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Fo_Cuss

Gaining Focus - Upgrade, mod, or replace?

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Hello everyone :)
I'm a new member, looking for some fundamental advice on gaining 'focus'.

I have a starter telescope 60/700 with 4, 9, 12, 20 mm Kellner type eyepieces.

It's okay (read rubbish), as it provides plenty of learning potential - it having major viewing issues that might otherwise have been masked (if it had been built to a higher specification).
Plus, there are no concerns over 'messing it up' - a liberating concept 🤨

The project
To gain a telescope that will focus.

Here's a very brief outline of where I'm currently at:

Tripod

It has a telescopic tripod with a 'U' bracket azimuth mount.

The first thing that I learned, was that the system resonance could be dramatically reduced, by not tightening the central horizontal axis mount shaft.

An O ring was fitted to the sloppy vertical axis lock shaft, and a plastic pad installed to the lock screw faces.
Then all the joints and lock faces were lubricated with silicone grease.

From a user perspective, the tripod was born again.
However, the horizontal axis lock screw was a beautiful fail.
When locked, the whole system resonated when touched.

Leaving the shaft to run free, solved the issue.
Resonance now only occurs in the telescope, and this, only for around 1.5 seconds.

It was a big lesson.
I'd noted one user (with a balanced EQ mount) talking about stabilisation taking a few seconds (after each touch).
I came to the conclusion, that the tripod and mount would ideally need to be heavy and rigid.

This drove home the concept of 'the gulf' between upgrades - perhaps applicable to all aspects of a telescope.

In my mind, I figured ... don't expect a 'fancy tripod and mount' to solve the problem - it would need to be an expensive 'fancy tripod and mount'.

I'm thinking of adding weight, to further absorb the resonance.
Also, a PTFE washer might be of benefit.

Telescope

Single lens.
The manual has no reference to it being Achromatic, but my guess is that all scopes are now sold with this type of bonded lens.

The image circumference is blue when viewed.

The manual states that the eyepieces are kellner type.

The focus tube slides in a plastic housing, on PTFE strips.
The rack is chromed plastic.
The pinion is spring loaded brass.
Tensioned up, it has no lag.
The adjuster rotary knob is too small - I intend to increase its diameter, to gain finer adjustment.
... however...

The view is not sharp, regardless of position.

Solutions (advice required)

My concern, about buying a better scope, is that the jump in quality might not be worth the money.

The Celestron 90/1000 is available for around £140 delivered.
A bigger aperture, and longer focal length, might dramatically improve the problem.
However, I watched a review video of the Astromaster 70/900, and the images were barely any better.

I'm guessing that the lens is the same, only bigger.

The 90/1000 is surely better still, but, as a consequence, I'm thinking that the solution lies with:

  • Superior lens - perhaps the same crystal, but coated.
  • Two lenses (doublet)
  • Superior eyepiece

The problem is 'lack of specifications', and lack of lens suppliers.
- every telescope site that I visited, listed every component that you could desire, except lenses.
I guess that everybody goes directly to China - I can do that.

Superior eyepieces are interesting, because @ 1.25" they are standard, so a good investment regardless.

The questions are :

  • Will a superior eyepiece solve the focus problem?
  • What coatings should I specify, for new lenses?
  • Is doublet the way forward?

Regarding my current tube

I'm thinking that I could simply add tubing, to build the diameter (for a larger lens and focal length).

I'd then have to add counterbalance weights (or move the mounting screws).

The doublet lenses, and superior eyepiece would probably cost less than £140, and I'd have a far superior scope compared to the single lens Astromaster - maybe :)

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Made some progress, and better understand the doublet type lens.

Objective Lens

The two types readily available are glued and airgapped.
At around €40 delivered (plus risk of taxes), I've found a 93/800 glued and a 80/900 airgapped with mounting ring - both multi-coated.

I have yet to draw out the angles, and look for tubing; but I presume that a suitable arrangement can be found.

My understanding is that greater focal length is beneficial, but perhaps more so the lens diameter.
Also perhaps; the air gapped lenses might provide better focus of all light beams.

Does anyone have an opinion on the 93/800 glued vs 80/900 airgapped?

Eyepieces

The eyepieces produced a good deal of researching ... many of the links coming to SGL :)
They range from €6 upwards.

The 3 types that I concentrated on was in the €35 - €45 bracket
Burgess €35

Kson Super Abbe €40
TMB  €45

All are multi-coated.

