Full apeture iris diaphragm?

[vlaiv]

I was just looking at prices of achromat scopes (for unknown reason ), and I noticed that there is very small difference in price between Skywatcher Evostar 102 and 120 - just under 30 quid (OTA only, VAT excluded).

Couple of nights ago I had a brief planetary session with my SW ST102 (F/5 achromat). I know, I know, that is probably the worst choice for planetary observation, but it was a quick peak of Jupiter, Saturn and Mars (best Mars I've seen so far ). Technique that I've used is to use 50mm aperture mask - this turns scope into 2" F/10. Both Jupiter and Saturn were very viewable at x100. Mars on the other hand suffered - scope has astigmatism in red, and since planet is red - it showed as very slight unfocused red blur around planet - detail also suffered (more than Jupiter and Saturn).

Given the small price difference between Evostar 102 and 120 OTA, I think it is well worth getting bigger scope - deepsky will benefit, CA on full aperture will be worse, but scope can easily be stopped down to 100mm giving CA performance of 102 and even further like 80mm or 70mm - where CA will virtually be undetectable up to x150 or so.

This gave me idea, but I don't seem to be able to find anyone making and selling these - Full aperture Iris Diaphragm - so one can "dial in" offending aperture, depending on target. It might be also useful for taming the seeing with larger scopes, when high mags simply won't work because of seeing.

So far I've found that Baader and Edmund optics make diaphragms large enough. Baader one is aimed at Solar Ha (again to achieve suitable F/ratio for Ha filters to work properly), Edmund optics is like regular one.

Both of these cost more than above mentioned achromatic refractors

Anyone have seen something like that in affordable range (it does not need to be full metal, or high quality, it just needs to work, it does not even need to be 0 aperture capable, but as close to circular as possible)?

[Paz]

I would be interested in something like that as I often stop down the aperture. How about if it was possible to make a 120mm lense cap with nested masked apertures of say 100mm 80mm 60mm and 40mm so that you could just take off the right number of aperture masks for the aperture you need. It need only be cheap plastic caps  so could it be done on the cheap?

[vlaiv]

2 minutes ago, Paz said:

I would be interested in something like that as I often stop down the aperture. How about if it was possible to make a 120mm lense cap with nested masked apertures of say 100mm 80mm 60mm and 40mm so that you could just take off the right number of aperture masks for the aperture you need. It need only be cheap plastic caps  so could it be done on the cheap?

I've already done something like that for ST102, I've found that PVC pipe plug is right diameter to nicely fit inside lens hood (some velour like self adhesive foil to help it fit nicely). I've made 80mm, 66mm (and probably one more).

Problem with self made aperture masks is that you really have to make sure it is circular and that cut is smooth - otherwise you are going to get all sorts of diffraction. Another thing is that having large number of masks is just cumbersome. This is why I think that iris type diaphragm is good solution - a bit like zoom eyepiece (without drawbacks of possibly lesser view quality) - you can vary aperture to suit conditions with slide of lever.