Making a planetary Dall-Kirkham telescope

[Stefan Buda]

I've been making Corrected Dall-Kirkham astrographs commercially for a while and prospective buyers often ask if the instrument is also good for planetary imaging. Unfortunately I have to disappoint them by explaining that the 50% linear obstruction, while perfectly ok for deep sky imaging, disqualifies the CDK for planetary imaging.
So I decided to design a complementary instrument of the same aperture, that is as good as theoretically possible for planetary imaging.
The case for the Dall-Kirkham:
My main criteria in deciding the type of instrument were, aperture, focal length, secondary obstruction size, thermal behavior, portability and maintenance (collimation).
The aperture had to be set at 250mm as that is the maximum I can manage with my equipment.
Focal length for this aperture needs to be around 6000mm for imaging at the theoretical limit, but that would mean an f/ratio of 24, quite possible but not very nice from a manufacturing point of view.
A typical SCT telescope has a linear obstruction of at least 32% and I wanted to do a bit better than that.
Thermal behavior means that the instrument needs to track ambient temperature changes well without trapping warm air or cooling too easily below ambient and getting dewed up.
Through portability I mean a weight of not much more than 10kg so that the instrument can be taken to locations for catching planetary events or avoiding local cloud.
Ease, and retention, of collimation, and also focus drift, can be dealt with using carbon fiber and precision mechanical construction.

All the above criteria come together nicely in a 250mm f/16 Dall-Kirkham OTA.
One other very important aspect in favor of the D-K is that the optics can be made to a very high standard, rivaling the venerable Newtonian. This aspect is a bit hard to explain for people other than mirror makers.

[Rod]

I’m very much looking forward to seeing this come together. A cassegrain will be one of my retirement projects so I’ll be interested to learn here. A couple of questions to start with:

I’m assuming you prefer the DK because the classical cassegrain has the complexity of the hyperboloid secondary and simple optics are easier to build to a high standard?

I’ve read some say the spherical secondary on the DK does not need collimating, although it must be well centred. Is that true?

Rod.

[Stefan Buda]

You are spot on about optical complexity. That is why I compared the D-K to the Newtonian. One of the great advantages a Newtonian has over any other reflecting telescope, and few people realize, is the fact that the secondary mirror can be mass produced to very high precision without any hand figuring. Thus the typical Newtonian has only a moderately aspheric mirror that needs hand figuring. The next in line of simplicity is the D-K with its spherical secondary that can be figured against a not too difficult to make spherical test plate. The primary of the D-K is faster than a typical Newtonian primary, but the departure from spherical is only about 3/4 of the equivalent Newtonian's primary.
The D-K does need to be collimated but the collimation is far easier than any other Cassegrain configuration. The reason is that the secondary mirror doesn't have an optical axis, only a mechanical one. Any line that goes through its center of curvature can be regarded as an optical axis. And that means it doesn't even have to have its mechanical axis accurately centered on the primary mirror's axis.

[sharpiel]

I.
Can't.
Wait.
Popcorn ready for progress updates :cheers:

[toc]

Can you call it a 'Budalon'? :). I dont think Takahashi will complain!

[Stefan Buda]

I'll try to post something later today, before you run out of popcorn. :)



I'll stick with DK250 for now. :)

[Stefan Buda]

The first metal parts made are the spider vanes. I had to start somewhere.
For the spider assembly I decided to use tension joints, rather than shear joints likes all the commercial OTAs use. Tension joints can't slip or rotate like shear joints and can be made more accurate and more stable, although more expensive.
I don't have a sandblasting setup to texturize the parts before anodizing for a matte finish so I tried something different: I put all the vanes (I made two sets) into a small tumbler, with chunks of carborundum, and I let the tumbler spin for a couple of hours. After that all the sharp edges were eroded away and the flat surfaces looked like they were sandblasted. Quite happy with the result.

[RodW]

Hi Stefan

Interesting project. Please excuse my ignorance but how long would the optical tube need to be for this DK250 at F16? That'a focal length of 4000mm.

I've seen your very impressive CDK250 which is nice compact design, so I'm just wondering how physically different this DK version will be.

Rod W

[Stefan Buda]

Hi Rod,

The OTA will be a truss type and have a length of 805mm from the front ring to the backplate. The focuser will add another 70mm. From the focuser, another 79mm to the focal plane.