Both the Burgess and the TMB, are housed in TMB Planetary II enclosures.
I note that this is a touchy subject (as reported here on SGL)

I have no idea what the difference is ... perhaps just the name.

The blurb for the Kson Super Abbe sounded great (as per usual), but some reviews suggested otherwise on SGL.
However, that was back in 2011 ... perhaps the manufacturing techniques have improved.

I note that both TMB types have a standard final lens diameter (and extendable eyeguard) - FOV 60 deg
... whereas the Kson final lens, reduces in diameter - FOV 30 deg - 50 deg

Without knowing anything more ... the Burgess seems to be a reasonable bet.

I'm wondering if the Burgess will also be okay for terrestrial spotting.
Their web page mentions 'light scatter'.

Does anyone have experience with the TMB type eyepieces?

Anyway, the project has gained direction.
I need to now make strides with the tubing, and the focus angle.

:)

 

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Lenses, Calibration, Tube Upsizing, Flocking

Eyepieces

I've added Celestron Omni Plossl eyepieces and 2x Barlow, to the consideration list.
The Barlow is 2 lens, and the eyepieces 4 lens.

I note SGL has an eyepiece forum (that I will be using).

I've already perused it, and found it fairly horrifying 😲
... expensive eyepieces branded as 'useless' 😁

Doublet

The starter scope objective lens assembly was dismantled, to reveal two 'loose' lenses, held in place by a plastic threaded cylinder.
Neither of the lenses were coated.
They were airgapped by 3 small strips of tape, at the perimeter of one of the lenses.

Upon examining the images of the available glued lenses; I could see no evidence of any airgapping method.
From this, I presume that they are not airgapped.
 

D93mm-F800-Glued.jpg.b80d0a748473537078271c9092009239.jpg


Research indicates that an air gap improves focus ... leading me away from selecting glued lenses.
... though perhaps they are better aligned for eccentricity, as I note that each lens is always marked at the edge.
Does anyone have any knowledge of this?

If this is the case; with the starter lenses being installed loose
... this may be another reason for the scope's poor focussing (with the lack of coatings).

A laser calibrator is available.
Perhaps this will be a required item, if focus is to be maximised?
 

Cattle-Laser-Calibration-Eyepiece.jpg.fcdb40fc56b476848c117fdbe5e83032.jpg


The annoying aspect of the objective lens, is that the simple plastic 'lens enclosure' is priced around €18 - the same price as the laser calibrator!!!
I guess it is their high profit item.

Tube Upsizing

This went very well ... ebay to the rescue, for rapid viewing of what is available.

It looks like the best bet will be stainless steel flue adapters.
They offer many options, but critically, 60mm/80mm and 60mm/100mm are available.

Of course; they match the spacer tubing
... meaning that the correct focal length can be achieved.

The key feature is that the 60mm end goes inside the 60mm tube
... and the large end goes outside the tube.

This should work perfectly; though an additional perimeter spacer will be required for a 93mm D lens.
A 100mm D lens would be ideal, but they all seem to be 104mm or 106mm :BangHead:

Flue adapters might sound a bizarre choice for a telescope
... and further, they are not swaged or spun.
Instead, they are welded.

However, they will be machine made, so effectively concentric enough.
Ha!
That laser calibrator is looking more and more like a required purchase :)
 

Pipe-adapter_60-100mm.thumb.jpg.bed0658002cdc6b0e4ad2f8708a7a53e.jpg

Pipe_extension.thumb.jpg.1a285d481c75e519ef5fdf782bd91d54.jpg


At this moment in time, I believe that 60mm/100mm is the best option.
This would accomodate a 93mm objective lens, with wiggle room for calibration.

I don't know how I will mount the lens (yet).
Something will surely fall out in the wash 😉

Flocking

The material that I've seen, is glue backed.
... so likely to be a very messy job.

€8 gets 0.5 m long x 0.7 m wide about 0.5 mm thick

Flocking-material.jpg.cf2fbff800db652ce9cea2dc4a22c2d0.jpg

This is when that new ultra black paint would come in handy.
I'll look into it, but I expect it to cost at least an arm and a leg.

I do have some matt black barbeque spray paint.
Does anyone have any experience of this as an alternative to material flocking?

Next step, is to visit the local hardware stores, to see if they have the flue gear in stock.
Having the kit in hand, is always useful.
Oh yes ... and try to finalise the eyepiece strategy :icon_scratch:

Edited by Fo_Cuss
spelling

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Actually ... the next step was to reflect.

