79 replies to this topic

[Oberon]

Like all obstructions, mirror clips create diffraction and scatter light, reducing contrast and in worst cases even visible spikes. Following on from our findings in this thread about secondary obstructions I thought I'd better run a few designs through Maskulator to see if some types of clips are better than others. For example, when I built Merope's cell I carefully machined a set of smooth gently curved circular protrusions hoping to minimise diffraction. 



But it turns out I was wrong.

According to Maskulator, those nicely rounded clips produce a relatively high level of diffraction, and other designs are better. As we shall see.



 

[Oberon]

Merope sits somewhere between the two top designs, which are the same except the RHS covers more area. As this is the worst set of 3 sets of 9 attempts (total of 27 alternatives attempted) you can see that Merope's nice smooth rounded clips are somewhere in the zone of being the worst of the worst.

 

Center LHS is also circular, assumes 20mm diameter washers, and looks better. But this is a 500mm mirror, so unrealistically small.

Center RHS are just plain square clips 20mm wide.

Bottom LHS is looking more promising despite being a set of pointed spikes.

Bottom RHS tries seriously but fails; the clips curve with their radius at mirror center; even the sides are curved. But to no avail.

Each of these designs suffers from being 4 clips at 90 degrees apart. This means any diffraction effects are doubled, as in a common garden variety spider, and more likely to be noticed.

Edited by Oberon, 07 September 2015 - 10:38 PM.

[Oberon]

This set duplicates the previous set in some ways, the key difference being each mirror only has 3 clips. Some of them I set 120 degrees apart, others are 90 degrees apart at the bottom (just because some people align their clips with their edge supports), but that's not important. What is important is that in each case 3 clips has less of an effect than 4 clips.



Predictably bigger clips are worse. So thus far we have established two basic rules...

Rule #1: Less is best; 3 clips are better than 4.

Rule #2: smaller is better (but must be big enough to protect mirror without chipping)

...but nothing really conclusive about shapes. Yet.

[Oberon]

Most designs in this set is remarkably better than either of the previous.

 

Until I tried the last two on the bottom I was pretty sure 3 x miniature volcano spikes were going to prove best, and so played around with sizes and tweaked the curves a little to see what worked. The results are pretty good, but not as good as the final two thrown in for completeness. The bottom two are just flat and straight, assuming a stiff piece of metal or plastic on its edge. The RHS bottom is 3mm thick and the LHS is 5 or 6mm thick (because I'd rather make a clip out of plastic than steel, and 3mm might be considered too thin for plastic).

Rule #3: don't worry about fancy shapes, just keep obstruction thin and to a minimum

Edited by Oberon, 07 September 2015 - 11:08 PM.

[Oberon]

Note: the exposures on all of these are greatly exaggerated to bring out the effects. The exercise is purely about comparing the relative diffraction effects of different designs with each other.

[Michal1]

Hi Oberon, great idea to start this thread. I was solving this question recently but I didn't find the time for playing with Maskulator. Based to certain pseudoarguments, I guess that these two mask shapes could produce spikeless PSFs (I will explain if they turn out to be good):

 

1) The clip masks are circular having the same radius as the mirror.

2) The masks are circular again, but the arc-lenght of each mask is 60 degees (for thee clips 120 degrees apart). Maybe this is the top-left figure in the post #3.

 

I would be thankful if you coud try these masks for the planetary Newtonian I'm currently building -- 200mm, f/10, 15% CO.

Edited by Michal1, 08 September 2015 - 07:20 AM.

[gdd]

Why not omit the safety clips? Make your cover about 1 or 2mm smaller than the mirror to be your circular safety clip.

 

Gale

[jtsenghas]

1) The clip masks are circular having the same radius as the mirror.

 

This is the one I'd expect to show the least effect, unless an entire circular edge retainer is used as Gale suggests, or unless a method is used to grasp the mirror (with clearance to avoid astigmatism, of course) outside of the mirrored surface.

 

Although the area covered would be slightly greater than usual edge clips, the total aperture outer edge arc length would remain the same as would the distribution of angles. Those angles would just be swapped slightly clockwise/counterclockwise at clip locations.

