Focuser flex in Bresser 10" Dob

[Astronomist]

after using my 10" Bresser Dob with a very heavy 28mm UWAN for a couple of months, I've noticed that the weight of the eyepiece/focuser causes flexure in the walls of the OTA, resulting in the focuser tube not running parallel to the optical axis. This is not a problem with lighter EPs, but with the 28mm the entire focuser can sag by as much as 5mm. Does anyone have any advice on how to stiffen the OTA around the focuser? ( preferably without drilling ) 

Thanks

Astronomist

[Mandy D]

2 hours ago, Astronomist said:

after using my 10" Bresser Dob with a very heavy 28mm UWAN for a couple of months, I've noticed that the weight of the eyepiece/focuser causes flexure in the walls of the OTA, resulting in the focuser tube not running parallel to the optical axis. This is not a problem with lighter EPs, but with the 28mm the entire focuser can sag by as much as 5mm. Does anyone have any advice on how to stiffen the OTA around the focuser? ( preferably without drilling ) 

Thanks

Astronomist

Bonding anything onto the outside of the tube over a large enough area to make a laminate is going to effectively stiffen it. The stiffness comes from the outside surfaces, so as long as it is stiff in tension it should work. You could try plastics or thin wood veneer, or roll some thin aluminium to the same radius.

Edited June 1, 2023 by Mandy D

[Ricochet]

The 10” Bresser has been out for a few years now and I don’t recall any previous reports with flex. I have the 8” version and the same 28mm eyepiece and have never noticed any flex. I also have a 21E and a binoviewer which must be just as heavy as the large eyepieces which again I’ve never noticed any issues with. It might be that the reduced diameter of my scope increases the stiffness or that as a cost/weight saving measure they have switched to using a thinner sheet metal at some point but my initial reaction is that if your tube is flexing then it is not up to spec. 

The first thing that I would do (that I suspect you have already done) is confirm that it definitely the tube wall that is flexing and not the connection of the focuser to the tube. You have the four obvious nuts and bolts that hold the adaptor plate to the tube but there are also four tiny grub screws in the side of the adaptor plate that hold the focuser. From memory they are something like 2mm at the top and bottom and 1.5mm on the sides. 

If you bought the scope new then it is still under warranty so before you make any modifications contact Bresser (.de) and find out whether they consider that amount of flex to be within tolerance. They have a good customer service department and it would not surprise me if they replaced the tube. 

[Astronomist]

Thank you for your replies, I have already checked the grub screws and bolts on the adapter plate, the flex is definitely in the actual tube walls. as far as i am aware at least one SGL member has experienced this problem on their Bresser 10"; (it may have been @Vroobel but i cannot find the thread) i think they were imaging so I'm not too sure how much weight there was on the focuser. I will contact Bresser and see what they have to say about it.

Btw my scope goes out of warranty in a few weeks, the 'couple of months' refers to how long I've had the UWAN. Hopefully Bresser replies quick!

[Vroobel]

Hi, 

Thankfully, it works that I got the post as well... 

I place a link to my post on the bottom. Yes, I'm imaging. I understood an origin of the problem after analysis plenty of final pictures after stacking. There was a kind of uneven shadow on the bottom of pictures caused by lift up mirror of the Canon DSLR (180* rotated image) that was unable to eliminate by series of flat frames taken, when the OTA was in a vertical position. It was caused by the gravity: the camera was changing it's position within a night (EQ mount). BTW, nobody was able to explain this effect looking at the pics. 

After my modification (unfortunately involving a lot of drilling) I never experienced this effect. Now I use a heavy cooled astro-camera, OAG with its camera, filter drawer and EAF auto-focuser. I don't think that your EP is heavier than my items together. Now my subframes taken over a night move across the picture only because of the guiding or GoTo imperfections. 

My conclusion is that the steel wall is not stiff enough and in my opinion @Ricochet is right in their suspects that their 8" OTA is stiffer because of its smaller diameter. 

 

 

 

 

Edited June 3, 2023 by Vroobel

[Louis D]

That amount of flex should be easier reproducible just by extending the focuser tube and pushing sideways on it.  That seems excessive.  Just adding a plate behind the focuser may just transfer the flexure further away.  Ideally, you want a stiff reinforcing ring around the entire inside of the upper tube.

Now I don't feel so bad having a Sonotube for my Dob's tube.  It doesn't flex at all that I've ever noticed.

[Vroobel]

BTW, I thought about a custom carbon tube, but going towards i.e. GSO/iOptron/StellaLyra 8" RC OTA seems cheaper. It's smaller, shorter, lighter and in the meantime it has a longer focal length. Of course, it's about the astrophotography. 

[Ricochet]

1 hour ago, Astronomist said:

Btw my scope goes out of warranty in a few weeks

Check that too when you contact Bresser. The Bresser Messier range have 10 year warranties but you might need to register with them.

[Vroobel]

8 minutes ago, Ricochet said:

Check that too when you contact Bresser. The Bresser Messier range have 10 year warranties but you might need to register with them.

It's a budget telescope, I don't think that they will fix the problem. 

[Astronomist]

thanks everyone, I didn't know the messier scopes had a 10 year warranty, mine was sold to me with a 2 year warranty, however inspection of the Bresser website reveals a 10 year warranty is indeed offered, not sure i really want it though, as some of the mods I have in the works at the moment would most likely void it  

Edit: Bresser got back to me very promptly, but it seems there is no obvious answer. I think I will just live with it as conveniently my only really heavy EP is very low power,  so there is no significant worsening of the view. Also, i measured the 5mm with the focuser racked out some distance and the tube horizontal (looking at a distant pylon), so in normal use at night the flex is not as bad.

 

Edited June 5, 2023 by Astronomist