Quark and cemented Triplets **A Warning**

[Zakalwe]

This has been pasted over on Solarchat, but I thought that it was well worth repeating.

This horrific image is the result of two hours solar viewing using a triplet APO for solar observing/imaging. I'm not suggesting for a minute that the failure had anything to do with the Quark...it appears to be a danger of using a telescope that has the lens elements cemented together (a practice on *some* triplets) for solar work. The glued-together lens elements cannot handle the different expansions of the different types of glass.

:eek: :eek:

The moral of the story is be careful when using triplets or any scope where you cannot confirm that the elements are not air-spaced.

[SkyGibbon]

Would be same for wedges then?

[Stu]

Ouch!

I've used my Astrotech triplet a couple of times with the wedge, fortunately with no ill effects. I guess it's air spaced then!

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[Zakalwe]

Would be same for wedges then?

I can't see why not. The damage is apparently caused by the different rates of expansion of the glass elements.

With the arrival of the Quarks device, it's possible that many more people will be pointing their 'scopes at the Sun. This is a salutatory warning to those with cemented optics.

[estwing]

Ekkkk!

[nightfisher]

I can't see why not. The damage is apparently caused by the different rates of expansion of the glass elements.

With the arrival of the Quarks device, it's possible that many more people will be pointing their 'scopes at the Sun. This is a salutatory warning to those with cemented optics.

I hope thats not your own scope

[Owmuchonomy]

Crikey! Not ever having ventured into solar observing I am going to ask a very dim question. Doesn't pointing ANY scope at the sun bring the inherent danger of creating dangerous local heating and damaging the internals. I always thought you had to filter block your objective using specialised materials.

[ChrisLX200]

Nasty. An air-spaced achromatic doublet is all that's required for narrow band anyway, using an APO will add nothing to the image quality. I would certainly not point any of my APOs at the sun, but my f/11 Vixen 90mm works wonderfully well with a white-light solar filter and I assume it would work equally well with a Quark. A useful warning for those who may be tempted!

ChrisH

[Zakalwe]

I hope thats not your own scope

Thankfully no. But the poor sod that found out the hard way had only had the scope for a few hours.

Crikey! Not ever having ventured into solar observing I am going to ask a very dim question. Doesn't pointing ANY scope at the sun bring the inherent danger of creating dangerous local heating and damaging the internals. I always thought you had to filter block your objective using specialised materials.

Not necessarily. A Herschel wedge is designed to dump the heat load whilst passing a small amount of visible light to the eyepiece. The heat load only become dangerous when the light-cone is getting near to the focal point. A design like a Petzval is absolutely not recommended for solar work as the rear elements can be damaged. The exception to that would be where a front mounted Energy Rejection Filter is in front of the objective (or relatively close behind it). The ERF reflects the heat back out of the OTA.

[Peter Drew]

I'm amazed that ambient sunlight is hot enough to cause that sort of damage to a cemented component, I would have thought heating of the aluminium cell was more likely, transparent glass/cement would not normally store such excess heat. 

[russ.will]

Something doesn't ring true here. The pattern of deformation looks far more like the suns energy has been focused on the centre of the elements, which it is not when facing the sun - No more than it is on every square inch of your skin.

I'm wondering if some sort of UV/IR rejection filter (or some such reflective rather than absorbing type ) is reflecting the light cone back out through the front elements? The focal point of that reflection would be some way ahead of the filter and the light cone would be very much narrower as it meets the front elements on the way out.

I am probably way off track, but there does seem to be a 'focus' about the point where the damage has occurred and I can't think why that would happen on the first pass of the sunlight.

Either way - Gutted for the owner.

Russell

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[Zakalwe]

Something doesn't ring true here. The pattern of deformation looks far more like the suns energy has been focused on the centre of the elements, which it is not when facing the sun - No more than it is on every square inch of your skin.

I'm wondering if some sort of UV/IR rejection filter (or some such reflective rather than absorbing type ) is reflecting the light cone back out through the front elements? The focal point of that reflection would be some way ahead of the filter and the light cone would be very much narrower as it meets the front elements on the way out.

I am probably way off track, but there does seem to be a 'focus' about the point where the damage has occurred and I can't think why that would happen on the first pass of the sunlight.

Either way - Gutted for the owner.

Russell

Sent from my iPad using Tapatalk HD using chubby fingers. Sorry.

I think that what we are seeing is the separation of the glue that holds the lens elements together. That would start from the edge where there is most movement due to the glass elements expanding and contracting.

[Stu]

I think that what we are seeing is the separation of the glue that holds the lens elements together. That would start from the edge where there is most movement due to the glass elements expanding and contracting.

If I am interpreting the picture correctly, the lens elements themselves are intact (you can see that from the reflection in the centre 'hole'), but the cement has melted around the edges, leaving the centre untouched. Is that what you are saying Zakalwe?

Either way, I agree that first pass of the sun should not do this, I guess the cement used had a low melting point?

Stu

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[pete_l]

Something doesn't ring true here. The pattern of deformation looks far more like the suns energy has been focused on the centre of the elements, which it is not when facing the sun - No more than it is on every square inch of your skin.

