Cheap 8'' primary mirror - where is the catch

[Raph-in-the-sky]

Hey guy,

I just saw a 8'' primary mirror for 40 Eur on a local second hand website. I was under the impression that those things were a bit more expensve than that.

If I decide to get it (and eventually build a strock), what should I pay attention too?

Kr,

Raph

[barkis]

What is the f ratio, f5, f6, or f 8 ?  The  mirror need to be a parabola, and of good quality ,
especially the shorter focal ratios. Ask where the mirror came from, and the maker of it too.
Is it Astro Buy sell uk by any chance? Being in Belgium perhaps it's a more local seller.

Many amateurs make Newtonian mirrors, and good ones too, but bad ones can emerge on the market,
It is cheap for an 8" but perhaps it's in need of re aluminising, which is not a cheap outcome.
Ask how thick the mirror is, and is it glass or some other type, such as Pyrex.
Ron.
 

Edited November 4, 2019 by barkis

[Raph-in-the-sky]

1 minute ago, barkis said:

What is the f ratio, f5, f6, or f 8 ?  The  mirror need to be a parabola, and of good quality ,
especially the shorter focal ratios. Ask where the mirror came from, and the maker of it too.
Is it Astro Buy sell uk by any chance?

Many amateurs make Newtonian mirrors, and good ones too, but bad ones can emerge on the market,
It is cheap for an 8" but perhaps it's in need of re aluminising, which is not a cheap outcome.
Ask how thick the mirror is, and is it glass or some other type, such as Pyrex.
 

It's f5.

It's not ABS UK... I'm belgian, so local to me 😉

Thanks for the advice. I'll gather all the info I can get and I'll come back here to see what you guys think about it.

[John]

For a commercial mirror (Skywatcher, GSO, Meade, Orion Optics etc) assuming that it and it's coatings are in reasonable condition and that it has no scratches / damage, that would be a very good price. If it needs re-coating before it can be used that will add as much £100 to the cost.

For a DIY mirror of undefined quality that would be about the going rate.

Does that include the seconady mirror ?. They are often sold as a set.

 

[Raph-in-the-sky]

4 minutes ago, John said:

For a commercial mirror (Skywatcher, GSO, Meade, Orion Optics etc) assuming that it and it's coatings are in reasonable condition and that it has no scratches / damage, that would be a very good price. If it needs re-coating before it can be used that will add as much £100 to the cost.

For a DIY mirror of undefined quality that would be about the going rate.

Does that include the seconady mirror ?. They are often sold as a set.

 

Thanks for the info.

I beleive it's the primary only.  How could I know if the coating is all right? Is it something I should notice straight away?

[John]

You will need to budget for a suitable secondary as well then. The size of secondary you need will depend on the focal ratio of the primary mirror.

You will notice if the coatings are in need of replacement. Holding the mirror up to the sky and viewing from behind it will reveal if the coatings have holes etc in them. Ideally there should be no light coming though the mirror although one or two pin hooles are OK. The top surface should look evenly bright with no flaking, odd patchas, stains etc. Often coatings start to deteriorate from the outer edges first so those are worth examining.

Can the seller post you some photos of the mirror ? - you could post them on here then.

 

 

[Raph-in-the-sky]

23 minutes ago, John said:

You will need to budget for a suitable secondary as well then. The size of secondary you need will depend on the focal ratio of the primary mirror.

You will notice if the coatings are in need of replacement. Holding the mirror up to the sky and viewing from behind it will reveal if the coatings have holes etc in them. Ideally there should be no light coming though the mirror although one or two pin hooles are OK. The top surface should look evenly bright with no flaking, odd patchas, stains etc. Often coatings start to deteriorate from the outer edges first so those are worth examining.

Can the seller post you some photos of the mirror ? - you could post them on here then.

 

 

Yes the seller posted some pictures. Can you have a look here (https://www.2ememain.be/a/tv-hi-fi-video/materiel-d-optique-telescopes/m1477470257-primaire-spiegel-d-200-f-5.html?c=08c285449651fa109c354bbabe740c1b&previousPage=lr) and tell me if you notice anything worth mentioning?

[John]

That mirror looks in good condition to me. Obviously it's optical accuracy cannot be determined until it is used.  Many folks like a central spot / ring in the centre of their mirror to aid collimatiobn but that can be added later.

 

[barkis]

From the pictures,the mirror certainly looks in a good physical condition Ralph. Probably worth risking the purchase. It will be the major part of your telescope build. Have you given any thought to the type of instrument you wish to make?

The Dobsonian type is very popular, and fairly easy to construct and use in a modest form. There is much help and advice on SGL should you need it. They excel in help and assistance here.  Good luck in your project should you decide to go ahead.

Ron.

 

 

Edited November 4, 2019 by barkis

[johninderby]

Judging by the thickness could be an older mirror and possibly recoated. At €40 looks to be a good buy.

