Reflection for dim galaxys

Topics on reflection.

The star party I went to in Sweden last weekend I had a the topic up on silvering optics as this was showed on the Oregon SP this year.
As I got it, silvering reflects better than aluminium, but after read about it, I’m not shore.
Its a astro club here do aluminium costings with good results at Uppsala university.
Ok aluminium less s overcoat do not get over 90% but last several years.

One in Sweden that was at the SP now had send his 16” mirror to a UK company before as the idea was get more reflection out if it, but they damage the mirror totally for him.
Its was recoated by the club here and I has look into it before and its nice stars/contrast as a example a CZ at f/4 by using Ethos 13 mm.

I knew CZ do aluminium with coatings, and not ’enhached’ coatings.
Guess he is little over 90% and last for years.
As I got it, aluminum/engached will get 95% and be Ok if done the right way. ( and as I got it, its hard find one that can do it right )

I’ll read on silvering and it was told to reflects up at 400 nm at 85% and at 500 nm 90%.
But with a costings and UV coatings/enhached it could do 98%.

I do not know life span and price on that, but I guess rather pricy.
So one can ask this, will it be a gain going from aluminium with ocercoat to silver with overcoat UV ? ( say a gain at +5% ) and the pice is secondary in the question.

My CZ 18” in Quartz with his coatings on ( 14;1 factor ) at f/4 with a Antares 4” Quartz diagonal is a ’overall telescope’ that will has a paracorr on and Ethos EP.
Feet on ground and star-hop/push-to vs ’help’ ( no tracking ) in a SDM chassie and all parts in CF less the chassie parts in plywood.
A fun eye candy instrument, still decent area for light to get in.

Next telescope will be a faint fuzzes galaxy haunt only 30” instrument.
Idea is less obstruction at only 11% and no coma corrector and Delites EP and at f/4 and the range of 15-20X per inch in MagX.
-Who will do the parabola is not set or material ( or design )
I can get a Pyrex 19;1 blank at ruff f/4 but I’m not shore vs sag here.

A question here vs the reflection.
My guess the parabola figureing is the king and no sag ( design ) but how about reflection for this faint fuzzes observing ?

Is the silvering as good vs the eye as anything, even if the nm range is not at 550 ? ( note, dim galaxy use )
So what about 90, 95, 98 % ?

Dim galaxys might like a real nice reflection.
Maybe its more imprtant than the very best figureing at a pupil at 1.5-2 mm ?
I’mean 15X per inch is not really high power and dim galaxys is not like planets.
Ok, even 15X require some good sky ( I’m aware, so this is at LaPalma - not Sweden use )

If I was into split stars only I could use a good smaller refractor.

If anyone has information on companys doing silver with UV coatings on bigger mirrors, let me know.
If I got it correct, home made silvering will has way under 90% at 400 nm, andvisual range is 4-700 nm.

I meet the owner of this company at Stellafane some years ago.
He he had a very nice 28” Kennedy and then a frech coated mirror.
Sky was not perfect but it shore was great, and I remembers some planeterarys that rock the sock’s off.

They do UV silver and aluminium overcoat and they told me recomendation is aluminium and with some R&D it can be up at 98% reflection and hold for many years.
But it’s a rather pricy operation well over 10K at 30”.

http://spectrumthinfilms.com/stf/

Remember that even if aluminum peaks, with overcoating, at 91% at 550nm, and silver might be 97%, that difference isn't going to really make a huge difference visually.
A difference of 0.1 magnitude (9.65%) isn't going to be decisive in the revealing of details in deep sky objects. This will affect, largely, only things at the very limit of the scope's ability to pick up photons,
and is likely to be in the realm of what percentage of the time a particular feature is visible to averted vision.

If you are truly pushing the limit to see something at the very limit of visibility (and I think very few ever really do that), then gaining a few percent in coatings and a few percent with simpler eyepieces might enter the picture as significant. A few here on this forum have seen small differences by doing so.

If, after a few years with the 18", you think that the extra few percent might help, then get the mirror recoated by someone like Terry Ostahowski with an enhanced aluminum coating.
Whatever coating you get won't last forever anyway.