Longer focal length refractor or make a aperture reducer for my f8.3

[Moonshane]

Hi all

I have been looking at some f15 refractors today (old classic ones eg the Towa 339) and lovely as they are, I'll be a dead man if I bring in another scope.

I therefore got to thinking about whether I could create the same 'effect' by making a mask for my 120mm 1000mm FL f8.3 achro?

I have roughly calculated that if I make a mask to leave a central hole of 67mm I would have an f15 scope or of 76mm to give me f13 and so on. I favour the latter I think?

I don't really have major CA problems but suspect the additional focal ratio may sometimes help with splitting doubles / viewing moon/planets etc and possibly improve the effects of bad seeing somehow??

Is it really worth doing this or am I talking/thinking rubbish?? Maybe I should just try it and see.

Cheers for any help / comments

Shane

[John]

Before I fitted the Chromacor corrector to my 6" F/8 refractor I did try stopping it down using the aperture built into the lens cap. That gave me a 110mm F/10.3 and it did reduce the chromatic abberation which gave some nice views. The problem is you also loose the light gathering / resolution power of the 6" aperture so I'm not sure I was gaining anything overall.

Personally I reckon your 120mm @ F/8.3 would outperform a 76mm F/13 on most objects but you can always experiment with some aperture masks and find out.

[Moonshane]

cheers John

I reckon you may well be right - of course!

will give it a try and maybe also with my dob too and see what the results are like. will be an interesting trial.