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OTA focuser hole enlarging


sbooder
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Hi All,

I need some advice on enlarging the focuser hole on my Orion GX250. The scope comes with a 1.25” focuser and the hole in the OTA is cut to match, I am putting on a new 2” Crayford and need to enlarge the hole and obviously keep it centred form the original hole.

Is it best to go the way of the Dremel and cut a square hole or buy a hole cutter bit for my power drill?

Thanks for any help.

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Thanks John, It sort of helps. The only problem I am going to have is the original focuser is fitted with a round baseplate and is attached with only three attachment screws, which as you can imagine are not located in an apposing pattern like the four bolts of a standard focuser they are in a tri-apposing pattern (if that is a word).

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That makes it a little more complicated but still not a big problem.

Cover the area around the focuser with masking tape. Then using a piece of A4 paper held against the tube ring on the end of the tube to give you a right angle, draw lines along the tube to mark the outside edges of the existing focuser hole. Then using these reference lines draw new lines along the tube for the hole size that you're going to cut out. Then a couple of marks to set the distance of the new hole down from the end of the tube.

Place the new base in position and mark out the hole and the bolt positions. You can use the sheet of paper again to check that the sides of the new base are aligned properly.

Hope this helps.

John

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buy a hole cutter bit for my power drill?

If you do, you also must have the tools to put something matching the curvature inside the tube. Trying to use a hole cutter if there isn't anything supporting the tube will invariably make the saw teeth twist the tube and then grab it, and that will usually fling your tube away very violently no matter how well it'd been fastened.

i.e. Do't do it.

I actually have a router so I made a couple of plywood cylinders which glued together could form a support structure behind the hole, and I then used a (very good) hole cutter bit to enlarge the hole in my Starblast, but it's actually more work than drilling a dashed circle of holes, connecting them, and sanding the remaining teeth with a dremel.

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If I am going to use a Dremel anyway, I might as well cut a square? Once the backplate is on will it make a tad of difference?

The problem with cutting a square hole is that where you want to drill the mounting holes for the base may end up being within area of the cut-out, or leave the mounting holes right on the edge of the cut-out. :)

Also as mentioned in an earlier post using a hole cutter is not recommended. While it can be done with the right preparations it's easier and quicker just to drill the holes. Attempting to drill the hole with a hole cutter in this situation could land you in A&E sooooo easily.

John

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I meant use a dremel with cutting discs not a hole cutter. If a square hole is within the confines of the aperture in the backplate it will still be big enough for a 2" focuser barrel?

I think I see what you're getting at, but it still might be a bit tight for the drawtube to pass through. You know rather than thinking in terms of a square hole why not an octagonal one. Should be easy enough to cut with the Dremel.

John

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I meant use a dremel with cutting discs

That will also tend to twist and grab the tube if you're not careful (and I like that even less than hole cutters because if the tube won't move, your arm will, and not in a pleasant way, unless it wears something in the front of the dremel tool, usually either the shaft of the cutter disk or the holder).

That is unless you cut very small portions at a time (i.e. make a sort of dashed line of cuts and then cut in between the dashes.) In my experience (which may not be typical) as long as both sides of the cutting disk are cutting and you're not cutting more than a third to half the diameter of the disk you're fine as long as you only move the disk down and up (trying to rotate or to translate it to cut along the circle will, however, typically result in something unpleasant).

Drilling with a drill and then cutting in between those holes with a dremel tool does work. For the remaining "teeth" that are left in the outline of the hole, though, a sanding bit works better.

Always use a mask - that aluminium is nasty when inhaled.

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Hmmm, just thinking... Wouldn't you need a bigger secondary mirror when you fit a bigger focuser? When I collimate my scope with 2" focuser, the secondary just fits underneath the draw tube, so must be made for that focuser size.

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You'll have less edge of field illumination at the edge, but up to illumination levels of 40% or so it's still useful. After all, 40% illumination still is a lot more than the 0% illumination behind the eyepiece field stop :).

Usually on fast scopes it's not too bad - illumination drop-off beyond the fully illuminated field is very gradual. And sometimes you can tweak the scope a bit --put a lower profile focuser on it like the KineOptics HC-2 and move the primary back a bit; that moves the focal plane closer to the secondary and that improves edge of field illumination again, letting you get more out of that same secondary.

On slow scopes designed for a 1.25" focuser and with an aggressively small secondary and a low profile focuser, though, the vignetting may become so severe it's not worth the trouble.

