Rebuild of old Newtonian Scope

[BadWolf]

I have a 40 year old 6 inch Newtonian in bits an pieces in a box - been carrying it around the country for decades. Have started a project to completely rebuild it with modern components (it has an old 0.965" focusser, for example). The mirror is an F/5 Coulter purchased from the USA many moons ago - I suspect Coulter is now defunct or merged with another company?
Long story short, is that the aluminizing on the 6 inch mirror and 42mm secondary have deteriorated badly and I need to have both mirrors stripped and re-aluminized. I have done some online searching, but having issues finding a business here in Australia that will provide those services. I am sure there will be a couple, but my poor internet search skills can't find them as yet. Can anyone out there provide the names of businesses that can help? I am in Brisbane, but can naturally ship the mirrors interstate. Any help or advice appreciated.


Thanks,

[astro744]

A couple of names I’ve heard on this forum are Saintech and Palmway Optical. Not sure if both still are around or offer thus service.

Before you re-aluminise do you know the optical quality of the mirrors? Ie. surface accuracy and smoothness. If of high quality by all means get them re-coated. Go for the best coat available to ensure not having to do it again for a long time.

If the mirrors are not that good optically then perhaps consider a replacement set of optics. To ensure a nice fit you just need to match the diameter and focal length of the primary.

[BadWolf]

Thanks for the information and advice. I will check to see if they are still in business. Good advice regarding the quality of the optics. I do intend to build a basic Ronchi tester and then verify the optical quality before re-aluminising. Thanks again……

[Saturnine]

Hi Badwolf

Palmway and Saintech are one and the same, based in Tuncurry NSW and they are definitely still in business, will be getting a mirror recoated by them soon.
For an 6" / 150 mm mirror it may be just as cost effective do buy a new set of mirrors, though stock in this country is an unknown , due to supply chain issues.

[BadWolf]

Thanks Jeff, I will make enquiries. I understand that new optics are cheaper, however the whole project is a sentimental one, so a few extra dollars are ok - unless the difference is "astronomical" - [pun intended].

[OzEclipse]

They are the old and new names of the same company. Dr Wayne Sainty started Saintech then later changed the name to Palmway Optical. They're located at Tuncurry and they do a superb job.

I have an excellent hand figured mirror that I had Palmway apply an enhanced coating + overcoating. Cost around $400 including two way freight 5 years ago. The previous coating was uneven, this recoat restored the mirror to it's former glory.
Joe

[BadWolf]

Thanks. I will be emailing them next week to get a quote. Good to hear they come recommended. Thanks again.

[ausastronomer]

Hi Stewart,

I understand that there is some sentiment attached to the scope and the original mirror set. As Jeff mentioned it will likely be cheaper to buy a new mirror set than get both of those mirrors recoated. Have you used the scope before or had the mirrors tested. Coulters reputation for producing scopes with very poor optics is world renowned. It's likely that the original Coulter mirror set will be very ordinary and to be honest, not worth recoating, if you actually plan to use the scope. In the larger apertures you could get away with an ordinary Coulter mirror, as most of the larger ones 13.1" and 17.5" were mainly used as "light buckets" for DSO's and not as planetary scopes. A 6" scope is more likely to be used on Solar system objects than on DSO's where mirror quality will be a lot more important.

Just something to think about! Not much point spending the time and money rebuilding the scope and getting the optics recoated to end up with a telescope that's a pretty ordinary performer due to the original crappy Coulter Optics. You'll be able to buy a GSO or Synta mirror that's brand new and a much better performer.

The other thing to consider is that a 42mm secondary is a 28% Central Obstruction and probably 1 size bigger than you need for a 6"/F5 tubed newtonian, which will further detract from planetary high power performance.

Cheers
John B

[ausastronomer]

Hi Joe,

What size mirror are you talking about ?