It's always a good move, when carrying out a project.
it's all too easy to press on regardless.
The concept of 'make a decision and stick to it' can easily end up with a wrong path being chosen.
... particularly when in a fog 😉

The eyepiece thread is producing good information ... more to come perhaps.

I took some time off, to re-examine the resonance question?
The fundamentals seem to be solidifying.

It appears that the key problem areas are:

  • Horizontal & Vertical Axis

The horizontal axis is via the two lock screws, through the 'U' mount, into the scope tube.
Screwed tight, the resonance period increases.

Learning this, was the breakthrough in understanding that the core problem revolved (ouch) around the axis 😀
Of course, it's obvious once you know (typical)!

The solution lies with the semi-loose lock shaft - connected from the tube horizontal centre line to the 'U' bracket (below the centre line).
 

Telescope_lock_shaft.jpg.7c820682b01b2869abd44ee497b36851.jpg


Yes, locking the shaft helps.
However; by unscrewing the tube horizontal lock screws, the scope has close to zero 'horizontal axial resonance' !
Nice!

What is happening here?

At first glance, you might think that, by locking the H axis, this would improve rigidity (locking the triangle).
However, for resonance, it is different.

If a tube was lying on the floor...
If you pushed it; it would simply move (it wouldn't vibrate)
If it was fixed in the middle, it would swing one way, then reverse, then reverse again - the undulations getting ever smaller.

The reverberations would continue for a very long time, because the energy of one swing, is mostly conserved by the metal spring.
... a bit like a pendulum.

Why not pivot around the lock shaft screw?

  1. Ive fitted an 'O' ring to the screw
  2. The scope is resting on the lock screw threads

In both cases, the enrgy is not conserved.
Hence, when the scope is tapped ... it vibrates for only a few milliseconds.

How to apply this knowledge?

The ideal would be (I think), to fit a second lock shaft on the opposite side of the tube, towards the rear of the scope ... using different geometry.
(If a second lock shaft was fitted in mirror image to the first, we would simply create a new pivot point around the 'U' screws)

A second lock shaft might provide an additional benefit...
The two lock shafts could be left almost loose, to allow free flowing movement of the scope.

Whether this development is required, is another matter, because the lockshaft itself, needs further development.
It is simply too crude.
The shaft doesn't slide without jerking.

I have some ideas on how to solve this problem....
 

 

 

Edited by Fo_Cuss

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Solved - Lock Shaft, Vertical Axis, Resonance

Lock Shaft

image.jpeg.53524e5a17662aa60cb35cf5276220d2.jpeg

The shaft slides through a rotating lock.
As the front of the scope rises, the shaft angle changes, and the lock rotates
... the screw acts as the axel, rotating in a hole in the U bracket.

The problem was that the bracket metal is too thin, and the hole too large, and it's spinning on a couple of screw threads.

Even after I had replaced the nut with a nylon lock nut, the screw axel had way too much play.
... and the turning force from the shaft, is always going to be one sided; causing the lock to waggle around.
This would cause the shaft to bind.

Rubbish!

There should have been turned a short spigot (then the screw thread) with a sliding fit into the hole in the U bracket.
It's less of a problem when a component is freely rotating (like the scope)
... but with the shaft pushing and pulling ... no chance!

I had no option but to fit a sleeve over the screw thread, and enlarge the hole to fit.

I ended up cutting down an old biro pen:
 

Biro_sleave_source.jpg.0a64961649f4b60cd33c9796973c5edf.jpg

Lock_block_thread_sleave.jpg.0b04e3f773ea694316f0337794e1f7d1.jpg


This was a right pain in the neck, but with the help of a butane torch, and reducing the thread diameter, I got it on.

Brilliant!

The lock is now forced to rotate concentrically, regardless of the unbalanced force from the shaft.
Smooth 😎

Vertical Axis - The Horror

The shaft was tapered, but the bore wasn't
... and the top diameter of the taper was slopping around in the bore.

It's only hope was that the U bracket shoulder could clamp down on the top face of the tripod
... but the clamp assembly was a failure - the design prevented any clamping action.

Consequently, I broke off the clamp limiter (central plastic spigot), allowing me to pull the U bracket down onto the top face of the tripod.

I packed the bore with PTFE and thick grease
... and by some strange good fortune, the clamp screw spins, without unscrewing itself (what!)

The scope rotates beautifully now ✌️

Scope Resonance

With the horizontal axis scope screws (up and down) loose, resonance had been effectively eliminated.
However, I wanted some clamping pressure on the scope ... enough to allow it to glide, and to hold it (when in position).