Stefan

[Stefan Buda]

I paid $300 for aluminium and $170 for carbon tubes. Looks like the total cost of materials for the OTA structure will be around $700.

The Pyrex cores I accumulated from trepanning CDK primary mirror blanks are coming in handy for making DK secondary blanks. I just have to reduce the diameter of one and slice it into two discs to get two blanks or a blank and a grinding tool.

[Saturnine]

Hi Stephan. The more I read of this project the more intrigued I become. What diameter will the secondary be, in regards to the central obstruction and also what will the focal ratio of the primary be, I imagine somewhere about F3 going by the length of the truss tube.

[Stefan Buda]

Ok, here are the numbers:

Primary radius -1854.28mm (f/3.7)
Primary conic: -0.73499 (prolate spheroid)
Secondary diameter: 62mm
Secondary obstruction: 68mm (increased by the secondary baffle)
Secondary radius: 550mm
Mirror spacing: 715.9mm

[Rod]

Hi Stefan

That horizontal cutter looks impressive. What speeds do the blade and blank spin at?

Looks like a very clean result.

Rod

[Stefan Buda]

I have a couple of these, very thin, diamond blades. I think they were made in Russia but I can't remember where I got them from. They are so flimsy that you need to clamp them between two metal discs. If properly clamped they can make less than 1mm wide cuts. In this case the clamping arrangement is a bit too small, to allow for deeper cuts, and the rim of the diamond disc wobbles a bit, making the cut a bit wider.
The RPM of the tool is only around 2000 as I'm driving it with a low RPM motor. The rotary table is hand driven.

[sharpiel]

I know your focus is astrographic...Any chance of a visual emphasis Bulon?

[Stefan Buda]

The DK is definitely not an astrograph and it is as good as it gets for visual planetary.
It is complementary to the CDK, as I mentioned earlier in the thread.

[sharpiel]

Gotcha. Upon re-reading I see how I misunderstood.

[Saturnine]

[QUOTE=Stefan Buda;1617066]The DK is definitely not an astrograph and it is as good as it gets for visual planetary.

I am presuming that it would still be good for planetary and lunar imaging though, which is what I'd focusing on with it, if I do manage to obtain one.

[Stefan Buda]

[QUOTE=Saturnine;1617077]

Of course it is. You don't need an astrograph for planetary imaging, but you do need one for deep sky.

For planetary imaging only two aberrations need to be corrected, and those are the spherical and the chromatic aberrations. Coma, field curvature and astigmatism don't come into play with the small field of view needed.
Astrographs actually compromise a little bit on the spherical aberration but not enough to matter at, say, one arc second resolution, but that amount of compromise becomes detrimental at, say, 0.4 arc second planetary contrast.

[Stefan Buda]

The next moves were to make 16 chess pieces and a length gauge.

For reasons that will become clear later in the project, the truss poles need to be precisely equal in length.

[RodW]

Very impressive Stefan. I take it you made the ball joint components in house?

[Stefan Buda]

Yes. I made them without a CNC lathe by generating the spherical shape on the milling machine. I did not take photos.

[Stefan Buda]

I had to take a break for a while from this project, but I'm cutting metal again.
I'm making the front ring out of a square plate, and I had to start by making a jig for holding it on the lathe.

[Saturnine]

Thanks for the update on progress, watching with interest the techniques you are using. Have done similar jobs when wanting a largish and disc or ring, getting some plate and cutting to a rough circular shape, making an adapter to bolt it to and spinning in a lathe and machining it circular.

[Leo.G]

Wow, did you have cigarette foil clearance on the lathe bed?
What size swing?


That's an amazing looking job but I've seen some of your other items and they all look amazing!
What size plate is that you are using?
Final dumb question (sorry).
The way you've cut the plate, I'm guessing not with the milling machine so it's been a decent size band saw?

