Bending tube Orion Optics UK 250 f4'8

[Acrab67]

Hello,

I have detected that the tube significantly loses collimation depending on the inclination of the tube. Presumably it is due to the lack of rigidity of the optical tube, since it is made of excessively thin aluminum. Has anyone solved this problem in any way without changing the tube itself? Thank you.

[Mr Spock]

I think it's more likely to be weak primary springs than tube flexure. They are easy to replace.

[Mandy D]

3 hours ago, Acrab67 said:

Hello,

I have detected that the tube significantly loses collimation depending on the inclination of the tube. Presumably it is due to the lack of rigidity of the optical tube, since it is made of excessively thin aluminum. Has anyone solved this problem in any way without changing the tube itself? Thank you.

I have been advised that OOUK tubes tend to flex and are best avoided for that reason. The mirrors, however, are amongst the best mass produced units you can find. I have a similar problem with my Skywatcher 300PDS, which definitely suffers from significant tube flexure. My fix for it is to buy two more tube rings, making four in total, installing them over a significant part of the tube length, evenly spaced and brace them with aluminium struts, one or both of which could be long Losmandy dovetails. The 31" in the link below is what I have in mind:

https://www.rothervalleyoptics.co.uk/adm-losmandy-d-series-universal-bars-various-sizes.html

Edited December 28, 2023 by Mandy D

[bosun21]

My 300mm reflector only suffered from weak primary springs. Once replaced with heavier springs I had no more loss of collimation whilst traversing the sky. Cheap and very beneficial upgrade.

[Saganite]

I used a OOVX12L f5.3 12" Dob for several years , moving it about the garden and rarely needed to collimate, in fact, as largish dobs go, I thought the OO was the best of several well known brands that I used.  I agree with Mr Spock, it is more likely to be the mirror assembly causing the problem.

[John]

I've owned 2 Orion Optics newtonians, a 10" F/4.8 and a 12" F/5.3. I am just an observer but didn't find tube flexure an issue with either scope even with very heavy eyepieces such as the ES 17mm 92 degrees which weighs over 2.5 lbs. 

I have occasionally read that imagers have had issues with tube flexure in OO newts though.

I didn't find the primary springs an issue but mine were older scopes which did not have OO's current mirror cells fitted.

Edited December 28, 2023 by John

[Acrab67]

Thank you.

I'm going to change the springs and see how it goes.

[scarp15]

Never personally encountered this issue having owned both an OOUK 14" F4.6 on their dobsonian base and continue to use an 8" F6 dob set up. The tube, trunnion blocks securely balance align and brace the tube. The tube is light, yet the mirrors are also light, particularly compared to alternatives. Nor had any issues with the mirror cell springs. These are the VX range, cannot comment on earlier models. In fact I would check the collimation on the VX14 prior to loading in the car for a dark sky trip as often dark upon arrival, yet collimation held just fine.    

[Acrab67]

Hi,

My optical tube in question is a SPX 250 f4'8 with 1/8 primary purchased in 2009. I sold it to a friend when I built a 16" Dobson with Ostahowski optics. The fact is that I have recovered This same optical tube a few months ago. I don't remember noticing any collimation problems in those years, although it is also true that I didn't use very sensitive collimation tools, just a 2" Cheshire Sight Tube from Catseye. Now I use Howie Glatter's 2" laser and tublug. The Tublug is very sensitive to any small change, which is not practically noticeable visually but is noticeable in the collimation tool. I have carefully checked and adjusted the mirror in the cell and I realized that I had changed the springs at the time for more rigid ones, in fact, it is impossible for the decollimation to come from there. Today I collimated it horizontally and raised the tube little by little to observe the drift of the reflection of the primary mirror circle in the Tublug. The variation is slight but it is there, and it definitely does not come from the cell. The only option is that it is either the tube itself, or the area of the tube near the focusing group that flexes somewhat depending on the height. I also don't know if the current VX series tubes are different in terms of wall thickness from the SPX series. I also changed the focuser at the time for a JMI EV-1n. I have also updated the rings fastening by 10" GSO studs (adding felt inside) of the tube that are fixed with a Losmandy type plate.

