In praise of the ST80

[Ags]

I have had an ST80 for a long time but have hardly used it because of lacking a satisfactory mount and lacking suitable eyepieces. I've finally figured out I can actually mount the scope on my AZ3 clone with both tube rings, and I have some good ES 82 and 68 eyepieces, so I have been able to have some satisfactory sessions with the "little" scope. Looking at the Double Cluster last night (using a 6.7mm EP for 60x) - and it was a very good view indeed - I was surprised to see the view was as bright if not brighter than it had been in my  old Nexstar 4SE 102mm Mak. 

That got me thinking about how much light each scope would let through - they both have a diagonal so we can ignore that. 

Mak 102: Because of the corrector plate spreading the light off the edges of the primary mirror, true aperture is ~97mm (rough guess), so 0.9 transmission. The CO is around 34% so that's another 0.9 transmission. Main mirror and secondary have 0.9 transmission each, giving 0.9^4 = 0.65 transmission, or 81% of the original aperture. So the real ideal light gathering of my old mak was approximately 83mm.

ST80: The ST80 has two layers of glass from its doublet primary, which must have a transmission of at least 98% each. One of those lenses is cancelled by the Mak's corrector plate, so the ST80 has 99% effective aperture, or 79mm.

Assuming my maths is roughly right, it's no surprise the ST80 can give similar views to the Mak of deep sky objects.