There is no value in it. The issue here is the heat loss of the metal tube - insulation is there to slow it, on the outside. Put the insulation on the inside does not stop this from happening and will actually keep the heat differential/heat plume for longer.
Also, there is no gain in putting felt on the inside of the OTA for the sake of "contrast". The problems of reduced contrast are 99% due to the materials used and design of the baffle tubes of the secondary and primary mirrors. Sure these are "black", but the problem is not them being black but how these surfaces reflect light at the shallow angles that light is coming on to them. Black anodised aluminium is actually very reflective and has a very high scattering ability when the incident light is at a very shallow angle. Shiny black plastic or even a matt finish plastic surface is equally reflective and has a very high scattering ability. The surfaces of the washers and bushings used to hold the central baffle tube to the primary mirror are also problematic. And not just on the outside, but on the baffle surfaces of these features - have a look up the rear of the scope without a diagonal and notice how reflective the inside of the central baffle tube actually is & how scattered this light is.
The inside of the OTA actually has next to no influence on contrast. Placing felt here needlessly risks the optics for nil gain and you also run the risk of having this felt material shedding these fibres and outgasing as this petro-chemical based product vents over time. This was a problem that plagued the 7" Intes Mak I had that a previous owner had stupidly done this and the felt was shedding fibres like a black labrador sheds hair in summer! Ironic too as Intes Maks have superior contrast to other mass production Maks and SCT's because these are designed and made very differently from these other scopes.
This is the main reason why refractors have better contrast compared to SCT's and most Maks, not because these reflectors have a central obstruction, but because of the poor design and manufacture of these internal baffle surfaces. Most people THINK it is because the reduced contrast is because of there being a secondary obstruction - NOPE! Eliminate these internal reflections from a scope and the influence of the secondary mirror on contrast you will find is actually bugger all!
Any doubts? Have a look at the two pics below. They show the inside of an SCT and an Intes Mak, both with a very similar angle of incidence of light into the OTA as this is the ONLY way to legitimately compare the two OTAs. Here you will see ALL the differences between these two scopes and why Intes Maks have so much better contrast than SCT's and Skywatcher Maks. These Intes Maks are not perfect either. There are some bespoke Maks that have refractor-like contrast because the internal baffle surfaces are designed and made even more differently, and they don't have baffle rings on the inside of the tube either!! My own 9" Santel Mak has no baffle rings on the inside of the OTA, but the baffle tube is made differently from Intes Maks, and its quality of contrast is just as good as Intes Maks.
If you want to improve contrast on your SCT and/or Mak, forget the felt. Use a high quality matt black paint, such as Black 2.0 or even better Black 3.0, on these internal reflective surfaces. THEN you will start to improve things.
Edited by maroubra_boy, 23 November 2022 - 04:40 PM.