Big trouble with brand new scope. so watchout.

[johnwhitehead]

New mirror faulty, 10 inch newtonian mirror transparent.

I recently purchased a new 10 inch f4.7 Newtonian reflector optical tube assembly (OTA).
I also own a 150mm f5, 200mm f5 newtonian as well as some other scopes.
First light for my new 10 inch Newtonian was of M4..(I use M4 as one of my sky referencers.)
Expecting to view a bright contrasty image I was very surprised to only see a washed out image with fewer and fainter stars than expected. I was quite dissapointed.
It lacked brilliance and contrast and the number of visible stars looked much like you would expect to see through a two inch reflector not a ten inch reflector.
Following a large number of experiments trials tests I observed a light from my torch on the lawn behind my scope originating from the torch I was actualy shining in the front of my scope down onto the mirror.
Next I found when I shone the touch onto the back of the mirror it illuminated the inside of the tube.
Then with my red laser spotter i found it would pass right through the coating on the mirror.
Being confused by this I did the very same test on both my other two scopes as well as a few of the others just to be sure. No light from either the torch or the laser would penertrate the mirrors of any but the new 10 inch scope mirror.

I reported this to the supplier who was completely uninterested in my report.
Following dozens of phone calls I was told I should blacken the back of the offending mirror like most people do. I had never heard of this before.
I did do as sugested blackening the mirror back. This made no effect on my problem with the washed out faint image i was getting. ( But did have some very interesting results which I will detail in a extra article.)

Following no action from the supplier after almost twelve months of phone calls I went directly to the manufacturer in Victoria.
After explaining what difficalties I had experienced and the explanation of the many tests I had run I was told if I was prepaired to wait a month or two because of Covid 19 I would receive from them a new mirror in a cell ready to replace the faulty one in my telescope.
A new mirror did arrive eventualy and to my pleasant surprise performed fully to my expections. The image of M4 was presented perfectally all stars showing plenty of brilliance and contrast. I could not have been happier.


( Report on the blackening of the mirror rear.)
Never in my 65 years of telescope making and observing have I seen a test presented to examine the quality of the mirror coating. and blackening the mirror rear.
After the blackening of the mirror back, I found viewing plenets I had two focus points very, very close together. The result of this it was impossible to reach a clean focus.
This effect seemed that there was first a image reflection originating from the mirror surface. Then a second image originating from the rear of the mirror because of the blackened back. and the image able to pass through the front and reflected from the back.
Once the black was removed from the mirror back there was no sign of the second image.
Maybe its time for a change and possibly every owner of a scope should test his new mirror for transparency in its coating because you cannot tell if it was done properly unless you do.
It would seem that any light getting through the mirror surface no matter how would be reflected back from metal cell parts on the back of the mirror resulting ever so slightly in focus difficulty.
( May-be my mirror was a friday night special ).
John Whitehead.

[Saturnine (Jeff)]

Interesting little adventure you've detailed, have never heard another story like it. When you looked down the tube did it appear that the mirror didn't have a coating or was very thin, should have been obvious if there wasn't any coating on the original mirror. Even thin worn coatings still give a reasonable image, lacking brightness of course.
To be given the run around by the supplier for so long is unforgivable in this day and age, you must have a lot more patience and tolerance than most and being advised to paint the rear of the mirror black is curious. Certainly blackening the edge of the mirror is a common practice to prevent stray reflections.
Good to hear that it has been resolved and things are as they should be with the scope.

[JA]

The reflectivity of most mirror coatings is in the 90/95 to 98/99 % range, also varying with wavelength, so even when new, that 1 to 10% that is not reflected, is transmitted in to the substrate and is either absorbed/reradiated as heat or re-reflected at the back surface and possibly passes back through, depending on the reflective coating quality and integrity. That's just part of the cost of doing business with a reflective surface; however, in your case John, that was in the extreme as you had a faulty perhaps "unsilvered" mirror. Good to hear it was resolved.

Best
JA

[johnwhitehead]

Originally Posted by Saturnine

Interesting little adventure you've detailed, have never heard another story like it. When you looked down the tube did it appear that the mirror didn't have a coating or was very thin, should have been obvious if there wasn't any coating on the original mirror. Even thin worn coatings still give a reasonable image, lacking brightness of course.
To be given the run around by the supplier for so long is unforgivable in this day and age, you must have a lot more patience and tolerance than most and being advised to paint the rear of the mirror black is curious. Certainly blackening the edge of the mirror is a common practice to prevent stray reflections.
Good to hear that it has been resolved and things are as they should be with the scope.

Yes just looking at the mirror in or out of the OTA it looked just fine.

[gregbradley]

You should name the supplier so others can avoid them.

Greg.

[Nikolas (Nik)]

Brand?

[FlashDrive (Poppy)]

Originally Posted by gregbradley

you should name the supplier so others can avoid them.

Greg.

+1 .. and even the brand .... please

[alexfine (Alex)]

that sucks !

[johnwhitehead]

I will tell you the make and supplyer in a personal email.
john_whitehead@y7mail.com