6" f5.9 Newtonian (Astro Systems) , ongoing project …… Hinds Mirrors

[Alan White]

@Stu what you suggest is exactly my path of thought.

@NGC 1502 indeed exceedingly fine mirrors, you initial suggestion of keep it classic is indeed the path I now intend. As usual I have to take a journey to reach my destination.

Recoating next then, and a 4 vane spider?or do I make a 2 vane curved one hmmm.

[SthBohemia]

@NGC 1502 what's your verdict on using silicon rubber? A ring of it around (near) the outer mirror edge and one or 2 others at various diameters. Not overly worried about mirror stress as the 8" mirror is annealed stress relieved pyrex 1.4" thick.

[NGC 1502]

9 hours ago, SthBohemia said:

@NGC 1502 what's your verdict on using silicon rubber? A ring of it around (near) the outer mirror edge and one or 2 others at various diameters. Not overly worried about mirror stress as the 8" mirror is annealed stress relieved pyrex 1.4" thick.

 

I have used silicone sealant intended for construction or repair of glass aquariums to secure a Newtonian secondary. It may be that regular silicone would suffice, perhaps experts in that field would know.  However given how disastrous it would be for the secondary to detach and fall on the primary, I’ve been prepared to pay the extra.  Richard Berry, a well known telescope guru many years ago, used silicone to secure the primary, so it would seem a good plan.

Commenting on your mention of stress released pyrex:-  Again I’m not an expert on that, but wouldn’t it be possible to stress the glass by improper mounting?   For instance, if edge clips as found on many commercially made scopes were clamped tight, even if the glass had been annealed and stress relieved, clamping down would surely stress the glass?

If silicone was used to mount a primary, I don’t think a complete ring around the edge would be necessary.  3 blobs on the back surface should suffice.  Advice from years back said support at 70% of mirror diameter from the centre, with thick nails under the mirror until the silicone had dried. This prevents the silicone from squashing flat. Once dried, remove nails.

Comments and corrections welcome 😊

 

[NGC 1502]

10 hours ago, Alan White said:

Reciting next then, and a 4 vane spider?or do I make a 2 vane curved one hmmm.

Hi Alan. Not sure what’s best. For deep sky fuzzies I don’t think it matters. But a top quality 6” is mainly a double star or planetary scope, that’s how I use mine.  I’ve never used a curved vane spider although many have. I think the choice comes down to this:-

The thick single arm secondary support you have holds collimation excellently, but gives a pronounced spike on very bright objects like Mars, Jupiter or Venus.

A thin 4 vane spider holds collimation adequately. On bright planets a much less pronounced cross spike.

A well designed and constructed curved vane- as said I’ve never used one, but people I trust like Gary Seronic says something like-  4 vanes concentrates the diffraction, curved vanes diffuse the diffraction across the whole field of view. Whichever you choose the diffraction is there, either concentrated into lines, or diffused across the field.

An expensive option with no diffraction (other than from the secondary itself) is a full aperture optical grade window that holds the secondary in place.  My only experience of that is my Edmunds Astroscan, but that’s definitely not a double star or planetary scope, but a low power wide field “sweeper” or comet catcher.  You almost certainly won’t be taking that route, but when contemplating it is an option.  Of course an optical window produces its own issues in addition to the cost, like how optically flat is it, how good are the coatings, where to I get one…..etc😊

DIY telescope making is a hobby within a hobby. But it’s a great hobby because the weather is totally irrelevant and you can spend many happy hours in your workshop, before binning it and starting again 😁 Yep, been there, done that!

Again-  corrections and comments welcome😊

 

[SthBohemia]

11 hours ago, NGC 1502 said:

t may be that regular silicone

I certainly would not use a general purpose silicon, probably would use something akin to Elastosil-27 (if memory is correct) and would swab the rear of the mirror sparingly, with nitric acid/distilled water to ensure good adhesion.

Re- Pinched optics due to overly tightened mirror clamps. I have not previously encountered/read about the problem with mirrors under 16" until joining SGL. Most mirrors I have made, largest 12.5", have been very securely clamped within their home made cells with no adverse optical degradation. However, all of those mirrors were a fifth to a sixth the thickness of the primaries diameter, and of high quality pyrex.

At times I have made curved spider vanes as suggested by N.E.Howard, 'Amateur Telescope Making' 1959. Somewhat undecided as to whether the extra effort was worth it.

Shudders, an optical grade window to hold the secondary... 😞 a double sided optical flat! Producing that would be akin to making a refractor doublet, would be easier to grind a Maksutov corrector plate than a wretched 2 sided flat.....

[NGC 1502]

21 minutes ago, SthBohemia said:

 

At times I have made curved spider vanes as suggested by N.E.Howard, 'Amateur Telescope Making' 1959. Somewhat undecided as to whether the extra effort was worth it.

Shudders, an optical grade window to hold the secondary... 😞 a double sided optical flat! Producing that would be akin to making a refractor doublet, would be easier to grind a Maksutov corrector plate than a wretched 2 sided flat.....

