150mm f10 "fuzzy" image

[alythall]

Hi

I've just bought a custom built 150mm f10 refractor. The build quality is good, Edmund Scientific lens, Celestron focuser, nicely baffled, but I've tried it out tonight and have been disappointed.

Using a trusted 25mm TAL plossl, I can't seem to "snap" into focus and the image is just a bit blurry. There's quite a bit of CA. I thought it would be relatively crisp and contrasty at f10.

I've used many refractors before and have never had this problem - could it be a collimation issue? Am also using a new diagonal for the first time (StellaMira dielectric), so is that the likely culprit? Any ideas welcome!

Thanks

Andrew

Edited February 6, 2022 by alythall

[mikeDnight]

Hi Andrew.

 How are your local seeing conditions? May be there is haze in the atmosphere?  That could exaggerate the CA problem and may make the view soft. 

[GazOC]

Although most popular 150mm achros are f8, f10 isn't particularly slow for a 6 inch refractor. You're going to see a fair bit of CA in that scope even if it's a well figured lens 

 

[Stu]

Time to roll this one out again, which relates aperture and focal ratio to levels of CA.

6” f10 shows as having filterable levels of CA.

[davidc135]

There may be a simpler explanation but your scope may be suffering from large spherical aberration.

It may be caused  by one or both of the two lenses that make up the objective being reversed on assembly. The radii of curvature of the four surfaces should be (very roughly) in the proportion of 2,1,1 and 4 going from sky to eye with 1 being the strongest curvature. So convex, convex, concave and convex most likely.

Excess CA would accompany the resulting SA.

You may not want to disassemble the lens and cell to confirm that just yet but it can be checked with some numbers that I'll try to fish out.

David

 

Edited February 8, 2022 by davidc135

[davidc135]

Vla Sacek in the excellent online resource 'Telescope-Optics.net' gives a precription for the commonly used Fraunhofer objective:

radii of curvature (x objective focal length)

R1 0.6 ie 900mm with a sagitta of (r^2/2R) = 3.12mm cx   over full aperture (r = 75mm) @ F/10

R2 -0.36                                                           5.20mm  cx

R3 -o.363                                                          ''            cv

R4 -1.51                                                          1.24mm   cx

If no other explanation is found a very basic plywood 'spherometer' could be made to check the above without dismantling the cell.

David

 

Edited February 8, 2022 by davidc135