Sky watcher 250P Flextube Collimation issue

[Grant93]

Hi there,

I've had my Dob for a few months now, and been out in the garden as much as possible enjoying the night skies as much as the clouds let me.

Really loving the views you can get through 10" of aperture compared to what my old Tasco use to give me (had it from being a kid).

The only issue I'm having is it struggles to hold collimation, I have to recollimate every time I set up. I take the scope off the mount every time I move it outside, and back inside when I'm done, placing it very steadily every time I put it down. After collimating it I make sure I see the perfect doughnut in an unfocused star, so I hope I am collimating it correctly. 

From what I read these Dobs seems to hold collimation pretty good, and shouldn't need it every use. Just wondering if I could be doing anything wrong - and hoping its nothing wrong with the scope itself!

Quick note, I dont think its this but it concerns me everytime - I tighten the primary mirror knobs finger tight every time, not too tight as I dont want to 'pinch' the mirror - Should I maybe try tightening harder? Or is it easy to pinch the mirror?

Thanks for your help in advance!

[johninderby]

The primary mirror screws just press against the mirror cell. You can’t pinch the optics by tightening them. They want to be tight but no need to overtighten. Are you tightening the lockscrews after collimation? 

Edited February 27, 2021 by johninderby

[Grant93]

12 minutes ago, johninderby said:

The primary mirror screws just press against the mirror cell. You can’t pinch the optics by tightening them. They want to be tight but no need to overtighten. Are you tightening the lockscrews after collimation? 

Yes I am tightening them after - Finger tight, this is where im wary of pinching the mirror, so I dont over tighten them. Might of used the wrong names in the above post

[johninderby]

No you can’t pinch the mirror by over tightening the screws  so don’t worry about it. They just touch the metal mirror cell underneath the mirror not the mirror itself.

[Mamm7215]

Dobsonians typically need collimation every time you take it out, even if just a small amount.  I have the same scope, I move it from my sitting room to my back deck gingerly every time I take it out.  I have a well collimated laser collimater and almost every time I need to make a small adjustment.  Takes 1-2 minutes tops.

[SMF]

25 minutes ago, Mamm7215 said:

Dobsonians typically need collimation every time you take it out, even if just a small amount.  I have the same scope, I move it from my sitting room to my back deck gingerly every time I take it out.  I have a well collimated laser collimater and almost every time I need to make a small adjustment.  Takes 1-2 minutes tops.

Second what Mamm7215 says, mines a solid tube OO UK and although it holds collimation well it does need a minor tweak most outings if bumped a little. The 200p on the other hand only needs you to look at it and the collimation can be way off.

Takes a few seconds to collimate both. I use a 1.25” Hotech laser collimator which itself is easy to calibrate if needed. I think they are one of the few laser collimators that allow easy access to the setting screws. 

Edited March 1, 2021 by SMF

[Grant93]

Cheers for the reply's.

Yeah I have no problem with the collimating, as long as its normal and nothing wrong with my scope!

[Ricochet]

I always used to have to collimate my dob at the start of my session, but the last time I collimated the telescope I tightened the collimation/lock screws more tightly. Since that time I've not had to recollimate my telescope. I took a photo of the collimation at the time and recently I wanted to post that photo. When I had scrolled back far enough to find it, I discovered it was taken in something like April 2019. So my advice of you are collimating at every session is to tighten everything up more securely and see if that helps the telescope hold collimation. 

[Grant93]

4 hours ago, Ricochet said:

I always used to have to collimate my dob at the start of my session, but the last time I collimated the telescope I tightened the collimation/lock screws more tightly. Since that time I've not had to recollimate my telescope. I took a photo of the collimation at the time and recently I wanted to post that photo. When I had scrolled back far enough to find it, I discovered it was taken in something like April 2019. So my advice of you are collimating at every session is to tighten everything up more securely and see if that helps the telescope hold collimation. 

