Advice on newtonian secondary job

[iPeace]

I have a newtonian scope. Collimating the secondary mirror has been a female horse of the nocturnal variety for me. I now have it in a serviceable state, but I'd like to make it a lot easier. My other newt has convinced me this must be possible.

I have few tools and my first inclination is to go for a commercially available drop-in replacement for the secondary holder (and spider vane, if necessary). This would involve either re-using the current secondary mirror (thus removing it from its current holder) or acquiring a new secondary mirror (which would make it easier to go back to the current state, should things go pear-shaped).

Either way, I'd have to glue a secondary mirror to a holder - something I'd very much like to get right the first time.

I'd be happy to take your thoughts on board before setting out on this particular adventure. Any advice on removing and replacing a secondary mirror, or indeed alternative ways of making it easier to collimate the secondary mirror, are welcome.

P.S. yes, I've used - and always will use - Astrobaby's collimation guide. This is not about knowing what to do, more about making it mechanically easier. The secondary on my other newt is an absolute breeze to collimate. I am willing to invest to make it at least that easy on the newt in question.

Thanks for reading and for any contribution you may have.

[Pig]

Can you not replicate the mechanics your other newt uses? ?

[Stu]

What is it about the holder currently which makes it so difficult? Some pictures might help.

I’m sure your will get some good advice on removing the secondary and re-fixing it. Positioning is obviously key, and ensuring any offsets are maintained. I’m not the right person for that though!!  

[iPeace]

6 minutes ago, Pig said:

Can you not replicate the mechanics your other newt uses? ?

Indeed, my first thought. This would involve replacing the spider vane and secondary holder with ones similar to those on the other newt. So far so good.

This leaves me with the prospect of (removing and re-) attaching a secondary mirror, something I've never done before.

P.S. Thanks anyway

[iPeace]

9 minutes ago, Stu said:

What is it about the holder currently which makes it so difficult? Some pictures might help.

All I know is that on the one newt, in order to adjust the alignment of the secondary, I can loosen one bolt and tighten another. It just works. On the newt in question, no matter what I do - and I've done all there is to do - I always end up in a situation where the bolt that needs tightening will go no further, while loosening either of the others has no positive effect. Now bored with this and convinced by the other newt that there's a better way, which I'd like to approximate.

Can't get any pictures just now, but asap.

[alanjgreen]

32 minutes ago, iPeace said:

All I know is that on the one newt, in order to adjust the alignment of the secondary, I can loosen one bolt and tighten another. It just works. On the newt in question, no matter what I do - and I've done all there is to do - I always end up in a situation where the bolt that needs tightening will go no further, while loosening either of the others has no positive effect. Now bored with this and convinced by the other newt that there's a better way, which I'd like to approximate.

Can't get any pictures just now, but asap.

Sounds like you need to screw the whole secondary mirror up the tube by turning it on its thread. Do you have a sight tube to view the secondary through the focuser?

If you loosen the collimation screws (you could remove one completely to see how long the thread is? ) so you have some play for collimation later. Then screw the whole mirror assembly back up the tube (viewing through sight tube to get in central). Now collimate from here.

 

You should only ever collimate using two of the screws (once you have it right the first time) this will prevent the whole thing from creeping up the tube as you continually adjust the collimation.

hth, 

Alan

[iPeace]

3 minutes ago, alanjgreen said:

if you loosen the collimation screws (you could remove one completely to see how long the thread is? ) so you have some play for collimation later.

Tried this. The central bolt goes through the spider vane into the secondary holder. When the central bolt is loosened so that the holder can be further up the tube, contact with the collimation bolts is lost and there's nothing left to keep the holder from flapping about. It's only when at least one of the collimation bolts is touching the holder that it stays still. It seems to me like there's something missing from this design...

So I'm now seeking a better solution, one that just works like the other.

[alanjgreen]

2 minutes ago, iPeace said:

Tried this. The central bolt goes through the spider vane into the secondary holder. When the central bolt is loosened so that the holder can be further up the tube, contact with the collimation bolts is lost and there's nothing left to keep the holder from flapping about. It's only when at least one of the collimation bolts is touching the holder that it stays still. It seems to me like there's something missing from this design...

So I'm now seeking a better solution, one that just works like the other.

Can you post a picture of it?

[iPeace]

1 minute ago, alanjgreen said:

Can you post a picture of it?