I discovered the problem.
The U bracket was too wide.

When the clamp srews were tightened, the U bracket became highly strung under compression.
This was the root cause of the resonance.

I fitted 3 washers to the side opposite the lock shaft, and 1 washer on the lock shaft side of the scope:
 

Scope_mount_3_washers.jpg.af83a84115fdee75627abe52779bddf2.jpg


This arrangement created a better alignment of the lock shaft.
Hahaha ... it maxed out the lock shaft function 😀

It also transformed the scope movement action.

The scope now glides up and down, at the slightest pressure - zero jerking at start of movement - Wow!
... and when the pressure is removed, it stops in position.

It's ...... Victory! 🌝

 

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Flocking the modern way - will it work?

I'd talked earlier about looking into a paint solution, thinking that the ultra black would cost an arm and a leg...
In fact, this is not the case.

Yes, the nanotube black is way out of budget, but there have been other developments.

A British Artist (Anish Kapoor) developed an ultra black acrylic paint, but refused to share it with the community.
This caused outrage around the world (presumably, all publicity is good publicity).

Another British artist (Stuart Semple) set about creating his own version - Black.
... available to everyone in the world except Anish Kapoor - hahahahaha!

Through feedback and further development Black 2.0 was released.
This absorbs around 96% of light, and costs £12 for 150ml covering a claimed 2m x 2m.
(a very reasonable price - no different to consumer grade black bases)

Apparently, this is a usable paint, self-priming, with a stable surface.

Black 3.0 followed (currently Black 3.2) - £15 for 150ml
This absorbs around 99% of light.
It requires a primer, and the surface finish scratches easily.
 

Black3.0_cut.jpg.80fbff9fd3259ebdff7fa6e309eff64b.jpg


Application

The good news is that both versions can be thinned with water.
In fact, multiple thin coats produce the blackest finish.

A matched primer can be purchased, though Black 2.0 is stated to be excellent, ensuring that only a small quantity of Black 3.0 will then be needed.
Otherwise, any acrylic primer can be used - presumably, the darker the better.

What this should mean, is that a solution could be swilled around tubes, multiple times.
Far easier than struggling with sticky backed material.
Also, it can be used for lens edges - simply painted on.

It should mean that everything that requires blacking, can be blacked.

YouTube Reviews

They are almost all appalling.
I can't get my head around why you would buy a hight tech paint, and not read the instructions :BangHead:
... and if you were producing a review video, it surely would be a must.
But no!

All but one, simply squeezed a glob out, didn't add water, didn't prime the surface ... and then complained about it.

I finally found one, that primed the object with Black 2.0, and finished it with Black 3.0 (a beta version).
It's worth a look.

He does an interesting test, wherby you cannot tell which way the object is rotating.
... and the finish is exceptionally black.

It doesn't stand up to laser light, but my guess is that it will be perfect for our needs.

I intend to buy it 🌝

Tubing Update

Managed to get into town today, to check out flue component availability.
Very limited - nothing anywhere near what I need.

On the bright side; I was able to get a feel of stainless flues.
Very nice and light.
There were some ferrous steel examples, that were extremely heavy, by comparison.

Overall, the trip was worth it, as it confirmed that stainless steel flues should be ideal for a scope 🌝

 

Edited by Fo_Cuss
  • Like 1

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Protostar flockboard is great but too expensive to import so I made my own version.

Get some acetate sheet 600micron thick and make a sleeve that will fit inside the tube. Then put the self adhesive flocking onto the acetate sheet while it’s sitting flat on a table. No need to tape the seam in the sheet when you slide it into the tube.

Get the proper flocking material that won’t shed fibres. Don’t use the high street flocking as it will shed fibres.

https://www.firstlightoptics.com/misc/black-velour-telescope-flocking-material.html

0B54C338-460F-4BEC-9412-3198CEDA2E7B.jpeg

FF39500C-7FBB-42BB-B360-3E9BCA4132D6.jpeg

Edited by johninderby

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Thanks John, for that advice and link :)
I may need it.
However, I'm very tempted to try Stuart Semple's Black 3.0

It's not that I'm looking to cut corners.
If the paint can absorb 99% of light ... it may be as good as gets.

... and there are further considerations.

  • I want to black the edges of the lenses ... therefore I anyway need a suitable paint.
  • The scope is to be a refractor.
    Hopefully, this will mean that all the light entering the lens, will be directed to the focal point.
    ... this presuming that the lens shroud has been appropriately flocked.
  • It will be a great experiment
    ... and has potential to form the basis for a genuine video review of the paint (much needed).
  • Also, it will add to the knowledge base for other DIY scope builders that follow.