I have a plasma cutter, only a cheap Aldi thing which will do 10mm steel and maybe slightly thicker aluminium, with the aid of a circular guide I can get an accurate blank in minimal time with minimal fuss, not that I have the guide but if I did. I reluctantly used it for the first time some time back to cut an old gas hot water tank in half to use as the slag tank for my plasma bench I'm yet to get to, I have everything here to do it. I was going to use an angle grinder till my son asked why did I bother buying the plasma cutter. I hadn't used one since I studied fabrication engineering at Young TAFE and didn't get to play much with the TAFE's expensive machine (no student did). I've fixed the electronics in a couple for friends but didn't have a compressor to play with them.
They are so fast, clean, effective and less overall scrap/waste.


Saying I haven't made the cutting table, I haven't welded the frame (stand) for the milling machine which came as a package deal with a combination lathe I bought my son, 3 years ago (health doesn't always see me getting to what I should get to). I got the separate table, vice, tools basically everything but the MOJO to weld the stand.

https://www.machineryhouse.com.au/k149?s=k149

Migraines and moving/mowing/welding/doing most average things don't mix so I'm just an old has been.

[Stefan Buda]

I had about 12mm clearance between the disc and the lathe bed.
It will be more difficult with the other truss ring which is 20mm larger in diameter.
The square plate was 307 x 307 x 16 and weighed 4kg. The finished ring's weight is 690g.
I still need to drill some radial holes and that will require another fixture.

[Leo.G]

Your work is always so impressive!



And the furnace to cast the waste?
I have some fire bricks and a gas rig here with intentions of maybe setting it up one day for quick, rough castings. Whether health permits it or not I don't know and won't till I get home from my doctors in 2 hours if I don't end up in hospital.


My engineer friend locally would often ask if I wanted bits of steel and alloy to play on the lathe, varying grades. He'd pull them from his scrap bin after someone changed the design on 100 something or others he'd manufactured for them, high grade steels of varying hardness. One day he mentioned he needed new pins on his backhoe at home and I asked "wouldn't this stuff you're throwing out fit?"
Well, problem solved, a prefect fit without the slightest modification.
To be fair he was/is in the process of clearing out his workshop and taking everything home to work from there in a huge shed he has and he has so much stuff and bits and pieces.
I avoid going to the shop too often because he keeps insisting I take stuff, things I know which have cost him a lot of money and I don't want to be that friend, I always offer him cash. I did take a few collars he was dumping, again, a hardened steel and said they'd be ideal for my 12 tonne workshop press. He then decided to take a set for himself (he'd normally just machine one on the day if needed) and also grabbed a bunch of them to give to his workshop neighbour. He didn't see the practical use till I mentioned something. Though for him to throw it out and buy steel next time he wants one and it will be charged out I guess is no big deal financially, for me, wow, the materials and gear he's given me.


I was shocked the other day when I bumped into him and he said he's had an offer of $3,000 for a lathe, a HUGE lathe which runs near the length f his workshop. I asked "didn't you spend $10,000 on the last chuck"?
Yes!
He said he'll get $3,000 for the turret lathe with multiple chucks and steadies included. Unbelievable, I spent more than that on a small Chinese thing.

[RodW]

Nice work Stefan. :thumbsup:

[Stefan Buda]

Thanks Leo and Rod,

I got the back ring of the truss machined too. This one weighs 20g less than the front ring.

[Stefan Buda]

The front ring now has its radial holes done too.
I want to get the truss rings sandblasted before anodizing.

[Leo.G]

Do you do your own anodizing?
I have a few things I've done or had done I must get to myself if I can find my black food dye. I used to do zinc plating and anodizing for a living at a place in Silverwater (Westwood Winters) back in the early 80s, back in the day of sulphuric acid only 2 tanks away from the cyanide tank, fun job, don't breath the dust in and hose before you sweep.
I never missed the big rubber aprons, gloves and boots when I left either.
They no longer use cyanide in zinc plating I believe.


Sorry, probably a stupid question (I have lots of them), do you sandblast to increase the overall surface area for a better finish or is it just for the matt finish and better grip/feel?

[RodW]

Ditto, I'm also curious as to why you would want to sand blast your beautiful machined finish. And isn't sand on aluminium a bit harsh? Thanks Leo for asking.

[Stefan Buda]

The sand blasting is only for cosmetic improvement. It hides the machining marks.
The anodizing is a bit of a problem in Melbourne as there are only two places that can do it. One is very unreliable, quality wise, and the other has a minimum charge of over $400, although they work to military specs and I never had a problem with their quality.

