Secondary Mirror recoat (now a discussion about minimum secondary size)

[Jethro777]

I have a lovely 110mm reflector with a 28/38mm secondary, where the secondary is getting a bit degraded.
Is there a place that can recoat the small secondary for me?

[dannat]

prob easier buy a cheap one

[Jethro777]

I am thinking the same - but I don't want a cheap one - I want a good one. :)
Yes, what do you reckon, it needs replacing?
How good do mirrors get as far as reflectivity is concerned? 94%?

[Wavytone]

Dielectric - 99%

[Jethro777]

Where can you get good dielectric secondaries?

[bojan]

I would wash it first.

[Jethro777]

Hey Bojan,
I don't think it will go by washing... Did you see the picture? Seems to be a scratched mirror with some side degradation to me. The primary, strangely enough is great.
Was also thinking a GSO which is 93, but hoping for something a bit better. I need a 27x38mm.
I managed to find a nice Intes Micro one, but it's 24mm

[bojan]

Still.. try with soaking it with tap water (slow drip), then apply mild dishwashing detergent and after that rinse it with demineralised water..

It may be quite OK (until you find adequate repacement).
Secondary is usually damaged while attemting to colimate with greasy fingers...

[dannat]

a good one from telescopes-astronomy in sa is about 60-65, average ones 35ish, orion optics uk sell decent ones but with post/exchange rate they are over 100

[mura_gadi]

Hello,

I was going to suggest Orion optics as well, they have a good list of secondary mirrors.

This article on secondary size for planetary and DSO/ visual and astrophotography applications is worth a read as well if your looking to change secondaries anyway.
https://garyseronik.com/sizing-up-the-newtonian-secondary/



S.

[Saturnine]

GSO 28mm Minor Axis Secondaries are available from Andrews Com and Bintel for $29 and $39 respectively or you could try calling Mathew at Telescopes & Astronomy in S.A. or Claude at Astronomy & Electronics, also in S.A. as they list secondaries among their stock, maybe have to order in though !

[Jethro777]

Great link there Steve, Gary explains it so so well.
And thanks for the references everyone to where I might hunt one down.

[JohnF]

In another thread I gave a link to a youtube on re-silvering mirrors -- My secondary is also in bad condition.

[Jethro777]

I may need some math/logic help here, if someone could help? :confused2: - The end purpose, is to calculate the minimum secondary size.



Okay..

So, I'm working this out for a Type 1 (1992) TAL reflector. So, it has a 110mm primary, currently has a 27x38mm secondary, and the official ratio I see around is f/7.3 (but there are two TAL versions - one original - one a later model white one, with a shorter tube which I don't have). (The scope is discussed here: https://www.cloudynights.com/topic/477161-classic-tal1-newtonian-from-1990/ - At the end of the article, Andie Howie describes some of the differences between this and the later version).

I raised the primary mirror in the tube 28mm, and the distance from the primary to the center of the secondary mirror is now 66cm (26"). This allows modern 1.25" eyepieces to be used.

"L"

The focal plane on a TAL is interesting. I need a moon or something, then need to project it onto tracing paper in the eyepiece holder without an eyepiece? I guess I can do that, currently, moonrise is a little late. For arguments sake,m let's imagine it's 13.5cm (5,3") - that's with the focuser about half way, and I can adjust/update the formula once I understand how it works.

How do I put all that together correctly, to find the minimum secondary size? :question:

[RPT]

Jethro,

Mel Bartels "Newtonian Telescope Designer" web page may be helpful.

https://www.bbastrodesigns.com/NewtDesigner.html#diagonal

Mark

[Jethro777]

What is the focal ratio of my scope?
I have heard it doesn't change - it's fixed - so that would make it f/7.3 - but I have raised the primary mirror 28mm - so does that make it change for the purposes of calculating a secondary mirror size?

I calculated it now - it seems to still be f/7.3
26" to the secondary, then 5.3" from the secondary to the focal plane.
The primary is 4.3".
So, 26"+5.3"=31.3"
So, 31.3 divide by 4.3 = 7.27

After plugging it all in, it seems as though the current mirror (1.1") is too small by far. The recommendation seems to be - (according to bbastrodesigns anyway) 1.83"

I'm comparing that with the page from Gary. He says it's L divided by F.

f = Telescope’s focal length divided by the diameter of the primary mirror) (in my case, 7.3)
L= Distance from the center of the tube to the focal plane (In my case, 5.3")

5.3 divided by 7.3 = .726??