Let's see how it goes.
We can but try 🌝

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Sounds good although seems their claims are a little bit exaggerated. Any paint though will perform better on a rough or ribbed surface when light hits it at an angle.

Here’s the results of comparitive tests in a lab.

 

408FCA03-0DA4-4A85-9B7A-4B785ABDCC3A.jpeg

00F1435C-E0E9-454B-A442-0B2517E94FAA.jpeg

Edited by johninderby

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Looks interesting, but I couldn't find the link to the experiment - tried loads of search terms.

Do you have the link?

 

 

 

 

 

 

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From what I can gather (from the charts)...
Black 3.0 is substantially superior to Krylon.

I note that absorption varies between 94% and 95%.
However, without the test report, the results are meaningless.

The YouTube videos show that, if the paint is not properly applied, the results will not be ideal.

Did the testers correctly create the coating?

Typically the answer depends upon who was paying for the test 😉

 

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Belt & Braces - Best Laid Plans

Solving Resonance (again)

Having got to grips with resonance, I moved on
... but the brain keeps working in the background.

The solution it came up with, was 'felt washers'.

It seemed hard to disagree, even though the scope does not now resonante to any great extent.
However, this is now, and the scope isn't finished.

Consequently, it does appear to be a sensible course of action to further separate the scope from the 'U' bracket
... and fortuitously add some damping to the clamp assembly.

Felt seems to be the most likely candidate for this task.
Having said that ... it is just theory; though sound enough to locate and purchase the washers.

Evidently, what I needed were 'fishing rod spool, clutch washers' (who would have thought it?)
 

Felt_washers_25x8_5x1.jpg.4de64c2975e134aeecf497b669f0e667.jpg


25mm x 8.5mm x 1mm

The idea is that these will be soaked in a light grease, and placed either side of the scope tube, between it and the U bracket ... allowing smooth rotation, whilst providing damping.

I bought them from Dave in Manchester.
You maybe know him ... he runs Trafford Tackle Repairs and has an ebay site purpldsere 
... a friendly helpful chap - the sort that you like to do business with.

Scope Expansion

The chimney flues were looking great, until I contacted the trader
... not in stock, don't know when....

Worse; nobody seemed to stock this kit.
... and finding a 100mm to 60mm reducer, is like finding rocking horse droppings :sad:

My search finally fell upon custom car exhaust tips.
... and out of that lot, I found this :
 

63mm-102mm-Exhaust-Pipe-Tip.jpg.cca8e55b71e2dd5ff2d4d1b86ee4d46e.jpg


It obviously need a lot of working.
The outlet end is rolled, and I will surely be forced to hone out both ends, to accept the 61.5mm scope tube, and the 100mm extension tube.

Talking of 100mm Extension Tube

I think that I am going to initially try a plastic downpipe.
Nobody seems to have the short length stainless, and I can get it in plastic just down the road for €5 (2m).

I need to examine it; but only 250mm (or so) is needed.
It should be rigid enough, and of course, it can be reinforced.

To be honest ... just getting a tube that will fit, is my main objective ... the rest will then fall into place.

Lens Mounting Shoulder

I was looking at 100mm internal circlips, but by now I was suffering 'spending fatigue' (typically a male condition) :angel7:
... I'll get over it :wink:

Once I have everything in my hands, an adjustable shoulder will present itself
... and with the laser calibrator, all will be well.


So the die is almost cast.
Money has fled from my account, in all directions.

Updates to follow 🌝

 

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Thanks for that link fozzy :)
Sourcing supplies in France is always very difficult.

As it stands, I will be trying Black 3.0
... because I've researched it, and ordered the product.

We will see how it goes.

If it is a fail, then I will resort to traditional methods 🌝

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The Optics

Doublet Objective Lens

Diameter (D): 93mm
Focal length (F): 1000mm
Structure: Air gapped
Coating: Multi-coated
Combination of material quality: 1 / 4-1 / 8
Diameter Tolerance: + 0 / -0.12 mm
Center thickness tolerance: ± 0.2mm
Focal length tolerance: ± 0.1%
Finish: 60/40
Eccentricity: 1 '

There is no point in providing an image ... we will see what we get, when it arrives.

Eyepieces

The eyepieces were discussed on the appropriate forum.

