Lukehurst-Nichol classic dobsonian modifications

[jetstream]

1 hour ago, Piero said:

As far as I know secondary mirror can cause astigmatism in the optical system when they are glued to the holder by a large amount of glue or silicon rather than three small points.

Any restraint can cause this -including my holders I read- but I just follow Astrosystems instructions and no issue. I'm not so sure about the 3 dab of silicone method- does  the secondary have an even gap all around?

As far as cells- my 200mm f3.8 OOUK has a nice cell that was maladjusted and the mirror was a bit too thick for it. As received there were used 3 dabs of silicone to "secure" it to the cell and the result was big astigmatism.

All contact points must support equal weight regardless of type.

The beauty of those fiber "button" supports is that they in themselves can offer "self" adjustment by the amount of squish the mirror imparts, ensuring even contact. My OOUK nylon tips were fussy to set up evenly and the mirror was jammed against the clips.

Obviously rotating the primary will show if it is the culprit, but there is always the slim chance that the sec might be an issue at the same time.

I'm puzzled by the transient nature of the issue though? Did the scope maker notice this when he tested it?

Edited July 26, 2019 by jetstream

[Piero]

The three dab of glue I mentioned were referred to the secondary mirror. 

I don't think these are the source of astigmatism, because if they were, astigmatism would be present all the time. I think the same for the primary mirror.

To me the fact that astigmatism is transient and last time there wasn't any (and the view was very good), suggests that a long cooling time can be the cause of this issue. This particularly considering the double layer of wood (mirror box + rocker box) and the current long daylight in the UK which causes the drop in temperature starting quite late.

In addition, the seeing was not good either in the first two sessions.

As I said previously, in the third session the airy disc was irregular only for 1/3, and the stars showed a kind of "half astigmatism". Considering the mirror box opening showed in the second photo of my first post, this suggests uneven cooling for the primary mirror to me.

Anyway I need more time for observing and testing.

Edited July 26, 2019 by Piero

[niallk]

7 hours ago, Piero said:

Regarding the collimation shift, I don't see how this would cause astigmatism though. 

I think thats right Piero - I was just explaining how I noticed the triangle was tight.  It was a pivot point around which the mirror slightly shifted when the weight of the mirror transferred more to the sling as the scope was lowered to the horizon.  With a thinner mirror, conceivably it could contribute an optical distortion due to stress.

The rear of my mirror box is completely open, and I'm in the habit of tilting the scope to the horizon, and gently pushing on the back of the mirror so that it swings a couple of mm away from the triangles.  Gravity self centers the mirror on the sling after travelling with the scope, and I allow it to rest back on the triangle contact points (on mine, 3 velcro strips keep the sling where it should be when transporting- if the sling goes slack when all the weight is on the cell when in my car).  Then I just point at say 60 degrees and collimate - only adds a few seconds to the process.

As you know, cooling (or heating up with those monster temperatures in the UK!) is key - for example I will give my 2" thick, 15" scope 2-3hrs with the rear fan on if I want to seriously look at planets.  I'm going to make a bracket for mounting an additional front forcing cooling fan one of these days...!

 

 

[Piero]

Thanks Niall, great post.

[Piero]

I think I found the main source of the problem.. 

[jetstream]

14 minutes ago, Piero said:

I think I found the main source of the problem.. 

Oh?!

Your keeping us in suspense?lol!

[niallk]

Tell us - you tease! 😂😂

[Piero]

 

The main cause of astigmatism is due to the incorrect installation of the Glatter's cable sling. There are no bearings allowing the sling to follow the mirror axis. Instead, the cable passes through the pads of the lower mirror edge supports.

Therefore, when the telescope is pointed at low altitude, the sling is not always parallel to the mirror plane axis. Instead, being trapped by the existing lower edge supports, the sling squeezes the mirror at the bottom, effectively disfiguring it, causing astigmatism. This also explains why there was no astigmatism when the Glatter's sling was loosened.

The two low mirror edge supports do work, so does the Glatter's cable sling. They don't work together, though.

In all this learning process, I also read an article written by Lockwood, an American experienced mirror maker. Interestingly, he considers Glatter's cable sling a rather outdated approach to mirror edge support. 

