Seeking Mirror ID

[Nomad Z]

I just acquired a used mirror set complete with cell and spider - see attached photos. I got it for the purpose of making a truss dob, and I'd like to find out a bit more about it.

The primary mirror is 300mm diameter, 27mm thick at the edges, and is on a 3-point cell. The secondary is 75mm across the minor axis, and is on a 4-vane spider with brass adjustment hardware at the ends.

Any thoughts as to the maker or what scope it might have come out of?

At this size, is it safe to assume it's parabolic?

I'm told the focal length is 1200mm - how can I substantiate that?

 

 

 

[Stu]

Isn’t that the Orion Optics set off eBay? Do you have reason to doubt it is OO?

If it has a serial number on the side then you could get the info from them, otherwise I would expect min 1/6th wave, should be a decent mirror.

[Nomad Z]

Yes, it's the set off eBay. I'm looking to verify what it is, if possible, because the 300mm mirror they currently offer is 37mm thick, not 27mm. They also only do a 9-point cell, and don't seem to sell spiders. I realise that this could be an earlier product (and maybe the hardware came from elsewhere). There are no indications that I can see to support it being from Orion. I've already written to Orion, but no response yet.

[Peter Drew]

Looks like an Orion UK primary to me as well. Similar rear surface and what looks like an Orion sticker label on its edge.

[paulastro]

Yep, my vote is for Orion Optics.  I thought I recognised the cell as being one of theirs.

[Stu]

1 minute ago, Peter Drew said:

Looks like an Orion UK primary to me as well. Similar rear surface and what looks like an Orion sticker label on its edge.

Yes, I agree Peter. It looks like an early OO cell, they are very different now but earlier ones I’ve seen are not dissimilar to this.

Pretty sure it is OO, question would be what quality it is which obviously will only be established by getting confirmation from them or getting it tested.

[Nomad Z]

The label on the primary can bee seen in the attached photo. The secondary has a 6-digit number written on the side in pencil.

[Nomad Z]

Thanks for the latest replies giving confidence to it being an Orion cell and mirror.

If it's an early one, is the 27mm thickness consistent with that?

[cjdawson]

You could verify the focal length, but jury rigging something together.  tilt the primary at an angle, suspend the secondary in collimation, and then hold an eyepiece where it should focus.  Using an artificial star or something like that to see if you can get the gear to focus.  I'm not saying that you'll get pin sharp focusing with this technique, but it should be good enough to give you the dimensions that you need to work too for your optics.     I'm sure if this is a bad idea someone will jump in and shout.

[NGC 1502]

You have good info already. I’d like to add -

The brass nuts on the secondary spider are the same as my 11 year old 10” OO Dob, which also has a 3 point primary cell. Some of the older OO mechanical parts were definitely a bit crude, but generally the mirrors were good.

Best of luck with the build, should be a nice scope to own.

Ed.

[Astrobits]

An easier way to get the approximate focal length is to mount the mirror vertically. as you would a shaving mirror, and shine a small torch into it . Pick up the reflection and move back or forward until the whole mirror is illuminated. If the torch is near your eye then you are at the radius of curvature which is twice the focal length. The old books suggest that you then dribble onto the floor to mark the point for measurement.

Nigel

[CraigT82]

28 minutes ago, Astrobits said:

The old books suggest that you then dribble onto the floor to mark the point for measurement.

Nature's plumb Bob... can't fail! 

[Nomad Z]

Just did the following...

Tried the torch filling the mirror and it was at a distance roughly twice the purported 1200mm.Even with a small LED torch, it was very bright. I then got a bit of ground glass, held it next to the torch, and used this to show the reflected image of the torch. When sharp, it was at 2300mm. I then remembered that I have a basic collimator which projects a semi-circle, so used this with the ground glass as well. The straight part of the semi-circle was sharp at 1150mm. Give or take 20mm for these measurements (definitely not out by 100mm for the torch on ground glass check).

I also measured the depth of the concavity using a steel ruler held edge on across the mirror and a digital calliper to measure the depth from the top edge of the ruler down to the decal. After subtracting the width of the ruler, I got 4.5mm, so a little more once the decal is included (+0.2mm?). That's slightly less than I get for a spherical mirror with a 2300mm radius (4.9mm), but I don't know how the depth would change for a parabolic of the same focal length.

So, 1150mm and f3.8?

 

[NGC 1502]

1 hour ago, Nomad Z said:

That's slightly less than I get for a spherical mirror with a 2300mm radius (4.9mm), but I don't know how the depth would change for a parabolic of the same focal length.

So, 1150mm and f3.8?

 

 

I think you’ll find that the difference between a spherical or a parabolic mirror would be too tiny to measure that way. OO mirrors were (are) good, so I’d be very surprised indeed if your mirror is not parabolic, and at least accurate enough for all practical purposes.

Ed

[Astrobits]

Difference between spherical and parabolic 12" f/4 is approx 0.005mm.

Nigel

[Gina]

That would take some measuring!!

[Nomad Z]

Orion Optics got back to me and, after some discussion, it looks like the mirrors were made around 2000, and probably 1/4pv. The 27mm thickness indicates that it's Corning glass, which is similar to the current Schott glass in terms of thermal expansion. Focal length at that time would have been 1180mm +/-20mm, so my measurements look reasonable. I'll have another go at measuring with a better setup when I get the chance. John at Orion said that it might need recoating given its age, which I'll keep in mind, but I think I'll aim to get the scope built first and try it out before I look at doing that.

[Dave_D]

the cell and spider design and finish are almost identical to my orion optics GX250 except my cell uses the same thumb knobs on the cell as on the spider