Some misgivings were expressed over the Omni Plossl suitability for a focal ratio of 10.75.
However, the overall view was that they would be fine, if well manufactured.

The other caution was the exit pupil, and potential eyestrain.
For myself, this should not be a major issue, as most of the viewing is expected to be via a digital format.

From this discussion, and maintaining a reign on the budget, I selected:

Celestron Omni Plossl Range : 4mm, 12mm, Barlow x 2 (1.25")
Broadband Multi-coated, Edge blackened

The choice of magnifications was based upon a balance of expenditure and desire to experiment.
The chosen range provides 500x 250x 166x 83x

I had read the SGL thread discussing this topic, and viewed various calculations.
... my scope producing theoretical useful magnification between 185 and 235

SGL suggested that 'viewing conditions' count.
... Here, in the South of France, we can experience very good viewing conditions.
Consequently, I chose the 4mm over the 6mm, knowing that I could achieve 6mm with the Barlow.
Plus, I will have the (perhaps limited) pleasure of viewing at 500x.
Seemingly it will be dark and blurry, though we will find out.

Eyepiece focal length: 4 mm
Apparent field of view: 50 °
Eyepiece exit: 6 mm

Structure Design: 4-elements
 

Celestron-omni-Plossl-4mm_eyepiece.jpg.4000c6e332b1f0dd06be84953719e438.jpg


Eyepiece focal length: 12 mm
Apparent field of view: 50 °
Eyepiece exit: 8 mm

Structure Design: 4-elements
 

Celestron-OMNI-Plossl_12mm_eyepiece_640x640.jpg.712e0427a44a4481876f4afeabc01851.jpg
 

Barlow x 2
Structure Design: 2-elements
 

Celestron-omni-2x-barlow-eyepiece.jpg.df3cbb7cda6448f881e40f7954ecad28.jpg


1.25" 90 Degree Diagonal Zenith Prism

I'm hoping that this manipulator of light will not overly degrade image quality.
From my limited experience with direct inverted, and mirrored images, the prism should make life much simpler.
No need to load Gimp, simply to view the image as it would be naturally seen.

I chose this partricular prism, because the design includes a protective grab band (over a screw against the eyepiece).
 

1.25_90deg-Diagonal-Zenith-Prism.jpg.fe0cbf2e2058725538e758a6b0be9ce5.jpg

 

Laser Calibrator

There were calibrators without a laser beam.

To be honest, I didn't research this topic.
I took a leap of faith, and assumed that this device will work as claimed.
 

Cattle-Laser-Calibration-Eyepiece.jpg.3be9dd9193d6c1e84cb7e950fbabb48c.jpg
 

Tomorrow - I will attempt to purchase the 100mm tubing...

Edited by Fo_Cuss

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Does the prism diagonal give a correct image like the eye sees it or one where up and down are corrected but left and right are reversed ?

Usually, the ones that give a fully corrected image have a flat top on the prism housing:

image.png.ef72ee99e5220e316dfddcb7b92fdc43.png

 

 

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I hope so John.

The full title was :
90 Degree Diagonal Adapter Erect Image Zenith Mirror Prism For Refracting Monocular Astronomical Telescope Eyepiece Lens

It could be that I have made an error.
My thinking was that the 90deg meant that it would flip and rotate the image (over the 45deg mirror).

Either way, it's too late now.
If I've made an error, I can live with that.

This project is effectively a prototype.
Prototypes exist to show up errors.

:)

 

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For astronomy I feel that the ones that are not fully corrected generally deliver better image quality (there are exceptions but they are very expensive). You just need to get used to the reversed left and right.

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Well John, I have just found the ideal video, that all potential buyers, of erecting elements, should watch :


Interesting points to note (over and above the actual rectified images)

From this test, there appears to be a gradual degredation of image quality, and an increase in magnification.
... the latter, no doubt due to the increased focal length.

In truth, it is a shame that the test was not conducted with a fixed camera, however a rule of thumb is likely gained.

I would have also appreciated a comparison with a 45deg mirror, as it achieves what the 90deg diagonal prism achieves.
... yet the chap states that the 90deg prism is the choice for astronomers (presumably over the mirror)

What's wrong with the 45deg mirror solution, I wonder?

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Tubes & Lens Mounting Solution

Tube

Managed to get to town today, and purchased a 1m length of 100mm OD - 96mm ID - PVC tube @ €3.29
Nice?
Not particularly, but I believe that it will be fit for purpose.

