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Old 23-10-2014, 01:59 PM
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5ash (Philip)
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Replacing the mirror cell springs on a newtonian

I have an 8" Newtonian and want to bring the primary mirror about 1-2 cm forward in the tube , will need to buy some replacement springs. What criteria should I use in selecting them. Since the mirror is ultimately held in position by push pull screws the strength of the springs is not so important , however what about the diameter? Also what do I ideally replace the existing adjustment and locking screws with?
Regards philip

PS the secondary is oversize so it's not an issue

Last edited by 5ash; 23-10-2014 at 08:04 PM. Reason: Corrections
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Old 23-10-2014, 03:19 PM
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Ah did u mean Newtonian? or is this some new design we have not heard about.
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Old 23-10-2014, 05:39 PM
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5ash (Philip)
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Ha, ha . At my age my eyes don't catch all the spelling errors and often the iPad introduces them without me noticing. Anyway any help with longer springs on a Newtonian would be of help.
Regards philip
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Old 23-10-2014, 08:06 PM
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5ash (Philip)
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Have edited out the offending spelling error
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Old 24-10-2014, 06:40 AM
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Shiraz (Ray)
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depends what brand it is, but Bunnings have assorted spring packs and bolts to do what you need. Be careful that the adjusting bolts cannot protrude far enough to touch the mirror (maybe even use tubular spacers over the adjusting bolts to stop this happening).

However, I would recommend that you seriously consider shortening the OTA tube. It is easy to do, keeps the standard hardware and will result in a much more stable arrangement. The lock bolts will hold everything fairly well when tight, but they are backed off to adjust collimation and that is when the floppiness of longer adjusting bolts becomes a real pain. In use, the mirror will also move around more with changing attitude if you have long bolts, which can affect collimation and CC performance.

Last edited by Shiraz; 24-10-2014 at 06:51 AM.
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Old 24-10-2014, 07:30 AM
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mental4astro (Alexander)
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Springs ain't springs, and collimation is a much more complicated equation

I agree with Ray about looking into shortening the tube is one better option rather than putting the mirror on 'stilts'. The longer bolts add their own flex into the collimation equation. This in turn compounds the tendency of coil springs to buckle, made worse by the longer span they need to cover. Coil springs are not unidirectional, and once they buckle (which is what you hear when you load them and the creek and groan), they effectively drop in the linear strain to where you want the load to go as they relieve the strain imposed on them out to the side. Heavier gauge coil helps, but the problem is still there and this is the main source of collimation movement during the night as a scope swings around and the springs are given a chance to release some of their strain, but just a heavier gauge lone is only half the story.

One way to control this buckling is the have the inside diameter of the compression spring fit snuggly around the bolt. This will reduce how much the spring can buckle out to the side. Trick is to find a suitable spring with a thick enough gauge to be stable and strong. Mass produced scopes have springs that are too large in their inside diameter, not even taking into account the wire gauge is too thin too. Remember, they are made to a price, not best for the job. This is why they are often replaced, but only replaced with 50% of their necessary properties (meaning the coils are thicker gauge, but the inside diameter is still too large so they can still buckle). In some smaller instruments, springs are done away with altogether and are replaced with a single O-ring!!! Then you cannot ever collimate the scope properly,

The only other way to improve collimation stability is rebuilding the mirror cell to beef up the entire unit and then be in total control of the collimation bolt diameter (work it out correctly and you also don't need a locking screw), and spring choice is more easily catered for. You then have the ability to use disc springs instead of coil springs as these are entirely unidirectional, so they never buckle and so always give 100% of their loaded strain in the direction you want, not prejudiced in any particular direction. Disc springs also help reduce the profile of the cell as they are very thin to start with.

Don't think of rebuilding the cell as overkill. Rebuilding it also allows you to set the primary mirror closer to the secondary as your original intention is! You don't need to cut the tube, and you can re-use the existing retension holes in the tube's base to hold the new cell in place.

This is what I did with my 8" f/4 Newt. I bought the OTA new, but the cell was damaged out of the box, so I got it for a bargain price as I was confident I could make a new (and better) cell. I used heavey gauge coil springs with it as I was not aware of disc springs then, but the coils fit nice and snug over the bolt - no locking screws here either. Massive bolt too, 1/2" - collimation doesn't move at all, Raised the primary mirror by 2cm closer to the secondary.

Hopefully this gives you some food for thought.

Mental.
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Last edited by mental4astro; 24-10-2014 at 12:23 PM. Reason: clarification & more info
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  #7  
Old 24-10-2014, 01:29 PM
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ZeroID (Brent)
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I put spacers under my springs, Turned up some 25mm wooden disks, replaced the bolts with longer versions and slid the spacers and springs on. Spacers don't move and the holes in the spacers and the base ring are a firm fit on the bolts so that ain't going to move. Plus if it is going to be permanent then you can glue the spacers in place.
No lathe ? plenty of alternative 'spacers' at Bunnings or elsewhere, and they don't have to be round anyway. Just have the right size hole through middle and they can be square or oblate or ....

It's an easy and cheap way to shift the mirror up the tube. Cutting the tube is permanent and irreversible.
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Old 24-10-2014, 09:37 PM
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5ash (Philip)
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Thanks for all the advice and suggestions, I took the mirror cell out and found that the mirror cell has 3 countersunk (glued?) machine screws coming out of the bottom of the cell( had to remove the mirror to access them . They would not budge and as the cell is very lightweight did not want to force them. I am now looking at getting a cylindrical thread extenders made that would move the mirror forward and still use the same springs. Now to find someone to make them!
Regards philip
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Old 26-10-2014, 09:03 PM
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Steffen
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Quote:
Originally Posted by ZeroID View Post
Cutting the tube is permanent and irreversible.
And not easy to get right! I've cut my tube three times already and it's still too short...

Cheers
Steffen.
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Old 28-10-2014, 06:33 AM
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ZeroID (Brent)
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Quote:
Originally Posted by Steffen View Post
And not easy to get right! I've cut my tube three times already and it's still too short...

Cheers
Steffen.
Turn the blade around and uncut it ....
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