Spider deforming tube

[Dave_D]

One thing that has been bothering me lately about my scope (Orion Optics GX250) is the rigidity of the tube, which is aluminium. I was sat in my room the other day and the sunlight caught the side of the tube. What i noticed, was that the tensioning bolts for the spider were deforming the tube as can be (sort of) seen in the pic. you can make out the reflection bending round instead of being a 'straight line'.

What i was thinking of doing, is shortening the tube and drilling the top ring, which is much thicker and stronger and mounting the spider there, but this would put the secondary quite close to the end of the tube. Would there be any benefit to doing this? it would appear to give much more stability to the spider, but looking at various open tube scopes, would still give better shielding.

[Lorne]

I wouldn't think the deformation of the tube is a problem as long as the secondary holder is exactly central in the OTA. Measure the length of the spider legs and try to adjust them all the same length. It MAY be that one leg is shorter - therefore too tight and causing the tube wall to bend in slightly.

Hope this helps,

L.

[JamesF]

I'm surprised to be honest that the spider retaining nuts are even done up that tight.  There's no real need for the vanes to be under significant tension.

James

[Dave_D]

I'm surprised to be honest that the spider retaining nuts are even done up that tight.  There's no real need for the vanes to be under significant tension.

James

they're firm but certainly not excessively tight, that's the problem y'see... while the tube seems to have good stiffness along it's length, i'm not convinced that there isn't a small amount of give radially. it's possible that previous owners may have overtightened the spider at some point, but that's just speculation.

[LeeB]

As long as its not flexing in use , don't worry, aluminium will deform easier and think of the weight saving.

[Daniel-K]

can you not just back them off abit of put some nylon washers on the outside of the tube? if it was my scope it would drive me bonkers. i got my scope 2nd hand and TBH the cosmetically is v.good except for on ding on the OTA which annoys the hell out of me

[michaeldurban]

hello dave,

I had the same problem on my flextube...

I took out the spider, and then reinserted it...and

overtightened the bolts.

as a result I dented the OTA and even warped it a bit...

I removed the dents and then put washers in between the OTA and

the bolts...

then I re-tightened the bolts...finger tight...

I made sure my sec. mirror was centered and have had no problem

with viewing or collimation...

As said above, as long as your secondary is centered you should have

no problem with your viewing..

but it is an exercise in caution...it is very hard to determine what is

tight enough...or not..

some forums say the spider vane needs to have a 'ping' sound to it, not

much unlike a piano wire..

this is highly deceptive!!

one tends to over tighten the vanes, to get that 'ping'

my recommendation... secure the bolts finger tight...

[Dave_D]

shortened the tube and fitted the spider to the more sturdy top ring. makes a huge difference in how easy it is to collimate the top end of the scope

orion optics spider is still utterly pants though.

just need a clear night now to properly collimate everything.

[Daniel-K]

Dave how much did you shorten it? i hope you can still get focus

[C7tsj]

Dave how much did you shorten it? i hope you can still get focus

Judging by the changes between the two photos I guess it has been shortened by around 2 inches?

[Dave_D]

Dave how much did you shorten it? i hope you can still get focus

the physical distance between primary and secondary hasn't changed so there's no change to the optical side., but yeah, just shortened the tube by 2.5 inches. one thing that really annoyed me was the spider centering thumb bolts, the part that goes through the tube have a diameter of 8.33mm. i mean, come on, who in their right mind does that? (ok, orion optics does) took me longer to find a 21/64" drill bit to get a nice snug fit than to cut the tube 

[ScottS]

Only down side is the potential ambient light ingress onto the secondary now that it is that much closer to the end of the tube. Just a thought! Mind you, a dew shield would easily cure this.

[Dave_D]

Only down side is the potential ambient light ingress onto the secondary now that it is that much closer to the end of the tube. Just a thought! Mind you, a dew shield would easily cure this.

no probs there, i made a 20" shield a while ago

[michaeldurban]

Man,..

I applaud your efforts...well done !

it's time someone took a hacksaw to Orion...hehe