Norme, thanks for your input regarding contrast.
"Veiling glare" - that is the term I was looking for!
You are right that I am aware of the different sources that dictate contrast. That is exactly what I was trying to convey, that there are two very different parts to contrast, and that many people are not aware of these and make comments that only refer to one of the two or only a glancing reference to veiling glare because it is poorly understood, and this only leads to more confusion and it becomes a pseudo-academic discussion because the conditions for contrast differences between different % size central obstructions just cannot be seen, and even be reversed if the scope with the smaller obstruction shows poorer contrast than the one with the larger scope when baffling in the smaller one is poorer - this why I mentioned this very type of example of my experience with two high end Maks. My 9" Mak has a smaller obstruction than my friend's 10", but the contrast is outstanding in my friend's scope but looks rather average in mine when the two are compared side by side with the same diagonal and eyepiece... Yet contrast in my scope is better than in mass production instruments. The differences in central obstruction counts for nil if the veiling glare contrast is not well controlled to begin with. You will otherwise not notice the contribution made by differences in central obstruction. It is for veiling glare why people flock their OTA's.
When veiling glare is REALLY well controlled, if a very bright object is kept just outside of the FOV it will not have an impact on contrast as its light will be suppressed. The object is still very much shining into the OTA even if it is outside of the FOV, but the stray light it will be causing will not be seen. Ever noticed the glare of the Moon impact on your viewing as a ghost reflection somewhere, or Jupiter or Venus?
Having the inside of the scope painted matt/flat black is only the first part. Just as not all scopes are created equal, not all matt black paints are created equal. Most paints used inside telescopes are just too reflective when the angle of incidence of the light source is very shallow (I mentioned this in my post No. 39). It is only when looking down the tube with the Moon also shining into it will you noticed the sheen that comes off this "black" paint. If you want to see what a truly flat black paint looks like and how it compares to what is typically available have a look for "Black 3.0". Now here is a paint that reflects just about zero at all angles of incidence. I have been experimenting with Black 3.0 with a good mate of mine, and the difference it makes is extraordinary. I am looking at painting the inside components of my 9" Mak with Black 3.0 as I now know that this scope will benefit from a blacker paint as I cannot change the design of its baffle tube and I have also identified some parts in the OTA that will greatly benefit from being touched up with Black 3.0.
Contrast differences between different scope designs stems more from veiling glare than anything else. And again it all boils down to the design of the baffling, the quality of masking, the materials used in the baffles (the secondary mirror usually has a baffle around it too, and all too often it is made of shiny black plastic... yet no one notices this???) and the type of flat black paint, the quality of the polish of the mirrors, etc. Again, comparing two different scope designs by looking down into OTA's with the Moon also shining into the scope will reveal A LOT. When you buy a more high end product, you are also buying into a better baffling system - it costs more as it requires more care to manufacture which means greater cost to the manufacturing process, and costs you more too - you get what you pay for...
I do remember looking down into the tube of my 8" SCT's while the Moon was also shining into it, and I do recall just how much of a glow there was inside the OTA. At the time I did not pay any attention to that because I did not understand what I was seeing. Looking into my 9" Mak, and this glow is far less prominent. My 127mm Mak also shows less of this glow than the C5 I had. The contrast in my Maks is better than the SCT's I've had. But I also now know what true contrast provides when this veiling glare is close to zero. If the veiling glare is not well controlled, then the relative size of the secondary obstruction actually counts for bugger all and is a mute point in discussions.
Sorry that this particular post is a bit nerdy and it has expanded the discussion in this thread. But it is important to understand the factors that REALLY control contrast, and that the size of the secondary obstruction if far less of a factor than most people give credit for.
If you want to see what excellent contrast really looks like through a scope, then make the effort to find those scopes that are renowned for it. You will then have a true appreciation for this. I thought I understood the causes of contrast, and I too thought much of it had to do with the central obstruction. Like I said, unless the OTA's baffling is really well designed and implemented then the secondary obstruction counts for little. Doug added flocking to his scope, to better control veiling glare, and only really one part of it.
Edited by maroubra_boy, 23 August 2021 - 09:39 PM.