Not just now, but I will, thanks.

[Chriske]

During course(35+ years) I never used that 'central bolt system'. Sorry guys but I think it's rubbish.
I let all coursemembers choose for themselves what secondary holder they're planning to install. A commercial(central bolt) or one of own design.
That choice is made after I explain and demonstrate how to collimate the two holders side by side. Not one course member has ever chosen that 'central bolt system' ever.

[iPeace]

1 minute ago, Chriske said:

During course(35+ years) I never used that 'central bolt system'. Sorry guys but I think it's rubbish.
I let all coursemembers choose for themselves what secondary holder they're planning to install. A commercial(central bolt) or one of own design.
That choice is made after I explain and demonstrate how to collimate the two holders side by side. Not one course member has ever chosen that 'central bolt system' ever.

I'd be interested in alternatives.

[Chriske]

I show a drawing assembly, so I can remove all parts that are in the way to make all very clear.
So for this reason I removed the (curved) spiders.
The red part is the actual secondary 'body'. On the perimeter of that body there are three small flats. The spiders are screwed against these flats. Six (M5) holes (threads not in drawing, sorry) are for fixing the 3 spiders(green bolts). Then at the front there are at 120° three holes for M5 grubscrews(yellow). These three 'frontscrews' should not be used during collimation toward the primary..!.
The three rear M5 (bleu)screws are there to correct that mirror's direction toward the primary mirror. But that is in fact the very last thing you'll be doing.

Before you do all that, and that is the beauty of this system, you first adjust the height of the secondary mirror in the telescope tube --> and at the same time you rotate the secondary mirror's direction to rotate that secondary mirror exactly toward the focuser tube. During that operation all yellow and also bleu grubscrews are fastened lightly, in fact so lightly that the thin rod can move between the setscrews. When the secondary mirror is in position you only fasten the yellow setscrews a little bit, just enough to hold it in place.
Make a peephole, use a laser collimator or whatever, the last thing to do now is looking from the focuser : adjust the secondary mirror's direction toward the primary mirror. To do that you ONLY have to use the three bleu(!) setscrews. While you're doing that the yellow setscrews act in fact as 'rotation' point. When all is well collimated the yellow setscrews can be fastened a little bit more as well. While fastening the bleu setscrews a last time you have to look for last minor corrections..

Warning, before you start this procedure, make sure the secondary mirror and its metal rod cannot slide out of the secondary body while at it..!!!!
That's why the very thin(white) rod on top of the rod. You can leave it there, as that nail (or thick wire) is hidden behind the secondary mirror.

It is a very stable system. But the biggest advantage is : easy to use...!

In the end I didn't applied the flats anymore, they're not really necessary.
If you make one of these please do not hesitate to make a variant of it and please show us.
In the early days I made these secondary bodies out of aluminium, later on I casted them in resin and nowadays I print them. For that body I use white PETG. Reason for this is when observing the sun black (or a dark coloured)PLA could deform during solar observing.

[alacant]

Hi. Have you proper bolts for the adjusters so that you can turn them by hand? m4 on every Newtonian I've tried. HTH.

 

[iPeace]

13 minutes ago, alacant said:

Hi. Have you proper bolts for the adjusters so that you can turn them by hand? m4 on every Newtonian I've tried. HTH.

Thanks, I do have bob-type bolt heads...and matching blisters.

[iPeace]

18 minutes ago, Chriske said:

If you make one of these

I appreciate this very much. It's a fresh approach to the problem. Depending on what type of spider I go for, it could work this way.

It will require a bit more DIY than I originally had in mind...

I would much prefer something I can just buy or have made, to be honest, but I'll have to see what I can find.

Thanks!

[Chriske]

[iPeace]

As a commercially-available solution, I think I like the look of this

[alanjgreen]

43 minutes ago, iPeace said:

As a commercially-available solution, I think I like the look of this

My Lukehurst dob has this arrangement (minus the locking screws)

[CraigT82]

My fullerscopes newt has an arrangement similar to the image above, only with a smooth bar rather than threaded bolt.  This smooth bar controls the longitudinal position and rotation of the secondary and is held by a grub screw. 

On the secondary holder are 3 screws which control the tilt of the secondary.

This is the easiest system for secondary collimation I've used so far... much easier than the synta affairs