Cheers
John B

[astro744]

Please confirm the 42mm is the minor axis of the secondary, i.e. narrowest width. It is an odd size for a US company like Coulter to use. Sizes used to be as follows in inches:

1, 1.3, 1.52, 1.83, 2.14, 2.6, 3.1, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12

Multiply each by 25.4 to get mm. If 42mm is the major axis then first divide by 1.414.

That being said there is only so small you can go with 6" f5 as there are physical restriction of tube diameter, focuser height and desired edge of field illumination. I have an Astro-Optical 6" f5.5 "Super Short" and they used a 1.83" secondary for it. GSO 6" f5 use a 50mm secondary and Teleskop-Express use 63mm but it is optimized for astrophotography. The detrimental effects of an oversized secondary are often overstated so don't be too concerned with 42mm as it is on the smaller end for a 6" f5 but it does depend on what focuser and tube you have. SCTs have far more obstruction yet still provide good planetary views. Ideally 25% and under is optimal but at 28% I wouldn't worry about it if the secondary is good.

Now after saying all that the modern offerings for Asia are metric and not a direct swap with existing secondary mirrors especially if they are held in place rather than glued.. It may not be that simple to simply change the secondary.

[ausastronomer]

I've looked through dozens of different SCT's over several decades (yes I can even go back to the crapola produced in a rush for Haleys Comet in late 1975 and 1976) and I am yet to get a lunar / planetary view in a SCT that comes remotely close to what you get from a high quality equal aperture Newtonian. The laws of physics preclude a SCT from being the optical equal of a Newtonian and that is consistent with my under the stars experience. Planetary imaging is a whole different ball game where the size of the secondary and some of the other factors that affect SCT's visually don't come into play. You might be more easily pleased than me, in which case the size of the CO wouldn't matter. If you're trying to eek out the last bit of performance for a given aperture then it does and ideally it should be under 20% and not under 25%. Under 20% is where the MTF curves of an obstructed scope approach those of an unobstructed scope. At 25% CO they aren't close. My 10"/F5.3 Suchting mirrored SDM has a 1.83" Protostar Quartz secondary and I can guarantee you there isn't a SCT on the planet that gets remotely close to it as a lunar / planetary scope. I don't even let them on the same field :)

It will all be academic if Stuart sticks with the original Coulter primary as it's quality is unlikely to be high enough for the secondary quality or size to make much of a difference.

Cheers
John B

[mura_gadi]

Hello,

Its handy to know the EP's you want to use, essentially you would want the 75% illumination field from the secondary mirror to be the same or slightly larger than the field stop of your lowest powered wide field EP.

Under 75% illumination and the edge will drop away sharply and become noticeable for visual use. However the edge of the secondary can result in optical distortion, so adding a mm either side can be a good recommendation, over a secondary that is mm perfect.

Illuminated FOV and secondary sizing can all be found easily using "Newt for the Web" program, well recommended.


Steve
Ps. Might be worthwhile asking around if any ATM'ers in the Brisbane area would test the mirror for you. If it still tests well then get the recoating done.

[astro744]

I wasn't suggesting an SCT will rival a Newtonian, merely a Newtonian with 28% obstruction is much less than an SCT will offer. I have a C9.25 as well as a 10.1" f6.4 Newtonian (Suchting), the latter with both 2.1" secondary and 1.83" secondary. I prefer the 2.1" for better illumination for my wider field stop eyepieces. The Newtonian with the larger secondary still gives exquisite views of planets with either Tele Vue offerings or my personal favourite for planets being Clave Plossl. I have teased out very faint detail on Jupiter with my 6" f5.5 Newtonian using a Vixen 5mm Ortho (0.965") before I had any Clave or other eyepieces and this with a 1.83" secondary.

I use Mel Bartels secondary sizing routine. Mel used to talk off axis illumination in percentage often referring to aim for 70% illumination at edge of field of widest field stop eyepiece you intend on using. Now he refers to magnitude drop (about 0.4-0.5 mag. drop).

