New Focuser for an old 4 “ DOB

[Frankskywatcher]

As some of you may remember that before I received my new 10” DOB someone gave me a 4” DOB and I was using it but it had a stripped out focuser .
So I ordered a new one because of the stripped out one and because the original one is also .97 and this new focuser is 1 1/2 “ .
So I removed the old one and thankfully the hole in the tube is big enough to accept the new one .
Now the only problem Is that when the focuser tube is all the way down in the tube, it hits the secondary mirror with still about a 1/4 to an 1/8 to go to be fully retracted down in the tube.
I am thinking of “building up” the outside area of where the focuser sits with some JB weld to raise it enough so that it will not hit the secondary mirror.
My question is if I do that will that change the focusing ability of the focuser ,or put another way, if I do that will it still be able to get into focus or will it not focus and then therefore become unusable ?
One way or the other it’s going on as there’s no turning back but I’m just wondering if anyone has any insight, thanks in advance.

[Lady Fraktor]

You can just cut the bottom section of the focuser tube to match the length of the old one.

[Frankskywatcher]

Wow I didn’t actually even think of doing that but I don’t think I can because the track for the gear worm goes right to the end so I’m thinking my only option is to build it up on the outside !
Thank you for offering up that suggestion this sucker is going on one way or the other !

[Lady Fraktor]

If it does not touch the mirror I would just instal it and try it out.
You will most likely never use all of the focus anyways so I would just try it out and see it it cuts off any of the view when in focus.

Of course the only way to truly solve the problem is to sell it and buy a refractor

[JayTee]

Why do you need to retract the focuser tube all the way down? It's kind of like going to the doctor and saying doc, it hurts when I do this as you quickly bend and unbend your arm. The doctors says, well, stop doing that.

[Frankskywatcher]

Gee I wish it was that simple ?
I m a newbie that’s why I’m asking this in that section and my limited scope ( no pun intended) on focuser’s is why.
I know for example that on my 10” DOB with one EP you need the focuser all the way out but swap that out for a higher power EP the focuser tube is all the way down to be in focus,so therefore if this one bottoms out when it hits the secondary with an 1/8 of an inch of focus tube to go will it still focus or be off.
I am just asking to try to get an informed decision before I put the new one on and thought I’m sure there are builders on this forum .
Any advise or insight would greatly be appreciated and thanks in advance .

[JayTee]

As a temporary measure you can put a thin piece of duct tape on the rack portion of the focuser tube. This will tell you as you're moving the tube inward that once it gets stiff to move you know you're close to the secondary.

[Frankskywatcher]

That’s a great idea, I’m going to put it on it either way like you suggested,however when it goes on it ain’t being taken back off with the JB Weld stuff .
I am going to try to take up the difference with a few rubber gaskets and some JB Weld to seal the thing up and down using the original screws and bolts from the old focuser .
As you can see in the picture the amount of “ slack “ in red or the difference at the top of the photo that I have to raise the whole assembly so as to not hit the secondary(red line at the bottom of the photo).
Again the thing I’m wondering about being a newbie is will that “difference “ effect it focusing it any way or is it inconsequential ?
Thanks again .

[Bigzmey]

Frank, try (gently) to see if you can achieve the focus with your EPs without focuser tube touching the mirror. If that the case you can add a raiser. The raiser would not be required for focusing, just a safety feature to prevent the drawtube hitting the mirror. it will not affect the focuser function.

[Frankskywatcher]

Frank, try (gently) to see if you can achieve the focus with your EPs without focuser tube touching the mirror. If that the case you can add a raiser. The raiser would not be required for focusing, just a safety feature to prevent the drawtube hitting the mirror. it will not affect the focuser function.

Thank you for eloquently zeroing in on what I’m wondering about my friend.
I have a couple of thin rubber gaskets ( a kin to a mason jar gasket) that I think will just bring it up enough so as to not bottom out on the secondary and IF it will not effect the focusing then I’m free and clear .
It’s kind of hard to be able to do the EP test because it not on the mount, it’s not secure, not light proof etc etc etc it’s going to be a attach it and hope it all works unfortunately

[Bigzmey]

Frank, try (gently) to see if you can achieve the focus with your EPs without focuser tube touching the mirror. If that the case you can add a raiser. The raiser would not be required for focusing, just a safety feature to prevent the drawtube hitting the mirror. it will not affect the focuser function.

Thank you for eloquently zeroing in on what I’m wondering about.
I have a couple of thin rubber gaskets ( a kin to a mason jar gasket) that I think will just bring it up enough so as to not bottom out on the secondary and IF it will not effect the focusing then I’m free and clear .
It’s kind of hard to be able to do the EP test because it not on the mount, it’s not secure, not light proof etc etc etc it’s going to be a attach it and hope it all works unfortunately

That works to.