Indeed I have that book along with several similar by Richard Berry, Reg Spry, Charles Frank….

Fully agreed an optical window would be an extreme solution, but it has been done!   Not an option with a basic workshop like mine but one for an optical expert with a relevant optical workshop and time to spare!

[johnturley]

3 hours ago, NGC 1502 said:

Indeed I have that book along with several similar by Richard Berry, Reg Spry, Charles Frank….

Fully agreed an optical window would be an extreme solution, but it has been done!   Not an option with a basic workshop like mine but one for an optical expert with a relevant optical workshop and time to spare!

Astronomical Equipment, Luton (not Astro Systems), used to produce Dall-Kirkham Reflectors with optical windows in the 1970's (see attached images), would be interesting to know how many of these they produced, and how many are still in use.

They claimed that these instruments would perform similar to a refractor of the same aperture, maybe a bit optimistic, or maybe not if you compare them to the refractors that were available at the time.

John 

Edited November 9, 2022 by johnturley

[JeremyS]

3 hours ago, johnturley said:

Astronomical Equipment, Luton (not Astro Systems), used to produce Dall-Kirkham Reflectors with optical windows in the 1970's (see attached images), would be interesting to know how many of these they produced, and how many are still in use.

They claimed that these instruments would perform similar to a refractor of the same aperture, maybe a bit optimistic, or maybe not if you compare them to the refractors that were available at the time.

John 

An interesting read, John. I wonder what they mean by “freedom from sky flooding “ (point 3). I suppose it might be to do with avoiding any background sky entering into the fov, but that would be a bit odd.

Also point 4 refers to a transfer lens, so this suggests a variant of the classical DK design that uses mirrors only (e.g. Tak Mewlon). 

 

[SthBohemia]

46 minutes ago, JeremyS said:

freedom from sky flooding

Refers to Cassegrains being in general, somewhat useless during daylight hours. Not exactly true as they can perform during daylight if baffled correctly...

[markse68]

The Cape Newise had an optical flat window and I think it was one of their weaknesses and probably the downfall of the design

Mark

[NGC 1502]

1 hour ago, markse68 said:

The Cape Newise had an optical flat window and I think it was one of their weaknesses and probably the downfall of the design

Mark

Years ago I got quite interested in the Cape Newise 8”.  This was at Kelling star party in north Norfolk when the vendor had them on display. From what I can recall the concept was good but the execution difficult.  Apparently collimation was very challenging.  Occasionally they can be found on the second hand market, if the price was very low I might possibly take a punt…..

Wow, talk about “thread drift”……guilty as charged😳

[markse68]

14 minutes ago, NGC 1502 said:

Years ago I got quite interested in the Cape Newise 8”.  This was at Kelling star party in north Norfolk when the vendor had them on display. From what I can recall the concept was good but the execution difficult.  Apparently collimation was very challenging.  Occasionally they can be found on the second hand market, if the price was very low I might possibly take a punt…..

Wow, talk about “thread drift”……guilty as charged😳

I think getting a decent quality coated optical flat window that size for a price acceptable to the end buyer caused a lot of issues. I have one and found it impossible to collimate- eventually  i discovered a big chip out the back of the primary causing distortion of the spherical surface seen clearly in a foucault/ronchi test. One day i will hopefully find the time to have a go at learning glass pushing and attempt to refigure it. Until then it’s useless  The Ocal collimator is invaluable for this sort of collimation I think and had the primary not have been damaged i’m confident I could have got it well collimated.

It had its mirror glued to the adjustable cell with 3 blobs of silicone rubber though too (back on topic  )

Mark

[NGC 1502]

Hi Mark.  That’s obviously serious damage to the primary.  So, was it originally supplied like that, or was it purchased then the new owner removed the mirror, dropped it causing the damage, then remounted the mirror, reassembled and sold it on?

Either way that doesn’t speak well of someone…..

Ed.

Edited November 10, 2022 by NGC 1502

[Stu]

Snap @Alan White. I blame you for ‘having’ to buy this 🤣

It seems beautifully simple and with excellent engineering. The mount moves very smoothly, no vibration. The mirrors are in great nick on this one so I’m just going to leave well alone. Might just try flocking it though.

Have only had a few quick looks using a 40mm Kellner, so no chance to check collimation properly but hopefully it will be ok.

Alan, have you looked at Protostar secondary vanes? Probably not worth the expense for the scope but maybe you could take some ideas from their designs? A single 180 degree curved vane is the shortest length so shows least diffraction, whilst still cancelling out spikes.

https://fpi-protostar.com/crvmnts.htm

 

[markse68]

4 hours ago, NGC 1502 said:

Hi Mark.  That’s obviously serious damage to the primary.  So, was it originally supplied like that, or was it purchased then the new owner removed the mirror, dropped it causing the damage, then remounted the mirror, reassembled and sold it on?

Either way that doesn’t speak well of someone…..

Ed.

