Diagonal slip

[wibblefish]

Im not quite sure if I describing this right but I have noticed since adding heavier EP that my diagonal will sometimes rotate. It is held into the telescope focuser by two thumb screws. Retightening these usually helps for a while but its fairly easy to knock the diagonal and itll spin loose again.

This is a skywatcher evostar 90 with a 1.25” diagonal. 

Any suggestions?
 

I wondered if there is a way to replace the collar(?) of the focuser with something that has a compression ring but Im not sure what to look for.

Edited March 15, 2021 by wibblefish

[wibblefish]

Hopefully this might make things more clear!

This bit has two screws but they don't seem to hold the diagonal tightly enough (i.e. it can work loose). Is there something I can replace this section with that perhaps uses a compression ring? (its a 1.25" diagonal)

Edited March 16, 2021 by wibblefish

[John]

I have a scope with a focuser just like that and I have just the same problem. Those little screws are not good at holding weight / gripping and are so small that applying any real tension to them is difficult, plus the screws mark up the diagonal barrel.

Ideally the visual back  (the black part that the diagonal fits into) would be replaced by one with a compression ring fitting. but I'm not sure that these are made for these smaller focusers 

A simpler alternative would be to replace the two tiny set screws with larger ones that have more for the fingers to grip and which have more contact with the diagonal barrel. This would entail re-drilling and re-threading the two holes in the visual back and fitting larger and more robust set screws.

It is an annoying problem. Perhaps someone else with a Skywatcher 1.25 inch focuser has managed to find a way to improve it ?. I'd be interested myself if they have.

Sorry that I don't have a handy solution but I can empathize !

[wibblefish]

@John thats a pain but I guess its not unusual so thats good in a way!  Visual back, thats the correct term then good to know maybe I can rustle up some more google-fu and see if someone else has diy'd something

[Pixies]

I was thinking about this. The ST80 has the same visual back. I haven't been able to determine the thread on the end of the focuser draw tube, but it's not a T2. 

@wibblefish, of you can find out the thread on the drawtube (the silver bit in your pic), or of anyone else knows, I'm sure there's a better visual back or an adapter that will fit.

[RT65CB-SWL]

I would be inclined to replace the the screws with the ones that have a plastic top/head, so can get a better grip when tightening or loosening up... or if you do not want to mark the nosepiece, replace them with nylon screws or nylon tipped screws. It baffles me as to why they have supply them with such small heads to begin with. 

[MrFreeze]

This should help : www.aliexpress.com/item/32800166770.html. A Svbony M42x0.75 to 1.25" adaptor - fits all Skywatcher 1.25" focusers and has a compression ring to avoid marking your eyepieces.

David

[Pixies]

That's M42/0.75 (T2) though, which matches the T2 thread at the end of the visual back. The thread on the drawtube is M43/1.0 by the look of it. Wrong 'sex' too.

It will work, but it will have to be screwed onto the end of the existing visual back.

Edited March 16, 2021 by Pixies

[Pixies]

4 minutes ago, Pixies said:

That's M42/0.75 (T2) though, which matches the T2 thread at the end of the visual back. The thread on the drawtube is M43/1.0 by the look of it. Wrong 'sex' too.

It will work, but it will have to be screwed onto the end of the existing visual back.

This is the beastie. Red is M43/1.0 and blue M42/0.75 (T2)

[MrFreeze]

Just screw it onto the back of the existing component (and remove the thumbscrews) - it will work fine. Never had a problem reaching focus with that arrangement, and we used our Evostar 90 like that for years. (I just went up into the loft to check - it's in storage up there now)

David

[Waldemar]

The Baader clicklock system may work for you: Baader ClickLock 1.25" Adapter with T-2 Thread | First Light Optics
Maybe you will need a M43x1 male to T2 male converter to reach focus: Baader M43a x 1 / T-2a Adapter (e.g. for Vixen) (baader-planetarium.com)

[wibblefish]

Thanks for the replies so far, I am still monitoring this thread just been busy  

I have a part similar to what @MrFreeze has suggested arriving later so I will update if that works. Not sure when I will get to test it (other than the diagonal is secure) though I might have a go during the day to make sure it can still focus!