References describing this issue:

https://www.cloudynights.com/topic/570249-whats-so-special-about-glatter-slings/#entry7767543

http://www.loptics.com/articles/mirrorsupport/mirrorsupport.html

http://www.jpastrocraft.com/cells.htm

Edited July 27, 2019 by Piero

[Piero]

For now my cable sling is loosened in such a way that it does not touch the mirror. 

In the coming months, I will think about whether taking it off completely. For a mirror of this size and weight, the current mirror edge supports do work well and do not cause any astigmatism nor pinched optics. This was proved in my last observing session.

I like the Whiffletree with rolling bearings proposed in Lockwood website. To me it seems a much nearer approach than having a sling.

 

Being this a public forum, I think it is also fair to state that the whole telescope works really well, and I am more than happy with it. We all can be picky at the details, and being critical is a good thing in my opinion as it allows us to improve things and make them work better. I did have a problem with the sling but this can be solved easily and in any case a sling is not required for the correct functioning of the telescope. When observing planets at low altitude I always collimate the telescope at the altitude of the planets before observing anyway. In my opinion, this is the most accurate way.

In my last session (with loosened cable sling), this telescope gave me the best view of Jupiter I have ever had. Considering the intricate and delicate details on that planet, I think this is a sign of very good optics and telescope structure.

Edited July 27, 2019 by Piero

[jetstream]

2 hours ago, Piero said:

For now my cable sling is loosened in such a way that it does not touch the mirror.  

Great catch Piero!

I just knew it was this

Yes that will do it but the cable will work- can you widen those clips -or- actually do you have a picture of the cell with the mirror out? are those pads integral with the clips?

ps - you might dump the pads and use velcro.

Edited July 27, 2019 by jetstream
idea

[niallk]

Glad you've found what was causing the effect, and that with a simple workaround, the scope is delivering great views 

[Piero]

The pads can be widen a bit to allow the cable sling to move with the mirror. They are attached to the clips, not part of them. I'm just not sure I want to go for that route or take the Glatter's cable sling out completely. 

Velcro would be needed of course. This is an easy mod. Said this, velcro does not replace the padding. The bearings do and make sure that the mirror does not move to the left or right of the cell. This is actually quite important if the ground is not exactly flat.

Edited July 27, 2019 by Piero

[jetstream]

3 minutes ago, Piero said:

This is actually quite important if the ground is not exactly flat.

IME it is really not an issue, a properly adjusted cable holds the mirror centered ie I can wheel my trusses over very uneven ground, and observe with them far from a flat place and no issues. Minor collimation is needed, but my center triangle is never outside the cheshire dot and in fact only the triangle tips will show off center slightly. Even putting the 24"on the trailer, pulling it with the loader, unloading etc didn't throw it way off.

My cells use wood for the edge limiters (soft, no hard contact) and are not far away from the edge, but a gap is always maintained.

I agree rollers CAN be a benefit if tested in a cell, a friend is choosing something similar to the image and the site- Aurora Precsion says Zambuto had input in the cell design

[Piero]

1 hour ago, jetstream said:

IME it is really not an issue, a properly adjusted cable holds the mirror centered ie I can wheel my trusses over very uneven ground, and observe with them far from a flat place and no issues. Minor collimation is needed, but my center triangle is never outside the cheshire dot and in fact only the triangle tips will show off center slightly. Even putting the 24"on the trailer, pulling it with the loader, unloading etc didn't throw it way off.

My cells use wood for the edge limiters (soft, no hard contact) and are not far away from the edge, but a gap is always maintained.

I agree rollers CAN be a benefit if tested in a cell, a friend is choosing something similar to the image and the site- Aurora Precsion says Zambuto had input in the cell design

Nice mirror cell! 

How much gap is there between the sling cable and wood edge limiters (clamps?) of your mirror cells?

[Piero]

A nice post by Nils Olof Carlin: "[..] To design a support for the largest mirrors, the sling may be worth the effort - for the next-to-largest, a whiffletree arrangement is fine, but for more moderate sizes, a 2-point support is fine by far, and easily made well. This includes a "normal" 18" mirror and a bit up. If you want an upgrade, practical and economical considerations come into play - Howie G-s sling might be the easiest to add to an existing mirror support."  (https://www.cloudynights.com/topic/570249-whats-so-special-about-glatter-slings/#entry7768997)

[Piero]

I took the mirror cell out. The mirror clips seem to be glued.. why this?! How does one take the mirror out if the clips are glued? 