[Albir phil]

If its only slight I would try it on a bright star and see if it affects the image,in the end it depends on how good you need it to be,if you replaced the focuser when you got the scope back could it be more heavy than the original,and possibly flexing at that area,just a thought 🤔

[bosun21]

I always collimated my dobsonian scopes at a 45° angle to try and compensate for any small mirror movement. Then again I have never actually owned an OO dob 

[Captain Scarlet]

I’ve had this with both 8” newts I’ve had. A 200p was terrible, I could see the laser dot move around as i changed altitude. It wasn’t the springs: I could alter the collimation just by applying finger-pressure to the tube around the focuser. I partially solved the problem by getting big air-conditioning ring-clips and positioning them around the tube one near the focuser one towards the primary.

And an OO VX8 showed the same. Again, not the springs. It had one of their new (very, very good) cells which have almost thread-strippingly strong springs. I solved that one by buying a Helmerichs carbon tube to replace the aluminium.

 

[Acrab67]

15 hours ago, bosun21 said:

I always collimated my dobsonian scopes at a 45° angle to try and compensate for any small mirror movement. Then again I have never actually owned an OO dob 

Me too

[Acrab67]

1 minute ago, Acrab67 said:

Me too

I've been considering it, but a carbon tube is quite expensive

[Acrab67]

15 hours ago, Captain Scarlet said:

I’ve had this with both 8” newts I’ve had. A 200p was terrible, I could see the laser dot move around as i changed altitude. It wasn’t the springs: I could alter the collimation just by applying finger-pressure to the tube around the focuser. I partially solved the problem by getting big air-conditioning ring-clips and positioning them around the tube one near the focuser one towards the primary.

And an OO VX8 showed the same. Again, not the springs. It had one of their new (very, very good) cells which have almost thread-strippingly strong springs. I solved that one by buying a Helmerichs carbon tube to replace the aluminium.

 

I've been considering it, but a carbon tube is quite expensive

[ONIKKINEN]

I used the hose clamp method as shown above by @Captain Scarlet to stabilize my VX8 aluminium tube and it worked well enough to fix any immediately obvious issues.

But then a bit later upgraded to a Helmerichs carbon tube which has been rock solid. Wasnt too expensive either at 380€ (if i recall that correctly), but the 10" version would probably be closer to twice that.

[NGC 1502]

Other things to try-  make sure that primary cell to tube bolts are secure, also spider to tube connections.  It’s well known that mirrors should not be clamped or pinched. However if there’s too much freedom for the primary mirror it can easily shift.  Primary mirrors can be carefully shimmed so no pinching occurs, but not excessive freedom either. My 10” f4.8 OO UK Dob rarely needs collimation adjustment, and I’m fussy to get the best views.

[Acrab67]

16 hours ago, NGC 1502 said:

Other things to try-  make sure that primary cell to tube bolts are secure, also spider to tube connections.  It’s well known that mirrors should not be clamped or pinched. However if there’s too much freedom for the primary mirror it can easily shift.  Primary mirrors can be carefully shimmed so no pinching occurs, but not excessive freedom either. My 10” f4.8 OO UK Dob rarely needs collimation adjustment, and I’m fussy to get the best views.

Everything is checked. I have to check if the tube at the base of the focuser flexes.

[Acrab67]

Has anyone by chance tried to mount a long dovetail, say, 600 mm, on one of these tubes, and found improvement?. Something like this:

 

Edited January 4 by Acrab67

[Acrab67]

Hello again,
I have made a dovetail with a sufficient length to prevent the tube from flexing too much and to keep the collimation of the optics within tolerable limits. Here is the result. In that aspect I have been very satisfied. The dovetail in question measures 70 cm in length. I attach a video where you can see that the collimation remains within more than acceptable limits.