[Leo.G]

While often refereed to as sand blasting it's actually bead blasting with various compounds and grades of beads available to suit different tasks/materials.
In a past life a company I was at we sold the cabinets and equipment to do the blasting and I did a couple of old rocker covers for a friends V twin Japanese motorcycle and they came out looking better than new. I should have bought a cabinet then but now own a little hand held gun, cheap Aldi thing I haven't tried yet and a suitable compressor.
I may yet build a cabinet for it, I'm sure I have the steel and only need the Perspex and gloves though I won't expect too much of the unit.


EDIT: I was replying here when my sick cat had an accident and I had to clean it up.


I know the gentleman who used to do it at the small arms factory locally (now Thalis I believe). He's said he may be able to get small things plated for me (he's now retired) but I haven't asked. When it comes to plating I'd expect if time permits it's better to save a few smaller jobs up and have them done at the higher quality place though anything urgently needed it's pay up.
This may also explain why a company I recently contacted somewhere in NSW didn't bother replying to my contact, not worth it to them to do the small slide projector parts. The amount of small one off's I did back in the day, all legit (down to a single bolt and nut, often free, just thrown in the rack with other jobs (huge tanks)), the old boss was a never say no person and wasn't a thief either.



I may be wrong in saying this, Joshua Bunn has in the past mentioned to me he does all of his own anodizing, perhaps it would be worth having a word with him but I just realised he's in WA and I think you are in Melbourne, postage would probably cost more than the job locally. but there maybe safer, better options available, I haven't looked. My son and I have played with a circuit board tank to nickel coat contacts on a circuit board and I'll be buying or making a smallish fish tank to do my own slightly larger projects.The chemicals are usually the dearer part but I've seen mention black food dye is quite suitable (and other food colourings). Play with scrap and the other important thing to remember is timing is everything.



https://www.dragonmetal.com.au/blog/anodising-aluminium-everything-you-need-to-know/




We've all seen your glass dome and I'm sure no one here really believes you leave machining marks, LOL


Sorry, not a sycophant, in awe of the end results of both you and Joshua with the work you've both shown here (I worked with some VERY bodgy machinists and some amazing ones too).

[Stefan Buda]

I can anodize small parts, but these are not small.

The expensive anodizer here in Melbourne has a lucrative contract related to the F35 program and can't be bothered with small jobs unless you are prepared to pay the ridiculous minimum charge.

[Peter Ward]

I feel your pain.

The CSIRO had a cold deposition mirror facility that I was prepared to pay for, (as I would not have to ship the optics offshore) but they point blank refused to service "external" (read actual $ paying) clients.

I digress. I am in awe of your work. :bowdown::bowdown:

[Stefan Buda]

Thanks Peter,

Yes, I don't know how we can compete with the Chinese, given some of the business attitudes around here.
Recently I dropped in to a fasteners place, on my way home from the northern suburbs, to get 4 M12 bolts, only to be told at the sales counter that they have a minimum charge of $20. I thanked the two men on the other side of the counter for explaining and then I let them resume their casual chatting.

[multiweb]

There's a place in Wetherill park in Sydney called WDC. They do all sort of fasteners and also stainless steel. Over the years I bought small bits and pieces from them. Always so helpful. They sell by bulk. Read 100s. When I come for 10 or 5 and go to pay they say don't worry about it. I always leave some cash for maccas for them.

[Leo.G]

Our local bearings and industrial supplies is much the same as what Marc mentioned. I often walk out with half a dozen stainless button head bolts for a gold coin when they don't even bother checking the price. Nice bunch of fellows.
Failing that I'd be going online but freight is often the catch and it's of no benefit when you're in the middle of doing a job and need them then and there.

[AstroViking]

Hi Stefan,

I don't know where in Melbourne you are located, but I've had good dealings with "Bayswater Bolts" on Canterbury Rd (Between Boronia Rd and Dorset Rd).

Usual disclaimer - not associated with them in any way shape or form, other than as a satisfied customer.

Cheers,
V.

[Stefan Buda]

Thanks Steve.

I do have my good fastener suppliers, not a problem. This was a place that just happened to be on my way, so I dropped in.

[Leo.G]

You mentioned you anodize small parts at home, I've yet to try it at home but my extender/adapter isn't exactly small, primary thing I want to do.