[Raph-in-the-sky]

21 minutes ago, barkis said:

From the pictures,the mirror certainly looks in a good physical condition Ralph. Probably worth risking the purchase. It will be the major part of your telescope build. Have you given any thought to the type of instrument you wish to make?

The Dobsonian type is very popular, and fairly easy to construct and use in a modest form. There is much help and advice on SGL should you need it. They excel in help and assistance here.  Good luck in your project should you decide to go ahead.

Ron.

 

 

I was thinking dob or strock... I'm not sure yet if I want to try this or not. I'm not much of a tinkerer but this would be a nice project

[Raph-in-the-sky]

1 hour ago, John said:

That mirror looks in good condition to me. Obviously it's optical accuracy cannot be determined until it is used.  Many folks like a central spot / ring in the centre of their mirror to aid collimatiobn but that can be added later.

 

So the only information I got is that it's 3.5cm thick, weighs about 2.5Kg and is lambda1 /6.... that doesn't mean much to me. Is that good?

 

[barkis]

44 minutes ago, Raph-in-the-sky said:

So the only information I got is that it's 3.5cm thick, weighs about 2.5Kg and is lambda1 /6.... that doesn't mean much to me. Is that good?

 

I'm not familiar with Strock, I assume you might be indicating an Equatorial mount but It's a wild guess.  .lambda refers to to the mirrors wavefront, and 1/6 is quite good.  The beauty of the .Newtonian pmirror Is, all the colours in the spectrum are all focused at the same point, therefore no chromatic aberrations. Refractors require two or three lenses positioned to eleminate chromatic aberration. 

Often, a mirror maker will engrave his/her name on the back of the disc. I think you would have noticed that though.

Ron.

 

Edited November 4, 2019 by barkis

[Raph-in-the-sky]

12 hours ago, barkis said:

I'm not familiar with Strock, I assume you might be indicating an Equatorial mount but It's a wild guess.  

Actually strocks are ultra compact dobs. Have a look here. It's in french but you should understand what I'm talking about. 

 

[barkis]

Very interesting Video Ralph, so the Strock is Dob. easily broken down into it's individual components for ease of transportation to an observing site. The Demo Vid. was instructional for sure, and it shows how compact the unit becomes.    I wonder about the reassembly on site though. One would need plenty of daylight time to rebuild it for use. I did like the 18 point suspension arrangement for the mirror. Not sure what size the mirror was for that particular setup. 16" perhaps?.  Many of SGL's 'DOB.MOB' own quite large units that also break down into component parts, but seem rather quicker to rebuild. These guys sometime go off to dark sites as a group for fantastic observational experiences. I think the Isle of Skye was a recent venue, and the reports they posted Indictated a great time was enjoyed by all. Anyway, I'm pleased I now know what a Strock is.  Thanks for the Video I don't know French, but it was self explanetory anyway.

Ron.

 

 

Edited November 5, 2019 by barkis

[Raph-in-the-sky]

18 minutes ago, barkis said:

Very interesting Video Ralph, so the Strock is Dob. easily broken down into it's individual components for ease of transportation to an observing site. The Demo Vid. was instructional for sure, and it shows how compact the unit becomes.    I wonder about the reassembly on site though. One would need plenty of daylight time to rebuild it for use. I did like the 18 point suspension arrangement for the mirror. Not sure what size the mirror was for that particular setup. 16" perhaps?.  Many of SGL's 'DOB.MOB' own quite large units that also break down into component parts, but seem rather quicker to rebuild. These guys sometime go off to dark sites as a group for fantastic observational experiences. I think the Isle of Skye was a recent venue, and the reports they posted Indictated a great time was enjoyed by all. Anyway, I'm pleased I now know what a Strock is.  Thanks for the Video I don't know French, but it was self explanetory anyway.

Ron.

 

 

I don't think I'm handy enough to build one but the idea of having a big scope you can take on a plane is very appealing. Can you imagine going to the southern hemisphere with a 8'' or 10'' scope! That's definitely making it to my bucket list!

[Raph-in-the-sky]

And the scope in the video is 10''

[barkis]

21 minutes ago, Raph-in-the-sky said:

I don't think I'm handy enough to build one but the idea of having a big scope you can take on a plane is very appealing. Can you imagine going to the southern hemisphere with a 8'' or 10'' scope! That's definitely making it to my bucket list!

Did that guy in the video build the Strock he demontrated perhaps.?
It looks a compact package for sure. Including a 10" mirror, perhaps the weight wouldn't be prohibitive as hand 
luggage on a plane. Not sure about hand luggage weight requirements  in an aircraft's cabin.

[Raph-in-the-sky]

11 minutes ago, barkis said:

Did that guy in the video build the Strock he demontrated perhaps.?
It looks a compact package for sure. Including a 10" mirror, perhaps the weight wouldn't be prohibitive as hand 
luggage on a plane. Not sure about hand luggage weight requirements  in an aircraft's cabin.

Yes, he did biult it

Hand luggage is  8Kg or 12Kg depending on the airline but in practice they never weigh it