Fpl51 vs fpl53

[vlaiv]

14 minutes ago, Elp said:

My Z61 is a doublet, doesn't have a colour fringing issue, not one that I've noticed. I've used an acro so know what it looks like. I've done visual and AP with it for planetary, lunar, solar and DSO.

To add to the mix, TS also offer FPL55.

CA rapidly grows with aperture size. If you look at "CA" index as first approximation (even if we use exotic ED glass), then 60mm F/6 scope will have CA index of 2.54 (that is F/ratio divided with aperture size in inches).

Now - we are not interested in achromatic performance of such scope - but we can compare two scopes by CA index if they use same glass elements. In another words, we can see what sort of 100mm doublet with FPL-53 glass we need to match CA index of 2.54 and to have same correction as small 60mm F/6 scope.

4" scope, in order to have CA index of 2.54 needs to be 2.54 = X / 4 => X = 2.54 * 4 = F/10.16 or ~F/10 scope in order to have same correction as 60mm F/6.

Yes, we know that SW 100ED/900 has very good color correction and is virtually color free. We also know that 4" F/11 with FPL-51 is also virtually color free.

However - we do know that both FPL-51 and FPL-53 4" F/7 scopes have some residual color - especially in photographic applications where sensor is more sensitive to short wavelengths than human eye.

[JonHigh]

Must admit that am leaning towards the 102 triplet. If theres not much in it re glass vs aperture and comes down to other criteria then the lighter scope with all the other accessories attached to it on the HEQ5 would give me better guiding and less stress on the mount?

Edited June 15, 2023 by JonHigh

[Elp]

Think an OAG will give you better guiding if you're not already using one. Less stress on the mount can only be a good thing, refractors get very heavy very quickly at larger apertures.

[JonHigh]

Yes. That’s what I was afraid of, so not a bad idea. More to work out. Thanks for the suggestion.