If you're using black are you using a specific dye?

I've only seen mention of black food colouring online (other than very expensive dyes) being suitable but haven't tried it yet.


It's different to working in the industry over 40 years ago where they had all of the precise chemicals and I no longer remember what was what. Plus they brought a chemist to get chemical mixtures right, I was happy not having to play with cyanide (I ran the zinc tanks but also did anodizing in another tank next to the one with cyanide) for a yellow/gold finish on aluminium products.

[Stefan Buda]

Yes, I use Sanodal Deep Black MLW from Clariant. I paid a couple of hundred dollars for a few hundred grams, over 20 years ago, and I still have enough left for a few more years.

Fabric dyes work but fade eventually.

[Leo.G]

Thank you sir!

[Stefan Buda]

The DK250 won't have a mounting rail for the dovetail plate, in order to minimize weight, and that means that the dovetail plate wont be adjustable for balancing the DEC axis. The DK won't need to cope with the wide variety of possible gear like the CDK, so the center of gravity won't shift as much.
To accommodate CG shifts I made the dovetail plate longer. The problem is that I'm only guessing where the CG will fall.

[Stefan Buda]

Finished the back plate. This is the most time consuming structural part of the OTA.
I decided not to sandblast the parts because I'm worried about distortions caused by the induced stress. It was difficult getting the back plate flat as every time I skimmed one surface on the lathe, the released internal stress distorted it. I was holding it at 3 points and had to alternate machining both sides, taking smaller and smaller cuts.
The back plate is very similar to the CDK250 version, the main difference being the addition of a pocket that will house the speed controller for the fans, the power jack and a red LED.

[Saturnine]

Very much looking forward to seeing the finished product, the design is a bit more complicated than I envisaged, with the cutouts for fans and switches etc. How much stress was there in the plate from when it was rolled, that it would distort when skimmed, would have been frustrating to have to re-machine each side of the plate.
Do you still have to do the mirrors for it or are they done already.

[Leo.G]

Lovely machining work, CNC or the old fashioned way?

[Stefan Buda]

The stress is always there when I make one of these plates.
The plate starts off at 16mm thickness and I machine it down to 14mm in stages so that I end up with no more than 12 microns runout near the edge.
I have two secondary mirrors ready for polishing and I'm modifying an old Pyrex test plate to become the first primary.


The old fashioned way except for the CNC engraving.

[RodW]

Morning Stefan

How far away is the first trial assembly of the mechanical components? What is still to be made before you can do this?

Rod W

[Stefan Buda]

Hi Rod,

I had the large aluminium components "sand" blasted, after all, but using a gentle-on-the-surface process using fine glass particles.
Yesterday I dropped off the same parts at Collins Anodic Treatments. I could not let myself be ripped off by Electromold with their $450 minimum charge.
So, fingers crossed. I'll find out in about a week whether they made a mess of my expensive parts. After that I will be able to assemble the truss structure.

Meanwhile I ran into a bit of a problem with my glass blank for the primary. The glass shows a lot of internal structure when viewed between crossed polarizers. Doesn't really look like poor annealing, but more like nonuniform glass composition. Maybe I will still put it through the annealing kiln to see if anything changes.
If I can't polish it to a smooth sphere, I will just use it as a reference surface for setting the spherometer when making the next primary.

[Saturnine]

Is that the glass blank that you got from me about 18 months ago or so. Hope the strain doesn't cause many problems when making the mirror though it sounds like it will.

[Stefan Buda]

No, this one came from a gentleman in Geelong. It was partially trepanned.
It will be good for getting all the grinding and polishing tools worked in ready for some good glass. I will be ordering some blanks from Germany soon.

[Stefan Buda]

I had a go at annealing the primary blank and, as I expected, it did not make any difference to the internal pattern seen in polarized light.

Exactly one week ago I did the casting of the disc for the polishing lap.
I covered the mirror blank with packaging tape to stop the plaster sticking to it. A few days later I was able to easily separate the two discs.

Today I picked up the 5 pieces of black anodized aluminium from Collins Anodic. They did a good job and the cost was reasonable at $99.
I'll be very busy the next 3 days, so I won't have the time to assemble the truss until early next week.

[RodW]

Great news about the anodising, that must be a relief.