It's an extrusion, so it is not perfect, however, of the three available, I chose the best one.
The good news is that they are slightly smaller than 100mm (by a few thou).

There were 2m tubes - 3mm walls, with outer and inner skin (filler recycled plastic inbetween).
I judged that the PVC tube would be fine; but if not, the 2m tubes are readily available.

From a previous project, I already had a number of precision injection moulded joints.
... It was just a case of having everything in hand, to see if a solution presented itself.

Lens Mount Concept

I was looking for a pre-made shoulder, upon which the lens could be placed.
An external tube joint provides this.
With an ID of 100mm (+ a few thou), it has a central shoulder 95mm ID.
 

tube-joint-100mm.jpg.1f8ed5e7fd3c8a2b7d3ef64e4ceb7834.jpg


The lens assembly (whatever that is) can be dropped onto the shoulder, and a band of 100mm tubing can lock it in place.
The opposite side can then slide over the 100mm scope tube.

This will create a removable lens (as per the eyepieces), and numerous advantages.

  • The tube extension can be laser aligned, and permanently fixed in place
  • Working on the lens assembly will be easy
    ... for initial setup, flocking, and future air gap tests etc.
  • The lens can be safely stored and transported

As stated above; the tube is not perfect; but the joint is (and it is rigid).
It will have a perfect position for alignment
... but laser calibration will allow this position to be marked
... and the laser can be easily carried, to confirm correct alignment in the field.

Overall, this seems like it will be a good solution to a problem that was of concern.
I certainly felt a wave of relief pass through me, when the concept revealed itself.

This leaves only the core lens assembly to figure out.

I already have some ideas on how this might be achieved, but this work will be completed when I have the lenses.

So far, so good.
Phew!  🌝

 

Edited by Fo_Cuss

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Felt Washers and Dampening Thoughts

The felt washers arrived, and were fitted.
They are entirely excellent!

I fitted an eyepiece, and camera; and adjusted the focus ... zero resonance!

Previously, each attempt at focussing involved 'adjust and wait'.
This was useless, as the eye prefers to pass through focus, and then back.
Now, I should be able to easily gain optimum focus.

This success made me think about damping the entire scope.

While felt would likely be the ideal; it would require numerous layers of paint, to make it neat.
Doable ... but a nice leather (and flexible glue) would also suffice, and at a stroke would look amazing.

It's just a thought, and it may not be needed (I don't have any spare leather).
... though, with the added length, and weight; the mounting and lock shaft will need to be repositioned to the new centre of gravity.
This will lengthen the eyepiece side of the scope, as compared to the objective lens side.

It's to be seen, what effect this may have on resonance.
One thing though...
The longer length, will lower the gearing for angular change.
It should make it easier to keep objects in view.

... and that's it for the moment.
I'm now awaiting the various items  🌝

 

 

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Doublet Lens & Black 3.0 Arrived

Black 3.0 - ordered 11th arrived 17th (UK to France).

Truly excellent packaging - thought has gone into it.
A standard jiffy bag, containing a carton wrapped in welded bubble wrap.

It remains unopened, as I will produce an in depth review shortly.

By comparison...

Doublet Lens - ordered 11th arrived 22nd (China to France)

Shipping time was fine ... fast in fact.
Packaging was poorly executed.

93mm_Doublet_as_received.jpg.ce32160d7f337c5f7821ca7ff2e43789.jpg

93mm_Doublet_Spacer_Ring_as_received.jpg.85d7e8d05fe2b5249a101c5a54339288.jpg


It was very safely wrapped in bubble wrap; but the damage was done during wrapping.
The ring was positioned between the lenses ... it obviously slid out, and was bent over, as the first layer of bubble was wrapped over.

This was not from the factory, but from a store that specialises in telescope type equipment (266 items).
... and not a cheap item (at around €50) in Chinese terms.

Another euro for a packaging box ... I think everyone would go for the box.
What a shame.

I'm going to alert them to the packaging failure.
We will see if they send me a replacement ring (though it is just usable)

Spacer Ring

Ferrous steel - possibly cadmium plated
Thickness varies between 0.2 mm - 0.25 mm

On close inspection, this variance will be due to a very fine 'stamping rag'.
I can eliminate this.

I presume that most people use it as is
... but if I am blacking everything, then the ring must be in the firing line, as it is right in the problem area.

I do wonder what the ideal air gap will be.
If anyone has experimented with this; I would be interested to learn of the results.
(I note that lens manufacturers quote wildly differing air gaps, though these are based upon centres, and not edges)

Clearly, with a blacking coat (or two), the air gap will grow.
We will find out 'to what', when the time arises.