 

My 10" mirror is actually 10.25" OD with an 1/8" bevel, yielding a 10" optical surface.  My mirror clips are locking clamps that are loosened during use that contact only on the bevel and scarcely overhang (less than 1 mm). That bevel is larger than it needs to be, but it seems to work to my advantage this way. I guess they would be a three-clip version of the first image of your third post, but only extending about 1 mm inwards.  I don't see any spikes from even the brightest stars from them.

[tim53]

For my 10" newt, I built a mirror cell out

of plywood and used two layers of veneer to make sides as high as the mirror around the circumference. The. I put double stick foam tape - the kind used to hold weatherstripping on a car at freeway speeds - at 3 places around the mirror to keep it from moving longitudinally. So I have no clips. I never point my scopes toward the ground, but I don't think it would fall out even if I did. I might have to destroy the cell to get the mirror out to re coat, but it's sealed so I can (and do) clean it in the cell.  The whole thing was made with scraps, so it won't be a problem building another one. 

 

Tim

[Nils Olof Carlin]

My suggestion: make the clips edge-on, and cut (perhaps 45 deg to the mirror plane) so that they will contact only the bevel, if/when they are called to duty. They can be thin and short, made of plastic if you like, and obstruct only by some percent of what spider vanes do. See #4, lower right.


Nils Olof

[Arjan]

That sounds like a good idea Nils Olof.

For ease of construction I've used screws as short as possible, with a piece of rubber hose as a nut. Probably a bit like bottom left in #4.

[Starman1]

I have movable clips.  They rotate into place to help preserve the mirror in transport, but they rotate out of the way at night.

Why, once the scope is in place for the night's viewing, would you need clips on the mirror?

Could my mirror fall forward out of the cell if the scope went below the horizontal?  Sure, but i have stops to prevent the scope from going that low.

Plus, my edge supports are in the mirror's center of gravity so the mirror does not come forward off the pads when it points low.

So my basic question is why observe with clips in place?

[Jeff Morgan]

Why not omit the safety clips? Make your cover about 1 or 2mm smaller than the mirror to be your circular safety clip.

 

Gale

 

Probably most people will find the loss of 2mm aperture to be absolutely unacceptable and heretical, even in cases where the overall correction of the mirror would be improved. More is always better, right?

 

In my own case with a Dobsonian telescope, the clips are only in place during transport and storage. I rotate them out of the way for observing. Not much risk of the scope going below the horizon and creating a situation where the mirror could pitch forward and leave its cell.

[Jeff Morgan]

Come to think of it, there is a risk for users of equatorial platforms. I know because it happened to me!

 

The situation was observing close to horizon. All was fine and good until it was time to reset the platform. The scope body itself slid sideways out of the rocker box and then the mirror did indeed leave the cell and picked up a nice surface scratch.

 

The solution was guides or keepers on the rocker box such that the tube can not move side to side. I have incorporated these into all of my Dobs since the accident.

[Oberon]

I have movable clips.  They rotate into place to help preserve the mirror in transport, but they rotate out of the way at night.

Why, once the scope is in place for the night's viewing, would you need clips on the mirror?

Could my mirror fall forward out of the cell if the scope went below the horizontal?  Sure, but i have stops to prevent the scope from going that low.

Plus, my edge supports are in the mirror's center of gravity so the mirror does not come forward off the pads when it points low.

So my basic question is why observe with clips in place?

I agree. I agonised over this decision when designing Merope, but worried that the day I needed the clips in place would be the day they weren't there. The old Murphy's law; if anything can go wrong one day it will. I considered a tilt stop, but know perfectly well the moment I am not looking a user will force the telescope past the stops, so it isn't foolproof.

I even built a clip that dropped into place triggered by tilt, but again worried that it wouldn't work under some unforeseen circumstances. Merope was designed to travel in the boot/trunk of my car; many years ago I rolled a car (3 times), which has a way of jumbling everything together violently. So basically I wanted Merope's mirror to be as secure in its cell as it would be in a travel case, with solid clips always there to protect against unforeseen circumstances, and designed a clip to meet all those requirements with minimal effects on diffraction.

It turns out I only got the first bit right.

Edited by Oberon, 08 September 2015 - 06:36 PM.

[Oberon]

Hi Oberon, great idea to start this thread. I was solving this question recently but I didn't find the time for playing with Maskulator. Based to certain pseudoarguments, I guess that these two mask shapes could produce spikeless PSFs (I will explain if they turn out to be good):

 

1) The clip masks are circular having the same radius as the mirror.