Looking forward to seeing the first assembly of the components.

RodW

[Stefan Buda]

On closer examination I found a few spots where the the anodizing has been slightly damaged by careless handling. I think the next truss will have powder coated rings.

Anyway, the truss went together nicely and it feels very rigid.
It weighs 2050g.

[RodW]

Looks good so far. How will the mirror cell attach to the truss section? Will you have a solid carbon tube section between the bottom ring and the rear cell plate?

[Stefan Buda]

Yes, it will have a solid carbon tube section. I designed it so that a CDK250 tube can be cut in half, to make two DK250 tubes.

[Moon]

Stefan - I'm really enjoying following your progress - thanks for the updates.

[Stefan Buda]

Thanks James, good to know.

Meanwhile I had a bit of a setback with the primary. I made a calculation error when setting up the Two Ball Spherometer and, after several hours of polishing, it turned out that the radius of curvature was about 10mm too long.
So I had to go back to fine grinding and deepen the sagitta by about 9 microns.
The good news is that the glass seems to be behaving as I now have a smooth and fully polished surface ready for figuring.

[Top_oz]

This is very impressive.

Can I ask what tolerances you are working to with the machining?

When you say the backplate was a challenge to get 'flat', what are you aiming for?

After dabbling with some machine work I have developed a much greater appreciation for tolerancing and metrology.

Flat for me is a multi faceted challenge of getting two sides flat, and parallel, and a certain distance apart. And then being able to actually measure the result with any degree of accuracy.

I've not found any specific advice for astro so very interested to find out how tight this work needs to be.

[Stefan Buda]

Hi Tim,

My OTAs are made to tight tolerances in order to eliminate the need for collimation of the primary mirror.
The back plate was held, on the lathe, against a sturdy steel ring, with three screws. The steel ring was about half the diameter of the aluminium plate.I used washers as spacers between the steel ring and the aluminium plate so that the plate was free to change flatness as internal stresses were released during machining. Flatness was tested with a dial indicator that has 2 micron divisions.
The finished disc was reading deviations of no more than 10 microns near the periphery on both sides.
If the primary mirror is adjustable for collimation then such accuracy is not needed.

[Stefan Buda]

I finished figuring the primary today. Unfortunately I got a bit over confident about airborne dust and ignored the north wind as I did some of the figuring.
As a result I ended up with a couple of scratches, which is very annoying although it won't affect the performance.
It was interesting to see how much easier it is to figure an f/3.7 mirror versus an f/2.7, as in the CDK. The interferograms are much easier to capture as the interferometer is not pushed to it's limit, and the processing of the interferograms is also easier.
The attached screenshot is a bit of a cheat because it shows only the spherical terms of the Zernike polynomials. If I enable all terms the Strehl ratio drops to 0.95 but it is way past the diffraction limit anyway.

[Leo.G]

You sir are amazing!
Will the scratches polish out?
When I win the lotto I'm going to have a clean room for various things I do ( I have been known to strip an occasional hard drive to resuscitate it and a lot of server gear and I strip down way too may camera lenses multiple times when I realise one cat hair made it's way in.). Owning (or being owned by) 5 cats means no matter how I clean or what I do I always find at least one stray cat hair. It will be a cat free room.
They always leave their mark somehow.

[Top_oz]

Thanks for the feedback on your machining. You're working to almost a magnitude lower than what I can achieve :)

[Stefan Buda]

No. I will keep this mirror for my own use.

The front end is now finished, minus the polishing and coating of the secondary mirror.
I used the same collimation arrangement as I did on the CDK but a bit simplified by using a single central spring instead of 3 distributed ones.

[RodW]

Looks perfect, very impressive!

[Leo.G]

Looking very impressive indeed!
It's a pity there isn't an applicable coating which could be smeared on and fill the scratches. I guess they would only be very light then it would have to be something suitable for the mirror coating to go over.
I've broken thousands of mirrors (that may explain something) working in the glazing industry early 80s but not sure on what would be suitable if anything.
If using for visual they wouldn't be such a problem would they?

They could be edited out with darks and flats for imaging I'd guess so still a win, win situation.

[Stefan Buda]

This is only a cosmetic issue. Scratched products don't sell well even when the scratch has no effect on the intended use.

