Impact of damaged mirror coating

[Matthieu]

Hi all,

I believe that my 16" dob mirror is in dire need of a recoat as per the attached picture. Note that this was taken at dusk in the garage with open door, mirror pointing East after sunset without any artificial light.

I haven't pulled the trigger as I'm not quite able to determine whether it would make a small or huge difference.

I find that on globulars and galaxies the brightness compared to the human eye doesn't really seem to align with the numbers as I expected the following light gathering ability:
- x165 for my Megrez 90FD
- x329 for my C5 (2x the Megrez)
- x3331 for my 16" (10x the C5)

Whereas to my eye, the jump from the 90mm to the C5 is very similar to the jump from the C5 to the 16". I can see the difference, it counts as an upgrade, but it doesn't feel like a massive upgrade.

Is my experience due to the failed coating or is it a case of brightness perception not scaling linearly?

PS: I had planned to join the Gippsland Star Party to compare to other scopes but the weather didn't play ball

[anthony2302749]

Hi Matt

Consider giving the mirror a clean first off. You will need to remove the mirror from the cell. Get a suitable tub that will take the mirror, probably something from Bunnings. Also get a large bag of cotton balls and dish washing liquid.

Put the mirror in the tub fill it with tap water so the mirror is just submerged. Add a little bit of dish washing liquid, general for me a couple of squirts, suds it up a bit.

Let the Mirror soak a bit 15min, half hour your choice.

Next step is cleaning the mirror with the cotton balls with the mirror still in the tub with the water. Take two or three cotton balls and dip them in to the water and wipe them across the surface of the mirror, left to right or Vise verse.

Once you reached the other side of the mirror from which you started discard the used cotton balls. Repeat with a new set of cotton balls until you have wiped the complete surface of the mirror.

Then do it all again 90 degrees from you first attempt, making sure to replace the cotton balls every time. Reason being that there is a good change that dust can be picked up and using a dirty cotton ball could scratch the surface.

Once you have cleaned the mirror remove it from the tub and rise of with tap water and dry with a blower to remove water droplets or very lightly dab with a facial tissue.

Same process with the secondary.

Once clean you can get a better idea of the state of the mirror.

Cheers

Anthony

[Matthieu]

Thanks Anthony,

I knew I had forgotten to clarify something.

I have already cleaned the mirror using the cotton ball method. What's left is actual corrosion that the cotton balls don't work on and missing reflective bits.

[Leo.G]

If that's all missing aluminium a re-coat is probably your best option.
One thing Anthony didn't mention in his thorough cleaning process is the final rinse should be done with deionised water (distilled water works). It's sold at Coles and Woolies for steam irons.

[Pierre_C]

Hi Matt.

I think you are on the right track with your calculations. I have checked some rough numbers using a different method (including adjustments for secondary mirror obstruction, glass transmission and mirror reflectivity) and have arrived at a similar conclusion.

While brightness increases by the square of the aperture, I understand that brightness also decreases by the square of the magnification. So, I think your calculations will hold assuming you are using the appropriate eyepiece in each telescope to compare for the same magnification.

I think your comparisons should be made at or above x80 to create exit pupils for each telescope of 5mm or less. Comparing at less than x80 may result in some light being lost to the image clipping the edge of the pupil with the 16” aperture, resulting in less apparent gain for that telescope.

Also, I think it may be more accurate to estimate the relative brightness between telescopes by how faint you can see, rather than comparing how much brighter things appear. You will be dealing with differences of roughly 1 magnitude and 2.5 magnitudes respectively, which at magnitudes 10-14 may be difficult to discern.

The apparent differences for larger apertures at this end of the scale may also be partly negated by light pollution, though I suspect you may not have this problem at your location.

So, perhaps recheck at x100 at your dark location and concentrate on comparing how much more faint stuff and detail you can see rather than how bright things seem at the same magnification?

In the end, I think what you see is what you get, and your eyes will be the best guide as to what works best for you and your equipment at your location.

I agree with comparing directly with other 16” scopes before making your decision. All this theoretical stuff is a guide only.

Regards,
Pierre.

[Matthieu]

Noted Leo and Thanks for the detailed explanation Pierre.

I hadn't thought about comparing magnitude. Would spotting galaxies work for that purpose? I understand that comparing stars would be ideal but I think my Interstellarum stops at mag 11. I could select galaxies between 10 and 13 magnitude and compare the reach of the 90mm, 5" and 16" in my skies to their respective theoretical limit (I don't think I've ever spotted a mag 14 object from here). Would that work or is the extended size of galaxies going to mess up the experiment?

I can then revert here for help interpreting the numbers.

PS: I unfortunately can't take my scope somewhere to compare it as it's $360 just for the return on the barge. I might as well just recoat and hope for the best rather than spend that cash.

[Leo.G]

While I've seen mention of a not so good job I have to say Dr Wayne Sainty did a fabulous job on my 8" Newt primary and secondary with a long lasting coat with high reflectivity.
Palmway Optical at Forster NSW


https://palmwayoptical.com/


I'm not sure if there's anyone closer, this is the one I'm familiar with.

[Pierre_C]

Hi Matt.

I think galaxies would be more difficult to compare because the integrated magnitude does not often correspond with the brightness at the centre of the galaxy or at its periphery, due to variations in size and structure.

Having said that, Tony Flanders has an interesting table of peak brightness (PBRT) for the Messier objects that can give you a sense of what magnitude you are seeing at the centre of each galaxy:

https://tony-flanders.com/messier-guide-index-by-number/