Replaced the mirror cell and now can't reach focus....

[Giles_B]

When a 9-point OOUK Mirror cell came up on ABS a few days ago I jumped at the chance to replace the sealed 3 point cell on my OOUK VX10 - a 10" f4.8 Newtonian - at a much more modest cost than buying new.

I've just spent half the day very carefully drilling the tube to install the new cell (this is the first time I've dismantled the tube), but now I've reassembled it I find I can't reach focus on the usual chimney pots and sundry that I generally test focus on in the day - I don't have enough in-focus at 34mm which is my lowest magnification eyepiece.

The position I needed to install the new cell in the tube meant that the mirror is about 10mm higher in the new cell than in the old one. I'd wrongly assumed this might be within a certain tolerance - the way I understood it I might move the focal point slightly, but I would just have to use more back focus, which is not a problem with my focuser. I don't understand why it's in-focus I am lacking.

The only ray of hope that I can see is that the view from my garden is quite restricted by trees and buildings - the most distant objects are only a couple of hundred metres away. So my hope is that I might still be able to reach focus on astronomical objects - this is the only reason I haven't reinstalled the old cell straight away - but it might be a while before clear skies will put me out of my misery.

Has anyone else ever had this problem, and am I likely to need to go back to the old cell?

Could anyone explain to me what I've done to the optical path and what steps I have available to remedy it - the old cell wasn't a big problem, but it did lose collimation regularly, so I'd rather upgrade if at all possible.

Photos attached to illustrate what I've done - on the second photo you can see the old set of holes are too low for the new cell, which would only fit into the scope if mounted a little higher.

 

 

 

Edited January 14, 2023 by Giles_B

[dweller25]

Well that has me baffled as I thought you would need a lot more out focus - which I’m assuming you have tried and it has not worked !

I look forward to other peoples thoughts……

Nice scope 👍

Edited January 14, 2023 by dweller25

[AstroKeith]

Yes I'm confused too. You description is contradictory.

You say you need more in-focus, but you also say the mirror is now 10mm higher (ie closer to the focuser). Which is it?!

Solutions are, (assuming the mirror is too hight)

Drill new holes for the primary cell to move it down 10mm, if you have enough tube length left!

Put a 10mm spacer under the focuser.

[johnturley]

If the mirror is about 10mm higher in the new cell than in the old one, then surely you would need about an extra 10mm of out travel on the focuser., which you can achieve with an extension tube if there is insufficient out travel on the focuser.

John 

Edited January 14, 2023 by johnturley

[markse68]

How did you measure the position of the mirror old vs new? Is it possible that even though the new cell is mounted further inside the tube that the actual mirror is further back due to the thickness of the old cell? You should have a fair bit of mirror height adjustment on the collimation screws- from the photos it looks about 10mm haha! You should be able to just reposition the mirror closer to the end of collimation adjustment screws to bring the focal plane further out at the focuser.

Mark

[Giles_B]

Okay - I feel less of a lemon now this has got other people baffled. There is definitely a lack of in-travel for the focuser - I can see the image coming more into focus as the focus tube travels down.

Unfortunately there is no way I could mount the cell any lower  - and, while it's a good suggestion, I did try to rack the collimation screws back - but thank you @AstroKeith for the idea of a spacer.

However... I do wonder , given the opinions here, if the mirror is actually lower than it was rather than higher - I'd assumed I just didn't understand how a Newtonian worked - but as @markse68 says I could have made a mistake here - my conclusion it was higher was based on a bit of dead reckoning  - checking the relative position of the mirror in the cells against the position they attached to the scope.

If the mirror is lower than it was it would be quite easy to raise it, because there is a second set of attaching holes in the cell - so this is the first thing I'll try tomorrow (unfortunately my astronomy time is out for today)....

Edited January 14, 2023 by Giles_B

[dweller25]

Which is lower - the top of the mirror clips on the old back or the new back ?
 

[Giles_B]

The old - The old back was 10mm lower.

But I wonder if I somehow overcompensated...

Edited January 14, 2023 by Giles_B

[dweller25]

So if the old back was lower then the new back is forwards, so you would need more out focus !

It just does not make sense.

I am assuming you have not changed the primary ?

[Giles_B]

No, same primary. It doesn't seem to make sense. But if the problem is lack of in focus it indicates logically that I must somehow have set the mirror lower with the new cell. I don't understand how. But I have now raised it, so will check it tomorrow, and see if it resolves the issue.

Edited January 14, 2023 by Giles_B

[dweller25]

Do let us know 👍

[Captain Scarlet]

At risk of insulting (for which I apologize if so), are you absolutely sure you’re getting closer to focus as you rack in to the minimum? I’ve been fooled by that before. IE actually you can’t get enough out-focus to catch your closer targets. Pull your eyepiece away from the tube at max out to see. My only other thought is that those OO cells do have quite a lot of travel in the mirror “height” (I have two, an 8” and a 12”, I think they’re extremely good). Perhaps push the mirror up as far as you can with the collimation adjusters.

Magnus

[Giles_B]

10 hours ago, Captain Scarlet said:

At risk of insulting (for which I apologize if so), are you absolutely sure you’re getting closer to focus as you rack in to the minimum? I’ve been fooled by that before. IE actually you can’t get enough out-focus to catch your closer targets. Pull your eyepiece away from the tube at max out to see. My only other thought is that those OO cells do have quite a lot of travel in the mirror “height” (I have two, an 8” and a 12”, I think they’re extremely good). Perhaps push the mirror up as far as you can with the collimation adjusters.

Magnus

Cripes you were right (no insult taken!) - physical reality is restored, at the cost of some embarrassment to me.... 😳

Shifting the mirror higher did not correct the problem. Once the weather dried out enough to get the scope out in the dark I was able to point the scope at a bright light source. It was much clearer than it had been by daylight that the image was resolving as i racked the focuser out. So it was back focus, and not in-focus that was the problem.

Huge apologies for spreading my confusion, and many thanks for prodding me in the right direction!

[dweller25]

 Very good 👍👍👍👍