2 replies to this topic

[Booney3721]

So reading all the posts on here about how something like the Canon 85mm 1.8 is horrible with star shape, and stopping the F ratio down to a F4 is idea to get rounded stars, however using the F stop, not the rings, will cause diffraction spikes, I was curious to see what rings I should get. I seen the math was lens lengthtl/f ration= size needed, and so for 85/4= 21.25, is this correct or did I read the math wrong? Trying to get quite a bit of time on Rho this summer, and all I have currently is either my AT60ED with the focal reducer/ flattner or using my 85mm lens I have on hand, but instead of using my ASI585MC Pro, I am electing to use my SL1

Also. Bringing this up, and I have asked before, how do you all go about attaching a guide scope and ASIair to your set up like this? I was looking at L brackets and using a Arca swiss plate for the Guide Scope, but then the question is where to attach the ASIair.

[Booney3721]

I guess I should add I am also not using the ASI585MC Pro because of not having the ability to use the manual focus on it, I just discovered I can't use a 50mm STM either on my ZWO, can only use it on the DSLR, so thats why I am electing to go to a dark site and use a DSLR instead. Rather get the data sooner than later before the summer heat starts making it hard to use a DSLR.

Edited by Booney3721, 10 June 2025 - 01:57 PM.

[ngc7319_20]

Your math is correct.  As for the image quality... I think you will need to try it and see what happens.  Placing the aperture in front of the lens is not quite the same as using the internal aperture.  At the field center they will be pretty equivalent.  But at the field edges the behavior may be different.  The glass area receiving light from a star near the field edge will be different for the stop in front of the lens vs. the internal aperture blades.  Try it and see how the edge of the frame look to you.

Edited by ngc7319_20, 10 June 2025 - 04:05 PM.