I think I mentioned earlier that I made a 10" f/16 Dall-Kirkham before, a long time ago. Well, fortunately, I still have the test plate I made back then for figuring the secondary mirror. This test plate was manually polished and figured with a Foucault test apparatus as I did not have an interferometer at the time.
Today I had a go at testing it with the Bath interferometer and the summary of the test is on the attached screenshot image.
The test plate was made much larger than the secondary in order to avoid possible edge figure problems. As the test results show, the inner 70mm of the test plate is quite adequate for the purpose.

[Rod]

Hi Stefan

That looks like a great result. Am I reading the graph correctly - it looks like the largest error at 35 mm radius is about 1/16 wave on the wavefront? If so, that’s fantastic. You were obviously very careful with your original testing.

I’m new to interferometry but now have all the parts to complete a bath. Can you analyse the interference fringes for the secondary with DFTfringe?

Rod

[Stefan Buda]

Hi Rod,

The test does contain artefacts as it is based on a small number of interferograms. If we take that into account then yes, most likely the central area up to 35m radius is about 1/16 of a wave on the wave front. If we include all the "noise" then 1/10 wave peak to valey should be read.
This was not an easy testplate to make, even though it is spherical, simply because it is f/2.6 and hard to see the full surface with a classical test aparatus. Something like an 80mm diameter would had been a better size, much easier to make.
For the secondary, if you can take good on axis images of the fringes then DFTF has no problem analizing them. You will need a good Fizeau interferometer so that the fringes are produced and seen in parallel light.

[Stefan Buda]

I started work on the carbon fibre sandwich tube and I'm very happy that I managed to get good quality 3mm Coremat. When I made the last two CDK tubes it was impossible to get the good stuff and I had to use whatever was available at the time. The main problem was the uneven thickness which required extra work and the result was still not as good as I would've liked.

I designed the DK so that one CDK tube can be cut in half to make two DK tubes.

Also I started work on a second primary mirror, this time using good quality glass from Schott.

[Stefan Buda]

Good progress with the CF tube(s).
The first pic shows the tube after it has been sliced up on the lathe.

[Leo.G]

That looks amazing Stefan!
Do you use a vacuum device when doing the carbon fibre?
I've not done anything with it (done a reasonable amount of fibreglass over the years) but I have been told you need vacuum formers for the CF?

[RodW]

Nice work again Stefan, great result.

So is final assembly the next step? I think everything has been manufactured if I've been paying attention.

[Stefan Buda]

No vacuum used just plain laminating.



To be made: Primary baffle, primary support, focuser interface, focuser extender, dovetail plate mounts, finder attachment plate mounts and dust covers.

[Leo.G]

That's a very nice job and I will re-visit my thought of using it for something if I remember what that something was. I was assured without the vacuum forming I'd have extreme issues. I just don't remember what I wanted it for back then.

[Stefan Buda]

The primary baffle is finished.
I used CF filled ABS filament for good heat resistance.
The bonding of the segments was done, one at a time, on the lathe so that each piece was running true before the adhesive gelled.

[Stefan Buda]

After bonding together all the structural pieces of the OTA, I used my wonderful Taylor & Hobson micro alignment telescope to see how much misalignment I accumulated up to this stage.
I measured a pointing error of 0.38mm at the centre of the secondary holder, which is good enough especially when considering that for a spherical secondary the optical axis does not have to go through its geometrical center.
For the next OTA, I will be able to easily reduce the pointing error by doing this test before bonding the CF tube to the truss section.

[Saturnine]

The scope is looking good and I'm impressed by your workmanship and attention to detail, looking forward to a first light report.

[toc]

The rabbit is impressed and so am I :lol:

Kind of a left field question, but how hard are SCT corrector plates to make? I dont think they can be hand ground can they?

[Stefan Buda]

Thanks Jeff!


The Netherlands Dwarf rabbit is my supervisor. :lol:

The SCT corrector plates are practical to make only in a series production scenario because of the fancy/expensive tooling required.
The plate usually gets sucked onto concentric supports by vacuum, so that the top side is deformed in such a way as to allow grinding and polishing to a flat surface. One the vacuum is released the surface will spring back to form the complex curve required to correct spherical aberation.

[Stefan Buda]

The backplate is coming together nicely. The fan speed controller works well.

