Results from testing two chemistries from Angel Gilding for silvering telescope mirrors. The original chemistry was not optimized for front surface applications but we seem to have made it work just fine. The new chemistry, labeled “Angel Chrome” was supplied to me and a few others to evaluate for use specifically for front surface mirrors.
Test run on December 21, 2021. Videos recorded, with relevant links to them in this report.
Fresh chemicals of both types: Angel Gilding original chemistry purchased in September 2021, new Angel Chrome received last week. Both were unsealed Dec 21.
Temperature: 45F in my garage.
Mirrors: Identical 16” rounds
PMRT for New Chemistry:
| Red | 97.0% |
| Green | 95.4% |
| Blue | 93.1% |
Spray time for Original Chemistry: 2 minutes, 1/2 liter each sprayer. Should have mixed more, I ran out. Half-liter works well on 12″ with material to spare, but this was my first 16″.
Spray time for New Chemistry: 10 minutes, 1 liter each sprayer. Again, I ran out of juice, but got the mirror well-covered.
Spraying Original Chemistry
Spraying “Angel Chrome”
Both chemistries gave excellent coatings. No difference in the final result. For me personally these two might be the best I’ve ever done, with practically no flaws and no haze (except for pits still being polished out at edge.) However, the reactions were quite different, with Angel Chrome (new) going on so slowly I wasn’t sure anything was happening for at least a full minute. (Remember, COLD.) I ended up using twice as much Angel Chrome to finish the same size mirror. If the weather was warmer (>70F) I would likely have diluted the original chemistry to 50% as I have done before and used the new chemistry full strength. It might be possible to repeat this experiment in a few months with warmer temps (and properly figured mirrors!)
How to clean glass really effectively: I cleaned both mirrors with only powdered chalk (precipitated calcium carbonate) and a clean rubber glove. Rinsing was aided by wiping with a second clean rubber glove to really squeegee off the microscopic chalk particles, followed by a thorough rinse.
Because these mirrors are in process they have a significant amount of residue on the edge: a dried mud made out of pitch and polish. It requires mineral spirits and 00 fine steel wool to remove. This step is only needed once in a mirror’s lifetime, unless you decide to silver in mid-process as I have done or if the mirror is re-figured later.
Removing Pitch and Cerium from Edge Using Mineral Spirits
Chalk Cleaning (THIS IS THE BIG ONE. I had to do it twice, partly due to being on-camera and being somewhat distracted. If you skip to 15:00 you will see just the second scrub, which is representative of what a good cleaning should be.)
Fixing a local bad spot at edge with chalk
Final Rinse-off Chalk + Sensitizing (First few minutes show the final removal of chalk particles with glove, after that it’s sensitizing. The mirror is squeaky-clean, and a cotton ball wipe showed it on the first mirror. You are watching the second mirror in this video.)
Speed Drying Water gone in seconds, no spots.
More cleaning and talking if you think this is educational/entertaining
If you need help with PMRT, see this video which simply shows how I take the image. PMRT Video
The image processing and measurement is covered in Photographically Measured Reflectivity of Telescopes. ImageJ software is free at https://imagej.nih.gov/ij/index.html
How to Eliminate Mirror Anxiety?
Thank you to Rhys Thomas for video, sound, lights, and bringing out the best in me.
Notes on PMRT, originally written to benefit Tom O.
Stable, repeatable readings are the goal. Obviously the RAW settings need to be locked down. I use these settings (Do not automatically brighten the image is not proving to be important, but do it anyway):
More difficult to control is the lighting uniformity. If you are getting varying readings using the saw RAW settings but on different images, then I would suspect lighting. Some examples:
Bad lighting
The bad lighting image shows a very non-uniform background source that is located behind the mirrors. I did this not for a PMRT, but just to show all the warts I was going to eliminate on my 8” and 12.5” mirrors. The problem here is that selecting matching areas on the background and the mirror is almost impossible.
More bad examples
I believe these are ones you sent me in July. Looks like a spotlight was used? One could in theory select the matching pair of spots on mirror and reference, but it just begs for error.
OK image
Here you got the lighting nice and uniform. The only problem with this image is that it isn’t taken directly facing the reference, and the reference isn’t very clean so it causes its own non-uniformity. Selecting the matching areas is harder when not taken face-on.
Good image
This image has uniform lighting, a clean reference, and is taken face-on. Selecting a pair of matching rectangular measurement spots is easy, as they will be symmetric about the line of contact of the reference with the mirror, as shown below.
By far the most sensitive error in this process is selecting equal areas centered on the right locations. You want each measurement rectangle to look at the exact same part of the reference, whether directly or in reflection. That’s why a uniform illumination and clean reference are so critical. Luckily, that’s not hard to do if you position things like shown above, and then use this simple process (BIG IMPROVEMENT OVER WHAT’S IN MY PAPER!):
Select the Reference area using the rectangle selection tool. Then go to Analyze>Tools>ROI Manager. Click “Add[t]”. This places your selected rectangle in memory.
Next, go back to the image and move the selection by dragging it to the mirror image.
Then in ROI Manager click Add[t] again, and then click Show All. You now have two equal rectangles. Make sure they are vertically aligned and symmetric about the line of contact.
Now go back to ROI Manager. Select both measurement rectangles, they should be blue:
Click on “More>>” then select Multi Measure.
Click OK and you will get a table:
Rows 1, 2, and 3 are R, G, and B respectively. Mean1 is the reference (assuming you made that rectangle first) and Mean2 is the mirror image. Ratio those and you have your results! I just copy and paste the table into a spreadsheet. (If all the ratios are >1 and not overexposed then you probably have Mean1 and Mean2 reversed, so just invert the ratio.)
I hope this helps you get consistent readings! Take a bunch of photos to get it down, and please let me know. -Rob