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Secondary mirror - how to work out which side is the optical surface ?


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Hi All,

Just getting back into stargazing. I have a small  home made 3 &1/2 inch newtonian which I have ressurected for possible solar use. But my secondary mirror fell off. a while ago. No problem I thought, I can just glue it back on to the secondary holder.

Er no I can't ! Because I can't work out which side of the mirror is the actual optical surface. Could anyone suggest some way to work out which is which ?

Many thanks !

Brutie

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2 minutes ago, Brutie said:

Hi All,

Just getting back into stargazing. I have a small  home made 3 &1/2 inch newtonian which I have ressurected for possible solar use. But my secondary mirror fell off. a while ago. No problem I thought, I can just glue it back on to the secondary holder.

Er no I can't ! Because I can't work out which side of the mirror is the actual optical surface. Could anyone suggest some way to work out which is which ?

Many thanks !

Brutie

With a glass mirror that reflects on both sides, you will have a double reflection on the reverse side and only single on the front side. The secondary reflection will be much fainter than the primary reflection and you might have to tilt the mirror and try under different lighting conditions to spot two reflections.

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Hi Mandy & Pitch Black Skies,

Ok Mandy is correct ....on closer examination I can just see a double reflection on the rear (it is certainly much  fainter than the main reflection) and a single reflection from the front of thr mirror surface.

Pitch Black - Yes I aslo thought that the optical surface would have a non reflective rear side but thinking about it I don't think that is so. With the optical surface on the front of the glass there's no need for any thing else on the front surface (ie under the reflective coating)

Anyway problem solved and I have learned something new. Thanlks to both of you for the prompt and spot on advice !

Kind regards,  Brutie

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1 minute ago, wulfrun said:

You might want to reconsider solar use. If the mirror fell off in storage, how easily might it fall off when exposed to the heat of concentrated sunlight? Not a solar expert but I thought Newtonians weren't considered a good idea?? Anyway, heat-proof glue as a minimum.

Newts are absolutely fine for solar. I use a 250 mm one for solar observing. You just have to place the filter at the opening of the tube so the mirrors only get a factor 10E-5 of the light and heat.

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Just now, Mandy D said:

Newts are absolutely fine for solar. I use a 250 mm one for solar observing. You just have to place the filter at the opening of the tube so the mirrors only get a factor 10E-5 of the light and heat.

Yes, I realised what I'd put and removed it. Too late, you quoted too quickly!

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I just removed the secondary holder and also removed the primary mirror  as both mirrors were very dusty. I'll wash them with distilled water  before re assembling. I can't remember what glue I used when I fitted the secondary originally (25 years ago) but I'll use silicon this time I think. My primary  mirror fixing is crude but seems to have withstood the test of time . This was a home brew project using some PVC drainpipe and a mirror set from the SPA following a Channel 4 astronomy documentary called "Earth Calling Basingstoke" many years ago. Well worth a look as I see it's on the internet these days.

Brutie

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8 minutes ago, wulfrun said:

No problem, I'll go hang my head in shame 🙂 Even more so as I also use a full-aperture filter!

I was honestly surprised when I looked at who had posted that silly response and saw it was you! 🙃 I'm sure I'll post something just as silly before too long. The comment is now immortalised ...

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2 hours ago, Mandy D said:

I was honestly surprised when I looked at who had posted that silly response and saw it was you! 🙃 I'm sure I'll post something just as silly before too long. The comment is now immortalised ...

Guilty as charged...one of those instances of engaging fingers before brain!

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