Now even more confused, lol.. C'mon - what have I done wrong?

[Saturnine]

Hi Jethro

Use this Newt for the Web calculator to work out your scopes dimensions.

https://stellafane.org/tm/newt-web/newt-web.html

The focal length of your scope doesn't change by moving the mirrors, the focal length is governed by the radius of curvature of the primary mirror which doesn't change unless you regrind the mirror. By moving the primary up the tube , all you are doing is moving the point of focus further outside the tube, as reflected by the secondary.
What also happens then is that not all the light from the primary is being concentrated on to the secondary. Some light will pass it and go straight out the front of the tube so you're effectively losing aperture. Your focuser will need to rack out an extra 28mm for focus to be reached. Doesn't matter if they are the original TAL eyepieces with a 32mm barrel or the standard 31.7mm ( 1.25" ) of most common eyepieces , the focal plane of the scope stays in the same place, it's the eyepieces that have slightly varying focal planes, that's why different eyepieces need to be re-focused slightly. Unless of course you have a par-focal set.

[Jethro777]

Thanks, that is helpful. So, the f/ratio stays the same.

>By moving the primary up the tube , all you are doing is moving the point of focus further outside the tube, as reflected by the secondary.

So, this is interesting - after moving up the primary mirror - I basically created a "problem" - that now some light reflected by the primary was missing the secondary - to compensate, I need a larger mirror. Is that right?

>Your focuser will need to rack out an extra 28mm for focus to be reached.

Previously, the 1.25" eyepieces could not be focused - only the original 32mm versions. After the mod, most eyepieces can be focused using the standard focuser as is.

What does this mean for my secondary mirror size calculations?

Well, I put the optics data in newt-web (Primary 4.3, Focal Ratio 7.3) and it calculated it as "Diagonal Range = 0.95 - 1.43"

A 1.5" mirror would give me a 35% obstruction, the original mirror (1.1") 26%, and it recommends trying to get 20%.

I was thinking of getting a 1.3" - Would that be advisable in your opinion given my situation?

[RPT]

Jethro,

There is a parameter in the calculation called "Max field diameter". The default setting in the calculation is 1.6 inches which is enough to fully illuminate a full frame camera CCD. You can change this setting to match the maximum field lens diameter of the eyepieces you intend to use. If for instance that was 0.75 inches that gives a minimum mirror size of 1 inch.

[Saturnine]

Yes you would need a larger secondary mirror but I still don't understand what you are trying to achieve. The Russians when they designed and built these TAL newts knew what they were doing and had the math all worked out for optimum performance.
I had one of these scopes in my possession for a while and it was a damn good scope for its aperture and by shimming the barrel of my 1.25 plossels with some masking tape to fit the focuser drawtube could reach focus with no problem via the travel allowed by the focuser.
If you really want to move the focal plane out as you have done by moving the primary up , then a larger secondary like the 31mm GSO would work fine. The slightly larger obstruction wont make much noticeable difference and the GSO quality is fine but there are other options too.

[Jethro777]

Thanks Mark and Jeff - I want to replace the secondary due to damage. When I do so, I don't want to get one that is too small, or too large. I've also adjusted the primary as per the recommendations the Russian users had on these first versions.

The travel is insufficient to get focus on modern eyepieces without the 28mm primary mirror adjustment.

As for its quality, they are exceptional. I love mine, and I certainly want to have a proper matching secondary. Does anyone know the strehl quality on the primary on these? I know it probably won't make a difference too much - but I'm emotionally attached to the thing!

>but there are other options too.

What do you mean Jeff? Tell me more. :)

[Saturnine]

Just meaning that there are other suppliers of secondary mirrors besides GSO but they probably need importing from the US or UK and are more expensive but supposedly better quality, especially Antares. Orion Optics smallest is 35mm which is getting a bit big as far as obstruction goes.

http://www.antaresoptics.com/
https://www.orionoptics.co.uk/OPTICS/ellipticalflats.html

[Jethro777]

I was actually thinking of the 1.3” Antares.
They don’t make a 1.1”.
I think a 1.3” probably would work well?