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I see some Apo objectives with correction collars to correct of different cover slip thicknesses (0.11 - 0.23). How useful are these if you always use #1 coverslips with a 0.15-0.17 range? Are there any other uses for this correction?

A cover slip thickness range of 0.15-0.17 is quite large.

Have a look at the links in the first post on this thread: viewtopic.php?f=5&t=2513&hilit

A micrometer with a range of 0.002 mm is a useful tool.

btschumy wrote:I see some Apo objectives with correction collars to correct of different cover slip thicknesses (0.11 - 0.23). How useful are these if you always use #1 coverslips with a 0.15-0.17 range? Are there any other uses for this correction?

For high NA dry objective coverslip thickness is very important, and even slight deviation from the expectation lead to surprisingly huge performance degradation. For example, just 0.02 mm deviation for NA 0.95 objective leads to 71% performance reduction (Table 1 at https://www.microscopyu.com/microscopy- ... correction).

So this correction collar is useful and maybe even necessary for high NA dry objectives even if you always use #1 coverslips (and you can also use the collar to correct for the sample depth). On the other hand, it's one more variable that needs to be adjusted.

A cover slip thickness range of 0.15-0.17 is quite large.

I'm surprised to hear you think that is large. I think #1 coverslips are allowed to be between 0.13 and 0.16 mm and #1.5 are 0.16 to 0.19 mm. Seems like every batch of coverslips I see has some variation. Just in case you misunderstood me, the variation is between cover slips not within a given one.

Maybe I'm just buying crappy cover slips.

I do have a micrometer that measure to 0.01 accuracy which seems enough for this application.

Very expensive coverslips are held to closer tolerances. However many professionals and hobbyists alike use a micrometer to separate their coverslips for exacting microscopy.

I suppose you could think of the bottom of a petri dish as being a coverslip. I have long working distance objectives with collars on my inverted scope. Here's looking up atcha!
Dale

.02 or 3 ,on the coverslip isn't going to make much of a difference, especially with a water medium, where the thickness of the water is the most important factor. The collar only factors in when the sample is precise and absolute precision imaging is demanded. With the objectives I own, that have correction collars, I find that I do a light trim, while looking through the eyepieces and the effect is usually very slight, if any, with well prepared water based mounts and I usually choose a thinner, rather than thicker coverslip.
Also, not all objectives are created equal. I have examples here, of dry apos , fluorites and high N.A. achromats of 40x .70, 40x .80, 43x .85, 45x .85, 50x .85, 60x .80, 63x .85, 80x .90 and 80x .95 , none of which have correction collars and all of which produce excellent imaging over a range of sample thicknesses. These objectives, have clearly been designed to limit the longitudinal spherical aberration near the front of the objective , which is the major reason for a correction collar.
I even have one 50 .85 oil objective, that produces as good an image unoiled as most dry 40 .65 do, so spherical aberration is not always as predictable as just looking at the N.A. and knowing it will be evident or less so, based on the number. Each objective needs to be evaluated, in order to get an idea of it's preferences and how it will handle various mounting conditions.

Apo, when viewing through the bottom of a flask with an inverted scope, that is when
longitudinal spherical abberation would benefit from the collars correction?
Dale.

that is a bit of a different scenario because petri dishes and the like are usually very irregular and thick, unless you invest in expensive dishes with precision bottoms, some of which are graded coverslip thicknesses. Your microscope objectives had to be designed to accomodate a great range of possibilities, mm differences over a long w.d. in your case, the glass is a disproportionate percentage of the total sample thickness.

The petri dishes were ok til I added the mechanical stage, now I want to fabricate rectangular
slides with sides. Like a big well slide. I can't free hand a petri dish on high power.
Dale

kind of like a little mini-aquarium with a good quality slide of known thickness for a bottom?

Exactly.

That's a great project. If I was doing it, since the slide, is in effect your cover slip, I would invest in a couple of very high quality precision slides with which to make these. I would go for the lower range of your correction scale. LImiting the amount of glass you have to see through, can only be good with dry objectives.