Objective spacers.

[Merlin]

Does anyone know how thick the spacers should be in a Fraunhofer 4-inch/f.12 objective? I need to make three.

[billhinge]

As far as I know there isn't a generic distance, it depends on the optical formula of you two lens and the desired effect wrt CA and spectrochromatism (change in spherical abberation wrt wavelength) - apparently increasing gap reduces SC but increases CA

I respond because I have an old 3" fullerscopes refractor, talking to someone in the know I believe these were tested on an optical bench and the spacing deduced that way

quote "From memory the 3" doublets were mostly edge spaced i.e there were no foil spacers the spacing was based on edge contact. Some were foil spaced but the thickness would depend on the particular objective – there was no hard and fast rule. The correct spacing really needs to be done on an optical bench to get proper accuracy. I’ve had a few of these over the years and they have all been optically very good. "

Having said that, when I created my own tin foil shims as an expt I didn't notice much difference, but the foil took up space in the cell edges and caused pinching in cold weather so I took them out

[Merlin66]

In "Telescopes, Eyepieces Astrographs" by Gregory Hallock Smith et al, p108 they discuss the optical design of a 200mm f15 Fraunhofer refractor and say the lens spacing is a variable and needs to be established in the design to finalise the other curves....

In their 200mm example the front lens is 25mm thick, a 10mm air gap, rear lens 17mm thick......

[Peter Drew]

One basic requirement of spacers is that they prevent inner surfaces touching so avoiding scratches. Objectives that have matching curves on the second and third surfaces need only the thinnest of spacers, aluminium foil is often used. Other designs may need thicker spacers to achieve this or to meet other design charcteristics.

[Astrobits]

Have you taken one apart or obtained two lenses independently?

In either case you will not only have to find the correct spacing but also the rotation of each lens with respect to the other one to remove as much wedge as possible in the lenses that might be present. If you have taken one apart there might be a mark ( possibly just pencil ) on the edges to show the correct rotation position.

Nigel

[Merlin]

Thanks Chaps. I lost the original spacers ( doh! ), but they were quite thick, say, a millimetre or so. I've read one report on the web where it states that, for a 6-inch/f.15 Fraunhofer objective, the spacers only needed to be the thickness of a postage stamp. Indeed, some people use postage stamps for that purpose. I'll sort something out.. .

[billhinge]

Yes Nigel is correct, my fullerscopes doublet has edge marks

[Merlin]

To avoid confusion, it isn't the edge marks I'm concerned about, it's the thickness of the spacers.