Thank you so much for adding bold to your comments. It adds context to your comments, context not required!:-)
Point by point:
Taking the scope apart? That is not required. That is not necessary. And that is not how one conducts a flashlight test. You just shine the light down the OTA. Furthermore, it is not done to see how dark the inside of the OTA is. It's done to look at the primary mirror.
My comments are in reference to a couple of the comments within this thread that apparently you are not familiar with, not specific to your flashlight test. For example:
Post #10 -- This is why learning to star test is important. I would never send an optic back for smudges or sleeks if it star tested well. I'd rather have a 1/6 P-V mirror that failed the flashlight test than a 1/4 wave mirror that was pristine. The only "light" test I've found useful is to set the mirror on a table with indirect light. Step away a few feet so that you are viewing the mirror from a 45 degree angle. If the mirror looks clean, it is and leave it alone. Then star test it. If it proves to be diffraction limited, be happy. If it is better than diffraction limited, celebrate! This has worked for me since 1960.
I believe the only way to test a mirror on a table would be to remove it.
Post # 22 -- A few weeks ago the 16" mirror in my 3 years old ASA newton astrograph received a new coat by Orion Optics UK.
Prior and after the new coat I made a image from behind the mirror with a light bulb in front of the mirror to see the difference in light shining through the mirror.
The ASA coating is a 97% coat (according to the ASA website) and the new Orion UK coat is also 97%.
The results shocked me, in the image the new coat is on the left, almost no light leaking through, right the old coated mirror: grey, streaks and smudges (ignore the black adhesive in the middle).
The images were taken with exact the same fl lenght and f# number of the lens, and also ISO , conditions, etc.
1) It is always the best to give a mirror a new coat.
2) Was the old coat always this bad? then don't trust telescope makers with their claim about quality.
3) Or a coating on a mirror can deteriorate over time, due to weather, moist, gently cleaning as I have done..., then recoat a mirror every 2 year.
4) Always check mirrors with light bulbs in front and from behind.
Please view this post and note the mirror sitting on a table with a light on one side and a pic taken from the opposite to determine how much light is coming through the mirror's old coatings. An interesting test, but in order to accomplish this test, I have to dig the mirror out of my SCT. If you buy a new scope, you aren't likely going to be taking the scope apart to check its coatings. My comment, "I know for myself that taking my new scope apart to shine a bright light at it and check to see if it is dark as it should be..." I am not referring to how dark the inside of the optical tube is, but rather referencing how dark the back side of a mirror is when a light is sitting on the other side of it in relation to this post (#22).
The flashlight test is not used to test the quality of the scope. It's done only to check the mirror to see if it's dirty. Nothing more.
I am aware of this, but, again, it is in relation to other posts within this same thread. For example:
Post #18 -- Glen has given the best answer.
Think if it this way. Suppose you had a C8. The secondary obstruction is about 5 square inches.
Now, each speck of dust is like a tiny obstruction, but suppose you scraped all of the dust on the mirror together and once concentrated, it made a square one inch on a side (one square inch). This would increse the contrast loss only to the same point as if you made the secondary obstruction only a few millimeters larger, and the difference would be so small that it would be impossible for anyone to see.
As for the qualiity issue, I do think that most people rather overstate the small effects that this or that optical issue can have on telescope performance.
But it is far more complicated than this.
To have a high Strehl, an optic must be very precisely made.
Just having 1/4th wave of Spherical Aberration by itself is not a fatal flaw, and someone looking though two telescopes, one with zero SA, and one with 1/4 wave, most people would struggle to tell them apart.
Ah, but here is the very sad reality. A manufacturer that is content to pass an optic with 1/4th wave SA will also tolerate many other small errors. These errors accumulate.
And when you couple some SA along with a slight turned edge, and a bit of a zone, and a bit of astigmatism, and then, you throw in a big obstruction (which by itself is not at all a fatal flaw) you can get a telescope that is a very poor performer fo the apeture.
So, this or that small error by itself is almost always of little consequence, but if there are enough of them, it does add up, especially coupled with some SA (which is one of the most serious errors) and big obstruction.
Dust, by comparison, is a tiny problem.
Post #24 -- I have to say I typically respect your opinions Uncle Al, but not this one. Of course, after use, an open optic such as a Newt will show dust. But a closed optic, like a SC should not. And when I said I always test a new OTA when it arrives, I do. Both optically and mechanically. Every OTA I have meets the manufacturers advertised specs. Diffraction limited (1/4 wave), no aberrations visible that aren't inherent in the design, etc. If people tolerate shoddy workmanship they deserve what they get. I do not.
I'd like to know if the scope I have is "putting out the best image" that can be expected, but for me, I'd have to have someone check this for me, and this is to what I was referring.
Hope this clarifies the remarks.
Ultimately, do the tests if you want to. If you don't want to, then don't.
I wonder, do you suppose on any of the Hubble servicing missions any astronaut shined a light down the front end to see how pitted the mirror had become from micro meteorite impacts? It would be a pretty amazing shot given the mirror is located down at the end of the shroud, but after 24 years in orbit, it could be possible.