Edited by sixela
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I used a block of wood I had lying around and cut a curve on the top using a handsaw that matched the inside of the OTA. Then I screwed it to the inside of the OTA using selftapping wood screws through the holes the focuser had been fixed with. A bit of careful measuring gave me the centre point of the existing hole and then I marked this on the wood block that was showing through the hole for the existing focuser and used a hole saw to cut slowly through the ota to the new size. The arbor of the hole saw was held central by the block of wood and the blade of the hole saw bit into the wood and was held in place.

My replacement focuser is very low profile so no problem with the secondary being too small.

Edited by angusb1
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I was thinking of using the foam from and old mouse mat as a baffle between the tube and the backplate to take up the tiny amount of space in the curve of the plate and tube.

Would this be too thick at 5mm? Taking in to account the give in the foam when mounting bolts are tightened?

Or I could just use a couple of layers of the self-adhesive flocking material I have left?

I will by the way be going with John’s drilling technique to enlarge the focuser aperture in the OTA.

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If you're using the TS curved base plate when you tighten up the mounting bolts it will tend to pull the tube wall up against the base plate so there may be no gap. If there is still a small gap some thin very soft weather-seal foam tape (the 10mm wide by 6mm thick stuff) down each side will do.

You don't want a big piece of 5mm mouse mat type foam under the whole base plate as when tightened down the base plate will have a small amount of movement.

John

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Thanks John, that worked a treat. I will post some pics later on. I found a very good way of marking the hole for drilling, the narrow part of the 2" extension tube fitted perfectly if not tightly through the existing hole, and then the focuser barrel fitted perfectly over that and it not only allowing me to mark the circle for drilling the new barrel hole but also made it precise for drilling the plate bolt holes.

Here are pics of setting up for marking of the plate bolt holes. I forgot to take any pics of the marking of the new focuser hole. Basically I took the barrel out of the crayford body and slipped it over the 2" extension that is sticking out of the OTA and drew around it with a pencil and then drilled the holes on the outside of the pencil line and then used the dremel to join holes, finally I again used the dremel to sand down the teeth left by the hols and smoothed with some fine wet & dry.

Works a treat.

fitting_crayford1.jpg

fitting_crayford2.jpg

fitting_crayford3.jpg

Edited by sbooder
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Had a chance to tryout my new (third hand but still in V good nick) Crayford Focuser last night.

I tried it at all angles, on Jupiter, Pleiades and M31, it was good on all subjects with no image shift or slack, as one would expect from a Crayford.

The only odd thing was, I could not use my 25mm unless I brought the eyepiece out of the focuser by about 1cm, but my 40mm worked fine, what’s that all about (in the vernacular of modern comedians!) ?

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Sounds like it went well, it sounds like your new focuser actually sits a bit lower than the old one so your 25mm isnt getting to the focal plane. A 20mm extension tube will help you there.

By far the easiest method for opening up a hole in thin steel is to chain drill and then file back to a neat finish.

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I had a similar problem when I replaced my focuser with a crayford. Most eyepieces would not achieve focus. It turned out that the new focuser has quite a bit less outward travel than the standard R&P. So an extension tube was the answer...

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By far the easiest method for opening up a hole in thin steel is to chain drill and then file back to a neat finish.

That is what I did, not as neat as I may have wanted but once i had folded some flocking around the hole it look top notch.

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I had a similar problem when I replaced my focuser with a crayford. Most eyepieces would not achieve focus. It turned out that the new focuser has quite a bit less outward travel than the standard R&P. So an extension tube was the answer...

The thing is, this problem was with the extension tube in!

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The only odd thing was, I could not use my 25mm unless I brought the eyepiece out of the focuser by about 1cm, but my 40mm worked fine, what’s that all about (in the vernacular of modern comedians!) ?

Different focal plane location with respect to the shoulder for both eyepieces. You can buy a parfocalisation ring to clamp on the barrel of that 25mm so that it won't insert to the end; that's often the most practical solution.

You could also try to move the primary mirror a bit closer to the back (start by loosing the locking bolts and tightening the collimation bolts), but 1cm is quite a lot, perhaps too much to do it merely with the collimation bolts.

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I think as long as the focus is too far OUT, then you can always use an extension tube or something like that. But if it was too far IN, then there is much less you can do. Can always make it longer but making it shorter is a lot harder.

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