can you change the secondary mirror?

[coffee_prince]

From all I have read it seems the secondary mirror will have a major impact on quality of the image, is there an overemphasis on eps etc when its the two mirrors that have a major impact?

In that case, how good are the mirrors in low end telescope like cheap dobs etc?

Thanks

[Doc]

Like all things in life you pay for what you get, but even the basic dobs have mirrors off 1/4 PV accuracy. Of course you can pay a lot more and go up to ultra grade mirrors of 1/10 PV accuracy but then they cost alot more.

[NGC 1502]

There are endless answers to the question of mirror quality.

I think that the mirrors are usually very good in Asian Dobs, and it takes excellent

seeing conditions (rare) to see the differerence in image quality between them and

higher spec optics.

Best regards, Ed.

[FraserClarke]

Also be aware that, because it is nearer the focal plane, the secondary mirror has *less* effect on the image quality than the primary mirror.

[Capricorn]

The secondary is simply to divert the optical path off to the side. They are simply flat and as long as the quality is fair I cannot see them making a great impact. They did when I was doing interferance systems and tests but that is a realm that exceeds the ability of the eye.

What we experience is the front silvering of the glass and that has to be good.

The problem of replacing is getting one that fits - is the right size and shape. Then attaching it to whatever mechanism that is on the scope. (Remebring that as it is front silvered you must keep your fingers off it at all times). Think that places like Galvoptics may sell them and if trying to locate an optical supplier try Melles Griot.

One problem I can see with a secondary is that they are sat in a nice high position in the scope easily accessed and many think of the main mirror and not the secondary. So they can get touched more easily and then acid from fingers and the aluminimum starts reacting.

[cootuk]

How long are mirrors expected to keep their optical qualities?

I think Orion Optics (who also supply primary mirrors and secondaries) say their standard mirrors should be good for about 10 years, and the hilux coated ones for 25 years.

From sone comments on here, even off axis chips can be tolerated as only a small central part of the secondary should be in the beam?

From what I can see of my secondary, it is effectivley held on by some sort of silicon sealant - there has to be some movement to allow for adjustment, with a few stickout blocks on the secondary holder to get the rough angle for attaching.

At what point would people send their mirrors off for resilvering?

As for 1/4 or 1/10 wave, then is the seeing in the UK, apart from dark sites, good enough to show a difference?

[dmahon]

The secondary is simply to divert the optical path off to the side. They are simply flat and as long as the quality is fair I cannot see them making a great impact

Isn't that like saying all diagonals are equal, from an unnamed £20 job to a TeleVue Everbrite?

[NGC 1502]

Post #6 - the life of mirror coatings is open to debate, factors that will probably

reduce their useful life include :-

1. Use near industrial pollution

2. Use close to the sea

3. Storage when wet/sealed from atmosphere

4. Over zealous cleaning

The question of when to re-coat mirrors is also dependant on several things,

including how fussy you are ! If they look mottled, or have bare patches,

or have many "pin holes" in the coating when held up to a bright light, then

maybe it's time.

The difference between 1/4 & 1/10 wave is not too easy to see, you need a

night when the atmosphere is very steady, this doesn't happen often, but

when it does, then more can be seen, particularly with an object like Jupiter

is being viewed, there is a lot of low contrast detail than can be lost with a

basic mirror.

HTH, Ed.