Advice: SN10 primary mirror replacement or new imaging scope?

[martinux]

Due to an unfortunate accident my SN10 primary mirror is broken. While it was an unwieldy beast and pretty much required a complete rebuild when I got it from the previous owner I really loved it.  

My understanding, thanks to discussions between owners of this OTA, is that the SN10 used a spherical mirror. However Meade's representative did not wish to confirm this "proprietary information" after stating that they could not sell me a replacement. 

So, I'm stuck with a bit of a quandary: do I roll the dice and attempt to get a spherical mirror constructed? (I doubt that non-parabolic mirrors are routinely manufactured and would probably come at a price-premium). I have read anecdotal discussions that the primaries are somehow "matched" to the schmitt correctors which, I must admit, makes no sense to me as I thought that a spherical mirror was manufactured with a uniform curvature. Thus, without more information, I'm thinking that this could be an expensive and ultimately unworkable solution. 

Alternatively, do I cut my losses and replace it with something like a Skywatcher 250P?

Any other suggestions? 

Many thanks for taking the time to read this and for any advice you can render.

[Ricochet]

Ouch, that is an unfortunate event to say the least. If Meade won't even confirm the shape of the mirror, then I guess they have not given the focal length of the mirror either (the focal length of the primary being different to the focal length of the telescope). As such it seems quite a risk to try to get one made that has the correct features. I think that unless you can find a second broken SN10 with an intact primary to combine the two scopes, purchasing a new telescope will be the best option.

[John]

My understanding is that the Meade SN's did indeed use spherical primary mirrors but what is harder to find out is the precise focal length of the primary and how it's figure relates to the meniscus lens and the secondary mirror.

Overall I suspect that replacement is going to be both difficult and costly, unfortunately.

 

[johninderby]

The cheapest way would probably be just to buy another SN10 OTA and use the mirror from that. 🤔

[martinux]

Many thanks for the replies. 

Unfortunately SN10's seem to be a bit of a rare beast. I had a search through the UK astrobuysell adverts and they don't come up very often (in this part of the world, at least).

I'm still concerned by the discussion of "matched optics"; I could end up with two sets of incompatible optics. Still, I suppose a wanted advert might be worth a try in case someone has their orphaned primary sitting on a shelf in their shed.

[johninderby]

Sold one a few years ago. Bit of a heavy lump. 😁

[Cosmic Geoff]

While doing some solar observing just now, it occurred to me that unless your mirror is smashed into tiny bits like an auto windscreen, you should be able to use the biggest piece to determine the focal length.  Set it up so that it focuses an image of the Sun back onto a piece of white card, and measure the mirror-to-image distance.  This will give you the focal length with a reasonable degree of accuracy.

Armed with the diameter and focal length, you can then get quotes for a mirror from amateur telescope makers and other optical specialists. Once you have an agreement in principle, you can give the maker the bits to play with, so that he can confirm the focal length and figure (spherical or ???).

Given that a MN 190, smaller than your scope, costs around £1000, this approach may prove cost-effective.

[Space Hopper]

Maybe worth giving John at Orion Optics a call.

I think OO used to have their own range of SN scopes, although it was a long time ago.

But i'm sure he'd give you some idea of costs etc.

The problem with buying another and swapping primarys is that these scopes are now pretty rare. You don't see them for sale very often.

 

[Captain Scarlet]

1 hour ago, Cosmic Geoff said:

While doing some solar observing just now, it occurred to me that unless your mirror is smashed into tiny bits like an auto windscreen, you should be able to use the biggest piece to determine the focal length.  Set it up so that it focuses an image of the Sun back onto a piece of white card, and measure the mirror-to-image distance.  This will give you the focal length with a reasonable degree of accuracy.

Armed with the diameter and focal length, you can then get quotes for a mirror from amateur telescope makers and other optical specialists. Once you have an agreement in principle, you can give the maker the bits to play with, so that he can confirm the focal length and figure (spherical or ???).

Given that a MN 190, smaller than your scope, costs around £1000, this approach may prove cost-effective.

an alternative method would be to stick the largest piece onto a block of, say, wood, so that the mirrored surface is more or less vertical. Also make another flat vertical surface, into which you have created a small hole, and attach a bright light just behind that hole. A headtorch taped to the back of the board would do. Then arrange the two vertical surfaces (the mirror and the screen with a brightpinhole light-source) on a countertop, facing each other. The light-source will reflect back off the mirror and when they are at just the mirror's spherical-radius apart, the reflected dot will be at its smallest. The focal length of the mirror is half the sphere's radius. I have done this myself.

On a separate note, I had a mirror made for me recently by Orion Optics. Before it was ready I asked them precisely what focal length the mirror would come out, as their website wasn't totally clear on that point. They replied "They come out about 1590mm =/- 10mm". So I would imagine the matching of corrector-plate and mirror may well be Meade grading their measured-after-the-event corrector-plates and mirrors, and matching pairs thereof.

Cheers, Magnus

[davidc135]

I see that the nominal focal length of the spherical mirror is 1016mm. Is this right? The corrector plate should then correct for 3.5 waves of spherical aberration for the pair to be matched.

A variation of 10mm focal length is around 1% and that translates to 3ish% change in spherical aberration (as it varies with the inverse cube of the focal ratio). 3% of 3.5 waves is approx .1 wave error at the eyepiece.

Magnus' method for measuring the Radius of curvature and hence f'.l. should easily be accurate enough. 

The above would give a first fix match which may well be fine but if the mirror maker was able to test mirror and corrector together (because of figure irregularities) the results might be really good although really expensive too.   David

Edited December 29, 2020 by davidc135

[martinux]

Many thanks for your input, everyone. Really great idea with determining the mirror focal length.

After looking at prices including OO's website for mirrors close to the required spec it seems to make a lot of financial sense to look at a new OTA, however I'll place a wanted advert and see what comes up. 

Clear skies!

[AL1]

Did you have any luck with the measurement.  I currently have a SN10 that I wish to keep hold of but maybe I can help with reference measurements, having said that I am new to astronomy so might not be much help.  I got mine just under a year ago and have seen two others on eBay since.

[martinux]

On 15/01/2021 at 19:44, AL1 said:

Did you have any luck with the measurement.  I currently have a SN10 that I wish to keep hold of but maybe I can help with reference measurements, having said that I am new to astronomy so might not be much help.  I got mine just under a year ago and have seen two others on eBay since.

Many thanks for offering, AL1, and apologies for not getting back to you sooner!  I'm not able to measure the FL myself as the mirror fragments were disposed of (short-sighted of me but it was a hazard at the time).

 

I had no luck with astrobuysell and a few things have come up that have taken much of my time in the interim but I'll have a look at ebay. Thanks for the suggestion.

[martinux]

Just a quick update for anyone who stumbles upon this topic (though hopefully nobody will ever have the same issue!):  

John at OOUK advised me: "The optics in these are meant to be spherical but end up being ellipsoids to match the irregular corrector plates." and thus it's not as straightforward as sourcing a spherical mirror. The cost to make a replacement mirror for the SN10 is greater than an equivalent OOUK VX10 OTA so I'll be looking at it, the Skywatcher 250P and a few other OTAs and weighing up the pros and cons before I commit.

 

Alas, poor SN10, you were a great scope.