Of course 20% is better than 25% and 15% is better than 20%, (f ratio dependent to avoid too much illumination drop). A smaller obstruction is much more easily achieved with a larger telescope than a smaller one. 25% obstruction has been talked about on other forums and is an acceptable target to aim for but of course less is better but whether you even notice it on most nights or at all is up to a number of factors.

I have looked through a 16" f5 (Suchting) with a 2.6" secondary and low profile Moonlite focuser and the image noticeably dimmed at edge of field. Obstruction was 16% and yes it gave a superb view of Saturn that night but as a general purpose telescope it could have done with one size bigger secondary at least. (100% illumination was very small).

[mura_gadi]

The .965" and an older style high profile focuser will limit the illuminated field considerably.

If your going with as complete as possible, depending on the ID of the focuser tube and the min/max height of the focuser, the .965" ep field stops are going to allow you to go pretty small on the secondary and still reach 75%(70%) illumination.

Fire up newt for the web and enter in as much data as you can in the specs' tab and read the help options, they are informative. Performance tab will show you the illuminated field values.

[OzEclipse]

Hi John,

My mirror is a 6 inch f7. I asked Palmway to apply both an enhanced aluminium coat plus a SiO overcoat on it. From memory I paid some tens of dollars to freight it to them, then they invoiced me $420 to apply the two coats to the primary and diagonal and freight back to me.

My mirror was originally hand ground polished and figured in the 1970's under expert mentoring by the late Cliff Duncan. The deal was that we we finished when he said it was finished not when I said it was finished. The scope had both sentimental value and the optics are really excellent so for me it was worth the expense. I would not re-coat a mediocre mirror.

I agree that the large Coulter optics mirrors in the 13.1" and 17.5" dobs were often not great, however Coulter also produced some very high spec optics at that time. The 12.5" Coulter cassegrain in Arthur Pages observatory at Mt Tamborine had incredible optics. So, I didn't make any assumptions about the quality of the OP's mirror but in my post I did make the point that my mirror is of excellent quality and so the added expense was worth it to me.

Joe

[astro744]

:thumbsup: Very well said.

[hamishbarker]

I have a chamber and can recoat your 6" and secondary, bare aluminium only though, I have not got Silicon overcoat happening yet. But I am in Nelson NZ. So you would have 2 way postage cost, circa $50 each way I guess, depending on how bulky and heavy. Best way to ship is to build a solid, slim wooden box with secure, lead-faced clips at 3 or 4 points around the mirror bevel.

I've successfully aluminized 4 mirrors so far (10", 6", 12" and a couple of secondaries). $50 plus $10/inch, so $110 plus postage.

For any other mirror makers or those needing recoats to old mirrors, the chamber can currently take up to 16". An upgrade to a bigger chamber is in the works to take up to 25.5".

Cheers,
Hamish

[ausastronomer]

That's great to hear Hamish. There are very few coating options in Australia and New Zealand at the moment. Wayne Sainty does an excellent job, but not budget priced, which is in keeping with the quality of his work. Outside of Wayne the cupboard is very bare. 10 to 20 years ago there were a few good choices including Isaac at Chi Qin Optical Coatings.

Cheers
John B

[BadWolf]

Thanks John. I am sure all your comments are very relevant. The project is really just sentimental value. As far as the quality of the optics and the cost of re-aluminizing, I will be running the Ronchi/Foucalt tests out of curiosity. I have been contacted by an IIS member who can do the job for far less than the more commercial services. Still assessing the issue at this time.

[BadWolf]

Thanks Steve, I will do all the math before starting on any serious rebuilding.

[BadWolf]

Thanks for the offer Hamish - much appreciated. I have made contact with member who can do the job, so once I finish the math and the planning I will get moving on the project.

[BadWolf]

You are correct about the secondary - my error. It is actually 46mm (1.83 inches in oldspeak). I am in the process of doing all the math at present. Thanks for the info.