I have been out on two "clear" nights so far and they clouded over as I was setting up  Think its been more than a month now since I have managed to get a session, pesky UK weather

Edited March 17, 2021 by wibblefish

[wibblefish]

Well I have had a quick go, everything fits but with the additional piece I cannot attain focus. Pesky short tube refractors! I have a similar issue when I add additional filters onto my eye piece or try to use a camera on the visual back. Back to the drawing board! 

Edited March 17, 2021 by wibblefish

[MrFreeze]

Short tube refractor ? Is this an Evostar 90/660 then ? The standard Evostar 90  will definitely reach focus.

If you are having trouble with filters on your eyepiece  preventing the eyepiece seating fully, this sounds like you have a restriction in your diagonal. Some diagonals have a restriction ring on the eyepiece  port, some others don't - unfortunately it's difficult to tell unless you can get hold of one to check it. Not the sort of thing companies advertise.

David

[Louis D]

4 hours ago, wibblefish said:

Well I have had a quick go, everything fits but with the additional piece I cannot attain focus. Pesky short tube refractors! I have a similar issue when I add additional filters onto my eye piece or try to use a camera on the visual back. Back to the drawing board! 

Definitely give that 3mm optical path length Baader adapter @Waldemar suggested above with the T2 threaded visual back attached to it.  It would replace the existing visual back and probably shorten the optical path length by 20mm to 25mm.

[wibblefish]

@MrFreeze yeah its a 90/660, primarily I can't achieve focus with to much glass (i.e. if I wanted to filter + barlow + EP), if I put the filter in the diagonal itself it seems to resolve the issue  I bought a new Altair Starwave 1.25" diagonal recently and the problem still persists so my gut feel is its just to do with it being a short tube refractor and I can't have to much "stuff" in the optical path due to the focuser restrictions unfortunately. Thanks for the suggestion about the part however it was definately worth a try!

@Louis D I am kind of loathe to buy something so expensive only to find it doesn't fit or won't achieve focus if it does  appreciate the assistance from @Waldemar and everyone else though and will report back if I find a different solution

Edited March 18, 2021 by wibblefish

[Louis D]

6 hours ago, wibblefish said:

@Louis D I am kind of loathe to buy something so expensive only to find it doesn't fit or won't achieve focus if it does  appreciate the assistance from @Waldemar and everyone else though and will report back if I find a different solution

Try removing the original visual back and just hold the T2 visual back up against the back of the bare tube to see if it will reach infinity focus with your eyepiece requiring the most in-focus.  Slowly pull the T2 VB away to see how much space you have for an adapter and can still reach focus.  This experiment won't cost anything.

Edited March 18, 2021 by Louis D

[Waldemar]

I agree the Baader solution is not cheap , but keep in mind that you will be able to use it on any future telescope with the right adapter.

[Louis D]

I literally couldn't find any other M43x1 adapters or step rings anywhere on the internet.

[MrFreeze]

I think this might work - https://www.aliexpress.com/item/33000127040.html . IF it really is a 43mm thread. It doesn't specify whether it is 42x0.75 or 42x1 at the other end either, and the threaded section isn't that long, so won't take too much weight.

David

[wibblefish]

Appreciated all, I havent had much time to investigate further 😔

[Louis D]

7 hours ago, MrFreeze said:

I think this might work - https://www.aliexpress.com/item/33000127040.html . IF it really is a 43mm thread. It doesn't specify whether it is 42x0.75 or 42x1 at the other end either, and the threaded section isn't that long, so won't take too much weight.

David

Almost assuredly M43x0.75 and M42x1 since it's meant to reverse mount lenses onto a Universal lens mount (M42x1) rather than a T2 (M42x0.75) mount.  The 43mm thread is almost certainly 0.75 pitch which is the standard for filters.

That means the thread pitches are backward relative the Baader adapter (M43x1 and M42x0.75) despite the diameters being the same.