2) The masks are circular again, but the arc-lenght of each mask is 60 degees (for thee clips 120 degrees apart). Maybe this is the top-left figure in the post #3.

 

I would be thankful if you coud try these masks for the planetary Newtonian I'm currently building -- 200mm, f/10, 15% CO.

Hi Michal1 I actually did your #1 test, but somehow it got missed. Here it is. Up there in the top 10 but nowhere near the best.

[Oberon]

And test #2 I had sort of done but was unsure how carefully I had aligned the curves to describe 60 degrees. So I did it again and made sure the obstructing arcs were exactly 60 degrees.

 

[Oberon]

This is an interesting comparison.

Following up in spirit Nils suggestion about very tiny clips close to edge I reduced the protrusion from 10mm to 5mm but increased the number of clips to keep it practical. On the RHS there are 12 clips equally spaced, but once again we suffer effects of duplicated angles. Center are 9 clips spread 40 degrees apart (which is the smallest odd number greater than 3 available to be divided evenly into 360) which does better.

 

Note: dimensions may be scaled around the standard mirror being tested here at 500mm

Edited by Oberon, 08 September 2015 - 09:31 PM.

[jtsenghas]

Considering how much those are overexposed, I would expect even the ones with duplicated angles to be nearly invisible at the eyepiece.  How bad, for example, are Merope's diffraction spikes from the crescent clips that appear disappointingly bad in Maskulator on Sirius or other objects?  Those clips aren't aligned with your spider, right?

[Oberon]

Why not omit the safety clips? Make your cover about 1 or 2mm smaller than the mirror to be your circular safety clip.

 

Gale

Basically because the cover would need to be about 5mm smaller to make sure the mirror didn't fall halfway through and jam. And masks actually require a very clean edge or else they won't be an improvement.

But yes, observing without clips is ideal. If that's a risk you want to take.

Another idea is to glue some sort of safety bracket to the mirror. Normally the bracket is free and unconstrained, not applying any force to the mirror. But should the mirror lift off its support, then the bracket would be caught by a constraining mechanism; anything, a loose cable tie would do.

[Oberon]

Considering how much those are overexposed, I would expect even the ones with duplicated angles to be nearly invisible at the eyepiece.  How bad, for example, are Merope's diffraction spikes from the crescent clips that appear disappointingly bad in Maskulator on Sirius or other objects?  Those clips aren't aligned with your spider, right?

I don't know. I'll put Merope back together soon now that we've settled into our new home and have a look soon. The clips are not aligned with my spider so I should be able to separate the effects.

But I agree. This exercise is first a matter of testing principles for the more retentive among us. Had I known what I now know I would probably have built Merope's clips differently.

[jtsenghas]

I agree wholeheartedly with your approach, and see how Maskulator can help you choose the best options. For astrophotography this may make a huge difference.  For visual use, at some point negligible is,....well.....negligible. Right?

[Oberon]

Even with visual use it should make a difference to noticing and appreciating subtle details on bright extended objects. I'm thinking mainly the Moon, Jupiter and Saturn. I was stunned at the difference Merope's 16" made to viewing the moon over my older 8" F7 for example, I hadn't expected that.

[gdd]

 

Why not omit the safety clips? Make your cover about 1 or 2mm smaller than the mirror to be your circular safety clip.

 

Gale

Basically because the cover would need to be about 5mm smaller to make sure the mirror didn't fall halfway through and jam. And masks actually require a very clean edge or else they won't be an improvement.

But yes, observing without clips is ideal. If that's a risk you want to take.

Another idea is to glue some sort of safety bracket to the mirror. Normally the bracket is free and unconstrained, not applying any force to the mirror. But should the mirror lift off its support, then the bracket would be caught by a constraining mechanism; anything, a loose cable tie would do.

 

A completely circular mirror retainer will not let the mirror fall through even if it is only 0.5mm smaller in diameter than the mirror. That is why manhole covers are round, so they can't fall into the hole no matter how hard you try. If the mirror has a bevel, the bevel can be completely covered by the mirror retainer with no loss of light.

 

Gale

[Oberon]

There is a reason why manholes covers are more than 1 or 2 mm larger than the hole.