Detaching the edge supports is even more difficult as there is no way to pass a key through that..



[jetstream]

2 hours ago, Piero said:

How much gap is there between the sling cable and wood edge limiters (clamps?) of your mirror cells?

Back soon Piero- but there is about 1/8"-3/16"...

[jetstream]

3 hours ago, Piero said:

The mirror clips seem to be glued..

Piero, are those top right angle holders thread on to the vertical supports? I see a machined hole in the side I think- is there an allen screw in there or something?

[Piero]

13 hours ago, jetstream said:

Piero, are those top right angle holders thread on to the vertical supports? I see a machined hole in the side I think- is there an allen screw in there or something?

Yes, those right angle clips on the vertical supports are attached with an Allen screw. I unscrewed them in order to take off the right angle clips and found out that these don't come off.. it is as if they have been glued to their vertical support. Maybe the paint went in.

Edited July 28, 2019 by Piero

[Pig]

11 hours ago, Piero said:

Yes, those right angle clips on the vertical supports are attached with an Allen screw. I unscrewed them in order to take off the right angle clips and found out that these don't come off.. it is as if they have been glued to their vertical support. Maybe the paint went in.

A quick firm tap with a large hammer should do the trick 😀 I know you know I am only joking Piero.

[Piero]

14 minutes ago, Pig said:

A quick firm tap with a large hammer should do the trick 😀 I know you know I am only joking Piero.

I spoke with David this morning. He confirmed that no glue was applied between the right angle holder and the mirror edge support, but that it is possible that some sprayed black matt paint is attaching the two components. He suggested to use an adjustable spanner.

[Pig]

Yes, paint can give a surprisingly tight bond especially on clean surfaces.

[Piero]

The adjustable spanner did the job! Very glad that David offered this clue!

Anyway, I decided to take off the Glatter's cable sling as it caused too many issues and didn't proved to be necessary.

To do so I had to remove the mirror cell from the mirror box, and remove the right angle holders from the mirror edge support. Therefore, I took off the mirror from its cell, partially detached the velcro from the edge holders, and remove the sling from the edge holders. I then removed the cable supports. To do this, I had to detach the mirror cell from the wood base. Finally, the mirror was put back with its cell into the mirror box.

Interestingly, collimation was not far off, despite of all this work! After collimating the primary mirror, I checked how this was retained at different altitudes, and there was no difference from before. Good! 

Very glad that now this issue has been sorted out once for all!

Thank you everyone for the support with this. I look forward to a clear night now!

Here are a few photos.

[jetstream]

Looks VG Piero!

Question- what function do those screws set up above the triangles serve?

Also, here are some numbers relating to mirror cells if anyone is interested.

 

Configuration	Description	RMS at COG	RMS 2.5 mm above COG
180° cable sling	cable on half the mirror circonference	1.8 nm	6.6 nm
120° cable sling	cable on a third of the mirror circonference	7.6 nm	12.0 nm
90° cable sling	cable on a quarter of the mirror circonference	10.7 nm	18.5 nm
90° edge bearings	2 support points at 45° from vertical	4.0 nm	10.7 nm
60° edge bearings	2 support points at 30° from vertical	8.9 nm	14.6 nm
120° edge bearings	2 support points at 60° from vertical	16.7 nm	39.7 nm
mirror on edge	one support point at bottom of mirror	15.6 nm	28.6 nm
45° whiffletree	4 support points at 22.5° and 67.5° from vertical	2.0 nm	7.0 nm
30°/60° whiffletree	4 support points at 30° and 60° from vertical	2.8 nm	8.4 nm
30° whiffletree	4 support points at 15° and 45° from vertical	8.0 nm	12.8 nm
14°/45° whiffletree	4 support points at 14° and 45° with unequal load (Alois Ortner)	7.1 nm	10.5 nm

[Piero]

They support the primary mirror. They look identical to those installed on the triangles of Orion Optics mirror cells.