I do think there's a little more to this. Have a look at the Daystar website (They make the Quark: an "eyepiece" for solar viewing).

On their page about their UV/IR filters they specifically state that their ERFs shouldn't be used with certain classes of telescope, including oil spaced triplets. One could suppose that the same caveat applies to this device, too.

If I am interpreting the picture correctly, the lens elements themselves are intact (you can see that from the reflection in the centre 'hole'), but the cement has melted around the edges, leaving the centre untouched. Is that what you are saying Zakalwe?

Interesting. My first look at the picture made me think that the patterns we were seeing were stress fractures in the glass (look at the point at about "7 o'clock"). But melted glue seeping in now sounds just as likely.

[Stu]

I do think there's a little more to this. Have a look at the Daystar website (They make the Quark: an "eyepiece" for solar viewing).

On their page about their UV/IR filters they specifically state that their ERFs shouldn't be used with certain classes of telescope, including oil spaced triplets. One could suppose that the same caveat applies to this device, too.

Interesting. My first look at the picture made me think that the patterns we were seeing were stress fractures in the glass (look at the point at about "7 o'clock"). But melted glue seeping in now sounds just as likely.

I think one of the reasons Petzvals are not recommended is that the second doublet is cemented, and because it is closer to the focal point there is more heat build-up. You are right, a cemented triplet probably should be considered in the same way as an oil spaced triplet as the material between the glass elements is likely to expand (or melt!)

It does look a little like a stress crack at 7 o'clock. I suppose it could be either, perhaps we will find out. The looks to be cement seeping from between the events at 2 o'clock ish too.

I wonder if anyone will pick up the cost for this? I wasn't an obvious blunder by the user I don't think?

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[chiltonstar]

I lost a pair of 16x50 bins many years ago climbing in the Alps; they were subjected to -25C at night and the differential expansion tore the cement in the doublets apart producing an effect a little like this. 

Chris

[Peter Drew]

I've purposely separated cemented objectives in the past, this required placing them in water and raising the temperature gradually, they had to be boiled for some time before the cement, Canada balsam, melted. I'm still sceptical that ambient sunlight could in itself cause the damage shown.

[Zakalwe]

 I'm still sceptical that ambient sunlight could in itself cause the damage shown.

Register at Solarchat and have a word with the owner. His comments were "After ca. 2 hours (more breaks than testing) I couldn't belive what I saw. The view was shocking..."

He originally thought that it was the lens cells pinching and causing the damage.

[Peter Drew]

Can't seem to find the post on Solarchat, what heading does it come under?

[Zakalwe]

Can't seem to find the post on Solarchat, what heading does it come under?

Here you go Peter:

http://solarchat.natca.net/viewtopic.php?f=4&t=11593

[Mark at Beaufort]

When I started to read this thread I became concerned because I use my 4" Astro Tech APO a lot with the Herschel Wedge. Lunt state this in their literature (see below) and clearly if you are going to use your refractor it really must be air spaced and not cemented.

I have now checked my Astro Tech manual and it states 'air spaced'

[John]

I guess this is the same reason that Huygens eyepieces were recommended over more complex types for doing solar projection - no cemented elements.

Does anyone still do solar projection these days ?. It used to be said to be the only safe way to view the sun but that was before effective front aperture solar filters were readily available to amateurs.

[Peter Drew]

Thanks Zakalwe. Makes interesting reading. I've had considerable experience with cemented or oil spaced doublets and triplets and this problem is a new one on me. Differential expansion or a tight cell could be part of the issue but you would have thought that differential contraction due to falling temperature would have more often caused similar problems in the past. The light transmission benefits of cementing/oiling components is well known, however cementing the inner component of a triplet which is usually a biconcave lens is a risky business. I have used my 5" and 6" doublets for long tracking periods for Ha viewing and have never noticed any undue heating of the objective whilst using internal ERFs. An internally mounted Baader D-ERF reflects most of the focused heat out of the front of the telescope if placed slightly more than 50% down the lightpath, the focal point outside the objective can burn your hand but does not seem to heat the objective. The resulting advice to avoid triplets for solar use, particularly the model affected seems worth following whatever the root cause. 

[russ.will]

Nice explanation Peter. That's sort of what I was getting at, but less eloquently!

Russell

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[Owmuchonomy]

Thanks Zakalwe. Makes interesting reading. I've had considerable experience with cemented or oil spaced doublets and triplets and this problem is a new one on me. Differential expansion or a tight cell could be part of the issue but you would have thought that differential contraction due to falling temperature would have more often caused similar problems in the past. The light transmission benefits of cementing/oiling components is well known, however cementing the inner component of a triplet which is usually a biconcave lens is a risky business. I have used my 5" and 6" doublets for long tracking periods for Ha viewing and have never noticed any undue heating of the objective whilst using internal ERFs. An internally mounted Baader D-ERF reflects most of the focused heat out of the front of the telescope if placed slightly more than 50% down the lightpath, the focal point outside the objective can burn your hand but does not seem to heat the objective. The resulting advice to avoid triplets for solar use, particularly the model affected seems worth following whatever the root cause.

So just to be clear, one would be fine using an air spaced triplet for solar viewing with Baader type solar film filter over the objective (external)?