I hope to achieve first light at the Messier star party in a couple of weeks time.

[Leo.G]

They look like good quality fans Stefan?
Cheap things never synchronise and set up harmonics and introduce a lot of unwanted vibration, I've seen it too many times in computer systems.
Overall it's looking amazing, I bet you're excited to try it all out.

[Stefan Buda]

Leo,
They are not the cheapest fans, but not top of the range either.
I hope to tune out the resonances with the speed controller.
Yes, I'm keen to try it although there are no planets now that are high enough in the sky to be worth looking at. I might have to test it on double stars.

[Leo.G]

Is the design better for planetary Stefan?
Yes, not top of the range fans but certainly not the cheapest will often put you in a sweet spot where you get the performance without the vibration or price. I've tried explaining this to people in the past who swear by their cheap eBay stuff. Though top dollar doesn't always win as the best either but you soon learn which brands to trust.


Sorry for my ignorance Planetary is in the title!
I must learn to read one day..........


I don't have a planetary telescope as such, I just naturally assume I can see everything with my telescopes, probably accounts for some of my lousy results at times.

[Stefan Buda]

No problem, Leo.

One thing I learnt early on, doing planetary imaging, is that a good finder scope, well aligned with the main OTA, is necessary, otherwise planet hopping with the camera mounted is difficult due to the very small field of view.

So I made a decent finder mount and knocked together a finder scope, using a 1.5" diameter achromat from my junk box, that will allow me to do the initial testing. The achromat has a brass housing and it is engraved Waterworth - Hobart. I did a bit of digging on the internet and discovered an interesting story about optics manufacturing in Tasmania: https://www.asap.unimelb.edu.au/bsparcs/exhib/omp/org/waterworth.htm
Also, I think I finished figuring the secondary mirror today.

[Stefan Buda]

The OTA went together yesterday, ready for testing, but it's going to be cloudy in Melbourne for a while.
I intend to take it up to Heathcote tomorrow or on Saturday to the Messier star party. It will be very embarrassing if it turns out that I made a design or manufacturing error and the thing won't even come to focus. I always wonder whenever I finish a new prototype.

[RodW]

Looks fantastic Stefan, well done!
What is the overall weight of the full rig as shown?
Friday could be clear in melb, according to my sources.

[Rod]

Congratulations Stefan.

Looks terrific! That came together really quickly. I hope to have a look through it one day.

Rod

[Saturnine]

I'm sure, with your attention to detail, that the scope will perform excellently but regardless, the telescope looks fantastic and well engineered.[

QUOTE=Stefan Buda;1625625 It will be very embarrassing if it turns out that I made a design or manufacturing error and the thing won't even come to focus. I always wonder whenever I finish a new prototype.[/QUOTE]

[Stefan Buda]

Thanks Rod, the overall weight as shown is exactly 11Kg. The OTA alone, without the finder scope and camera/filter wheel, is probably close to 10Kg.

Thanks Rod, I'm off to the Messier star party and if you are attending too, then you will be able to look through it, providing that I can make it work. :lol: Not much to look at though, with the planets so low.
[/QUOTE]
Thanks Jeff, fingers crossed - so to speak, as I'm not superstitious.

[Stefan Buda]

Back from field testing.
All went well, no mistakes found.

Unfortunately the seeing was horrible for planetary imaging and I came home without any good data captures.

The seeing was nowhere near good enough for capturing data for a Roddier test so I had to rely on visual star testing and a Ronchi eyepiece to assess the optics. I detected a very small amount of spherical aberration that I will have to track down and correct.

Managed to see Sirius B at 400x and the attached images were captured through red and blue filters respectively. Each image is a stack of about 2000 unsorted frames, as Autostakkert was unable to sort them.

[Rod]

Hi Stefan

I really enjoyed having a look through your new scope. It looks fantastic of course. I was really impressed be the sharp images I saw visually. The Tarantula was very clear and bright. Lots of contrast. Thanks for showing us the pup. I hadn’t seen it at the eyepiece before.

Looks like it will be another great product. Hope it goes well for you.

Rod.

[Stefan Buda]

Thanks Rod,

Shame about the poor seeing though. I've hever done planetary observing from that location so I have no idea how often good planetary seeing happens there.