Lens 1 - Front

Diameter : 92.90 mm
Thickness : 6.30 mm

Face 1 : Convex - radius unknown (small)
Face 2 : Convex - radius unknown (large)

No visible imperfections.
The claimed 'blue multi-coat' is NOT clearly visible, when the lens is angled.

Strangely with lens 1, when viewed through the camera, the blue sheen was very easy to see :

93mm_Doublet_Lens_01_Blue.jpg.00cfe292c9879b93c539ddca2feaea97.jpg

Lens 2 - Rear

Diameter : 92.96 mm
Thickness : 9.70 mm

Face 1 : Concave - radius unknown (small)
Face 2 : Convex - radius unknown (large)

No visible imperfections.
The claimed 'blue multi-coat' is clearly visible, when the lens is angled.

Strangely with lens 2, when viewed through the camera, the blue sheen was quite difficult to see

93mm_Doublet_Lens_02_Blue.jpg.c2141627cc51df7559f68326ff29a297.jpg


I am torn between whether both lenses on one face, have an aspheric finish
... it could be an optical illusion.

Both lenses were dirty.
This after I had breathed on one :

93mm_Doublet_Lens_dirty.jpg.4affa440774b1421f1f2dc544a312c84.jpg


Both lenses have a mark on the edge.
I presume that this is to indicate how they best mate together.
Ha!
I must remember this, when blacking the edges 😁

Overall, it's a win.
A locally manufactured lens set would cost at least 10 times the price paid.

The problem is that they do not come with a technical data sheet.
However, we shall see how they perform.

I've now got to figure out a way of mounting them 🌝

 

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Home Engineering - Making a Lens Mount & Enclosure

Assuming that a lathe/milling machine is not owned, and neither a 3D printer
... 'home engineering' is based upon the premise that 'it has already been made'.

The difficulty is finding it; though usually something turns up that can be repurposed.

In this instance, I have settled upon :

Lens_mount_components.jpg.edbcfe21bd7b687c120a2f2d3686b451.jpg

  • Lens goes in the grease tub.
  • Grease tub goes in the adapter.
  • Adapter goes in the tube connector.
  • Tube connector goes over the scope tube.
  • Whisky container goes in the grease tub (compressing the lenses).

The paper lap joint must be removed - slide a scalpel down the edge, and remove excess with hair dryer.
The steel base plate will be cut out with a sharp knife, to leave the pressed steel rim.

Assembly Plan
(Blurred areas, to be removed.)

Lens_mount_assembly_plan.jpg.e6e2b010e669ea1cea408b3f3a029fc0.jpg


The great advantage from using a grease tub as the lens holder, is that it comes complete with a 'screw on' lens cap.
Nice!

The grease tub radius, perfectly matches the lens chamfer, to provide ideal fixture.
The radius (externally) locates onto the tube connector 'inside rib', acting as a shoulder to the lens holder.
Fortuitous!

What complicates the plan, is the cardboard whisky container (lens compression cylinder).

The lens compression cylinder, requires a locking system - typically 'screws'.
... not great in cardboard, and there is no space available in the steel rim.

Therefore, the material structure of the cardboard, must be upgraded, and reinforced.

Thinned lacquer could be used; but better still would be a two part polyester resin (thinned with acetone).
The resulting 'resin cylinder' can then be reinforced with fine fibreglass sheet (wallpaper).

Lens_compress_cyl_strengthen.jpg.9912e720035d3649648ad8eb1112aba3.jpg


Depending upon need, the outer wrapper can be removed, and replaced with another fibreglass sheet.
However, regardless of this; the final working aperture diameter would be 90 mm.

At an initial diameter of 93 mm, this resulting aperture size cannot be bettered.

The fixing method is yet to be determined, but hollow 'leather rivets' spring to mind.
A dowel ended screw could simply locate in the rivet centres; locking the cylinder, causing no stress, and zero distortion.
... at the same time, the screws would fix the lens holder, to the adapter, to the enclosure.

All birds dropped, and barely a musket fired !  😀

Remaining would be, to find a 100 mm end cap, to close the tube connector, and...
we would have a fully enclosed removable objective lens.
(actually, two end caps ... one for the telescope tube)  😉

Method

Hahaha ... it's all easier said than done 😅

I have designed a general fabrication method
... but it is fluid.

We will see how I do it, when I do it  🌝

 

Edited by Fo_Cuss

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