Now that I know I can't pinch my mirror by doing this, I'll try it next time! 

Cheers!

[Pixies]

7 minutes ago, Grant93 said:

Now that I know I can't pinch my mirror by doing this, I'll try it next time! 

Cheers!

Not sure about the SW, but when tightening the Bresser lock screws, it effects the collimation. So keep checking as you finish off and use it like a final fine-tuning.

[John]

38 minutes ago, Pixies said:

Not sure about the SW, but when tightening the Bresser lock screws, it effects the collimation. So keep checking as you finish off and use it like a final fine-tuning.

I've found this with a few newtonian primary cell designs. I leave the lock screws undone on my 12 inch dob now, unless it's going in the car somewhere.

 

[alacant]

On 28/02/2021 at 00:11, Grant93 said:

Should I maybe try tightening harder?

Hi

Assuming you're leaving the telescope with the poles extended, the best way we've found is to forget the retaining bolts and instead fit stronger springs, 1.6mm wire compression springs replacing the three existing items and fitting the same over the retaining bolts giving 6 springs altogether. The latter remain loose and are there simply to retain the 'passive' springs. Collimate once with three screws instead of twice with six;)

Finally, remove the mirror and fix back in position with a blob of silicone sealant over each of the cork mounting points, thus preventing lateral movement.

Springs available here.

HTH

 

Edited March 2, 2021 by alacant

[KP82]

1 hour ago, Pixies said:

Not sure about the SW, but when tightening the Bresser lock screws, it effects the collimation. So keep checking as you finish off and use it like a final fine-tuning.

It's the same with my SW 200P based on my own experience. I have to tighten each one of the 3 lock screws a little bit at a time rather than fully tighen one and then move onto the next one. Otherwise the collimation gets slightly shifted.

Edited March 2, 2021 by KP82

[tripleped]

I have the same scope and I learned that those 3 little tighteners where the truss bars attach to the scope tube need to be pretty tight as they can loosen with vibrations. Noticed that mine were too loose and once I tightened them down collimation is very close each time I take out the scope. Still needs some tweaking each time but that’s to be expected with this scope. And yes don’t be afraid of overtightening the mirror lock screws. Took me awhile to learn that.

[SMF]

To be honest I find if you want perfect, and I mean perfect, right on the nose collimation (which isn’t absolutely necessary) then I find no matter how hard screws are tightened or otherwise, even with a solid tube it’s easier to just tweak or check the collimation each session. Takes less than a minute. It’s part of the reflector/dob experience.

So many aspects affect it, temperature, vibration, repeatability of the collimation tool being used, etc, that I find it easier to accept a quick collimation is just part of the set up procedure. And that’s only if it bothers me or the view is noticeably affected. 

[faulksy]

when you colimate, try and use the same 2 screws all the time. either pushing or pulling. other wise you will run out of adjustment because you will be just pushing the mirror up the tube 😁

[Grant93]

So happy with all the advice being posted in here, its very helpful and reassuring to know my equipment is fine and I seem to be on the right track. Too many to reply to everyone! 

But just to say, I'm happy to collimate every time, like everyone says its a 1 minute job, hardly any hassle - just wanted to make sure everything I'm doing is normal.

Thanks again!

[alacant]

23 minutes ago, Grant93 said:

a 1 minute job,

Indeed it is.

I think however it is more important that after your 1 minute tweak, the collimation holds at any telescope angle though the session, not simply at the angle of the telescope at the time that the collimation is performed. We haven't found a cheap Newtonian that holds out of the box without modification. 

But hey, we're imagers and so probably have no right posting here anyway. Hoping it may help though.

Cheers

Edited March 2, 2021 by alacant

[Grant93]

11 hours ago, alacant said:

I think however it is more important that after your 1 minute tweak, the collimation holds at any telescope angle though the session

I do regularly check throughout the session on bright stars, it always holds during the session, never had any issues with that!