I bought it from the original owner who said it had been returned several times for collimation to the creator Peter Wise.  I’ve no idea what happened to cause this damage but can only assume the scope must have been dropped and the mirror moved enough on the silicone pads to make sharp contact with the aluminium cell and nobody noticed 🤷‍♂️ The missing shard was still attached when i removed the mirror. It’s a real shame as i could see it’s promise as a scope despite its terrible distortion. One day i’ll hopefully get it sorted.

I think the Newise was a case of one man biting off a bit more than he could chew. It was an innovative design but plagued by production issues and although quite expensive to buy, I can see why with all the custom parts which are actually very nicely made.

Mark

[SthBohemia]

18 hours ago, markse68 said:

It had its mirror glued to the adjustable cell with 3 blobs of silicone rubber

After looking as closely as I could via your photograph as posted a few slots ^, am I correct in thinking that the medium of the mirror has inclusions of 'bubbles'??

[markse68]

8 minutes ago, SthBohemia said:

After looking as closely as I could via your photograph as posted a few slots ^, am I correct in thinking that the medium of the mirror has inclusions of 'bubbles'??

most pyrex blanks do 🤷‍♂️

Mark

[SthBohemia]

5 minutes ago, markse68 said:

most pyrex blanks do

I have never seen or used an annealed pyrex telescope mirror blank with any inclusions! I assure you I have made quite a few scope mirrors over the years. Have seen. one or two glass mirrors made prior to 1950's with a few bubbles. Brings to mind certain articles I have read regarding making sure the mirror you are about to grind is stress relieved and contains no bubbles...

[NGC 1502]

10 hours ago, markse68 said:

I bought it from the original owner who said it had been returned several times for collimation to the creator Peter Wise.  I’ve no idea what happened to cause this damage but can only assume the scope must have been dropped and the mirror moved enough on the silicone pads to make sharp contact with the aluminium cell and nobody noticed 🤷‍♂️ The missing shard was still attached when i removed the mirror. It’s a real shame as i could see it’s promise as a scope despite its terrible distortion. One day i’ll hopefully get it sorted.

I think the Newise was a case of one man biting off a bit more than he could chew. It was an innovative design but plagued by production issues and although quite expensive to buy, I can see why with all the custom parts which are actually very nicely made.

Mark

 

Hmm….I’m thinking for that sort of damage to occur to the mirror still within the tube, then the external damage to the tube would be massive and very obvious.

Agreed that this was a case of the designer biting off more than he could chew.  Nice try, shame it didn’t work.

Ed.

 

 

[GriffinUcos]

My apologies if posting here at such a late date is inappropriate.

Alan, thanks for the images and the great information. I have an FC1 in poor condition and am currently soaking the primary in white spirit to remove it from the back plate. I was going to build a mirror cell as described by Richard Berry in "Build your own telescope", but this thread makes me wonder if I should leave well alone and just glue it back on.

Did you get the mirror re-silvered and if so, who did you use?

Thanks

[Alan White]

Never too late to ask and your most welcome to do so.

The project has stalled, the mirror is sat wrapped carefully stored for a project one day, but not that soon.
I would still soak it off and have an adjustable cell myself, but that will not all fit in the existing tube, due to the length and also wanting to change the spider to a thinner 4 vane job.

My present thinking is buy a SW 150 f8 with a messed up secondary and shorten the tube and use all the bits to make the f6 hinds mirror live again.
So far its got no further than this removal and thinking stage sadly.

Good luck with what you do and please make a posting of progress.

[skybadger]

On 10/11/2022 at 09:40, markse68 said:

missing shard was still attached when i removed the mirror. It’s a real shame as i could see it’s promise as a scope despite its terrible distortion. One day i’ll hopefully get it sorted.

Have you confirmed that the absence of the shard has made any difference at all ? On one hand it could cause a relaxing if surface tension which will change the figure, on the other it's a small mirror, have you tried a NY sort of test of figure?

[GriffinUcos]

Thank you, Alan.

Like you I believe it will probably be a slow project. I have the mirror off the backplate but not cleaned up yet. I don't have a serial number on the rear but in cursive script, "fl 35.58" which = 903mm hence f 6.02

There are some scratches on the mirror face so I'll look closer under a bright light and see how deep they are. If not to bad I'll probably send it for re-aluminising. It seems a pity to have to refigure such a good mirror.

I'll post as things take shape. 

[NGC 1502]

Interesting thread resurrection😊

I’m inclined to think it would take a true optical craftsman to refigure to the same high standard as the original.

I also have a 6” Newtonian mirror made by Henry Wildey, a contemporary mirror maker of the same era as David Hinds.  A while back I was very tempted by an AstroSystems 10” on sale at ENS Birmingham, don’t know if it’s still available, scared to look because my temptation will be beyond resistance😊

Yet more thread drift!

[GriffinUcos]

NGC 1502 the 10" is indeed still available. What a scope and what a bargain! Go on, resistance is futile.

You are correct about the re-figuring. I might have chanced it with a suitable tool blank but not from scratch. Far too good to ruin.

I like thread drift, now I wonder if Ryanair would class that 10" as cabin 🧳. 

Have a great weekend all.