[Lady Fraktor]

The gasket sounds like a good idea to try, I would avoid the JB weld until you have determined the final position.
The buy a refractor comment was meant as a joke.

[Frankskywatcher]

As some of you may remember that before I received my new 10” DOB someone gave me a 4” DOB and I was using it but it had a stripped out focuser .
So I ordered a new one because of the stripped out one and because the original one is also .97 and this new focuser is 1 1/2 “ .
So I removed the old one and thankfully the hole in the tube is big enough to accept the new one .
Now the only problem Is that when the focuser tube is all the way down in the tube, it hits the secondary mirror with still about a 1/4 to an 1/8 to go to be fully retracted down in the tube.
I am thinking of “building up” the outside area of where the focuser sits with some JB weld to raise it enough so that it will not hit the secondary mirror.
My question is if I do that will that change the focusing ability of the focuser ,or put another way, if I do that will it still be able to get into focus or will it not focus and then therefore become unusable ?
One way or the other it’s going on as there’s no turning back but I’m just wondering if anyone has any insight, thanks in advance.

[Frankskywatcher]

UPDATE 4/7/22
So as I’m sure most of you are aware,I was gifted old four-inch dob but the problem was it had a stripped out focus are very near the end of the rack just before where it would go into focus.
I had managed to find a svbony 1 1/2 “focuser for 25 bucks, hardware not included from China,they were going for 50 here in the states.
So the last couple of days I finally got around to trying to removing the old .97 stripped out old focuser and install new 1 1/2 .
As you can remember my main worry was that because I went up and size the new focus was hitting the secondary mirror when all the way down in the tube and I had to bring it up maybe an 8th to 1/4 of an inch.
So as you can see I got these rubber gasket and they did the the job,they brought it up just enough so now when the focuser is all the way down it stops right before the secondary maybe just a 16th of an inch.
To my surprise it does focus alright and came into focus with both a 9mm Plossi that came with my 10” DOB and the 15mmWAE that MAKUSER was so kind to gift to me .
They both focused at the exact same spot but what I did notice was that the focuser is like 90% extended out
I tried to get it to focus going the other way ,the focuser going into the tube towards the secondary but it would not focus.
So I guess the positive side I now have an older scoped it will take inch and a half lenses it does focus.
I guess the negatives are that I don’t know what the negatives are until I get different lenses or lenses for the length of the 4 inch tube.
I won’t really know if it focuses back towards the secondary but as I said at least it does focus,I can use 1 1/2 lenses !
Thank you everyone for all your advice and direction.

[Bigzmey]

Congrats on your first telescope mod Frank! You did the test on daylight targets, right? At night on infinity targets (which all sky objects are) your focuser travel will be a bit shorter. In any case as long as it is reaching the focus it does not matter if it racked out 90% or 40%. Enjoy your better focuser and 1.25" EPs!

[Frankskywatcher]

I have been out all night playing !!
First off that EP MAKUSER gave me got a workout on the Orin Nebula and star clusters and even a little lunar searching!
Secondly to answer your question, yes I used the 4” DOB during the day and focused it on a sign like a quarter mile away and lined it up with the red dot
As I stated earlier, the focuser tube is extended a good way out but like you said it focuses and it’s now 1 1/2 so I’m happy my friend !

[Frankskywatcher]

The gasket sounds like a good idea to try, I would avoid the JB weld until you have determined the final position.
The buy a refractor comment was meant as a joke.

Yes as you can see the gaskets worked well and yes you are correct again I held off on the JB Weld and I totally got your joke it was amusing !
Thanks you again for the link to the Lunar atlas my friend !

[Richard]

so how is your 10 inch dobs I found after messing around with a 4 inch I just gave it way , I never tried to change the focuser as its a mission on these scopes and expensive here in RSA and if you have decent .965 eyepieces not really worth it , however on my 60 odd year refractor changing to 1.25 was easy and worthwhile , my Tasco 10TE is quite nice , but its a 76mm so all my other scopes blow it away , but it looks nice

[turboscrew]

Good. I wouldn't attach much anything permanently. There can always happen something that requires changing that specific part you have superglued in place. I can imagine the swearing then.
And good if you solved the problem. I've "hit" my focuser a couple of times such that it rolled in. Not intentionally, of course, but by accident. I think I would have said words that makes even sailors blush, if the focuser would have broken the secondary, and maybe a bigger part of the secondary would have dropped on to the main mirror and ruined it.