moon Filter

[Crossway]

I don't know what special optical  attributes a budget  moon filter is supposed to have but a £15 one I  used to use seemed to be nothing more than a piece of coloured glass .  I had heard that  "proper" ones made from polarised glass.

I made my own moon filter from a piece of  Perspex acrylic sheet.....moderately tinted with a neutral  effect,   It was protected on both sides by the original paper  overlay and  perfect for what I wanted.  

The result is very satisfactory .  Glare from the full or near full moon is greatly reduced and consequently the contrast is also improved.  So in performance it is certainly no worse than the fifteen pounder. ......and because I made it myself , it is probably better!!.

[Ben the Ignorant]

23 minutes ago, Crossway said:

I don't know what special optical  attributes a budget  moon filter is supposed to have but a £15 one I  used to use seemed to be nothing more than a piece of coloured glass .  I had heard that  "proper" ones made from polarised glass.

It's not just colored glass. It is optical glass, more transparent and purer than window glass. When you look at window glass from the side you see it's greenish, and with some attention you can notice that looking straight through. Compare the sky's tint through a window and through air only, the window dims the view a little, and changes the natural color.

Now a piece of clear glass as thin as a filter can't absorb a lot of light, and that's its job anyway, but since optics makers work with purer glass (BK-7 mostly), that's what they make filters with. The antireflection coating is also an extra that justifies the price. 15€ or £ for an optical element that preserves the scope's image up to 600x is really cheap, think of the smoothness the glass must possess in order to respect the image formed by fine telescopes costing thousands!  

It is far better polished than regular glass or acrylic, and its surfaces are strictly parallel. Do a test: aim at a very bright star through a window, and through air only, and defocus in both cases. With the window in the way you'll notice the defocused pattern is criscrossed by lines, they are the marks of rough polishing. The acrylic sheet may dim the view enough but at higher powers or with more attention you'll notice a loss of sharpness.

Also, a polarizing filter is not more proper than a colored filter for moonwatching, it's just different.

[Prador]

48 minutes ago, Crossway said:

nothing more than a piece of coloured glass

Filters costing several hundred, even thousands look like coloured glass. The difference is the much higher quality glass, very tight manufacturing tolerances, polishing, specialised coatings etc.

51 minutes ago, Crossway said:

The result is very satisfactory.

That being said, if you are happy with the views then that's a win for little cash outlay 

[vlaiv]

Justification of a cost for particular item is very deep topic (storage, cash investment / tying it up, transportation, labor and material costs, ...), that surely does not mean that there are not "over priced' items out there (but another aspect is desirability / availability) and reasonably priced ones.

I'm all for DIY approach if it cuts costs and provides you with what you need. Even when it is not the case - I support people that enjoy DIY as a hobby on its own.

As long as you understand difference between regular colored piece of glass / plastic vs optical filter, and you are happy to accept any drawbacks - that is perfectly fine in my book.

Optical filters are plane parallel (meaning thickness does not vary much to disturb light wave front and degrade optical performance of the system), minimize light scatter and reflections by use of coatings, etc ... But only way to attest if these are important / have impact on your use is to check it / compare.

[Crossway]

Ouch !  I walked right into that one didn't I and have  come out of it with some useful knowledge .  Thanks for replying guys.   I have no longer got my £15 moon filter but would have been looking at it in a new light ......and not even going to mention puns!.

Pete