will wear over time.

The problem with the extension is that unless you pull it all the way out it will more than likely be cocked. Unfortunately that leaves a sizable gap in the focus range -- there is a 20mm range in which focus can only be achieved with the extension at a point other than the ends of its travel. My favored eyepieces achieve focus in this range and thus are almost guaranteed to have axial alignment issues...

Hmmm... After re-reading your post a few times, I think I understand what you mean. The drawtube would be slightly tilted (i.e. cocked) and the eyepiece would no longer be axially aligned the focuser and therefore not be fully illuminated by the secondary mirror--is that correct? That is not ideal, but how bad would the effect be? Would they be able to reach focus and if so, would you see vignetting or just some light cutoff or would there be some other issue?

Just checked -- I have the 08881.

Since the two diagonal holes can be forced to line up so they sort of work, it seems like the holes in general must be very close. Perhaps things are just off by a millimeter or so. If you can confirm this by measuring the distances between the holes or perhaps by inspection when you try to line things up, you might find that a small amount of filing in the correct place on all four tube holes will allow the focuser to be installed with all four bolts.

This might be why they changed the focuser. Perhaps the specs and tolerances were such that a percentage of the scopes ran into this issue.

The problem with the extension is that unless you pull it all the way out it will more than likely be cocked. Unfortunately that leaves a sizable gap in the focus range -- there is a 20mm range in which focus can only be achieved with the extension at a point other than the ends of its travel. My favored eyepieces achieve focus in this range and thus are almost guaranteed to have axial alignment issues...

Hmmm... After re-reading your post a few times, I think I understand what you mean. The drawtube would be slightly tilted (i.e. cocked) and the eyepiece would no longer be axially aligned the focuser and therefore not be fully illuminated by the secondary mirror--is that correct? That is not ideal, but how bad would the effect be? Would they be able to reach focus and if so, would you see vignetting or just some light cutoff or would there be some other issue?

My children are now the primary users of this scope though I do use it for quick viewing sessions.

I don't know the exact ramifications but I do take steps to minimize any such issues and I've not noticed any in the views. (It just bothers me that the design has this weakness.) I typically pull the extension all the way out which squares it up and then I carefully push it back in just a little bit and try to keep it a squared up as possible while doing so. Once the extension is set like this I don't need to change it through the observing session. Since all but one of my primary eyepieces are parfocal (T6 Naglers and a 16T5 which are all parfocal plus the one outlier being a 31T5), I typically just make sure I collimate after I've extended the drawtube and focused using one of the T6s. Thus the scope is collimated at the point where nearly all the EPs achieve focus and the drawtube doesn't move much through the night.

My children use different EPs and to be honest I haven't been as worried about things being perfect for them. I still try my best to keep the extension squared up but they don't have parfocal oculars. On the other hand, they are happy just to have an excuse to stay up late...

I needed to pull the extension tube partway out to reach focus when using my DSI camera. The cocking problem was quite an issue until I discovered a solution to it.

I cut a piece of 2" schedule 40 water pipe that was the correct height to make a collar for the slide out portion of the focuser. A hacksaw cut it in half lengthwise and, after heating it, I was able to form it to the exact size of the slide out section of the focuser by pressing it against the focuser. I use a rubber band to hold the two sections of plastic pipe to the focuser and it makes a solid platform for the camera.

Phil

Just checked -- I have the 08881.

Since the two diagonal holes can be forced to line up so they sort of work, it seems like the holes in general must be very close. Perhaps things are just off by a millimeter or so. If you can confirm this by measuring the distances between the holes or perhaps by inspection when you try to line things up, you might find that a small amount of filing in the correct place on all four tube holes will allow the focuser to be installed with all four bolts.

This might be why they changed the focuser. Perhaps the specs and tolerances were such that a percentage of the scopes ran into this issue.

Thanks ThreeD for the good idea, as soon as I get a chance I will look at how close the screws are to the holes and see if they could be filed down.