Below is my setup for getting dark field illumination with low power objectives.
On the left are the N.A. 1.25 Abbe condenser lens cells. In the middle is a homemade darkfield stop made from black construction paper, using a pencil compass, scissors, and an X-acto stencil knife. On the right is the condenser body with Iris diaphragm.
The darkfield stop is dropped into the condenser body on top of the annular ledge just above the diaphragm. Then the lens cells are screwed back on top of the condenser body, but not quite snugged up tight. That way the stop won't bind movement of the diaphragm.
Then the condenser is pushed back into the fork mount.
Below is the slide:
Below viewed with an A/O Spencer 10X 0.30 apochromat objective. The relay lens to the digicam is a 10X Compens eyepiece.
The colors are interference patterns from details too small to be resolvable by the objective. When a 20x objective is used, the colors are mostly gone, and when a 43x objective is used, the colors disappear entirely.
The optics in my cheap little digicam just cannot do justice to what can be seen at the eyepieces. There is just a bit of field curvature, but much less than appears on the camera image. Tiny pores (punctae?) visible on some of the diatoms cannot be seen in this image.
Near the edge of the circle at the 4 o'clock position, there is a small round diatom with what appears as an orange center. The tiny pores just visible to the eye look like an intricate wheel spoke pattern. By centering that diatom in the field, using much zoom on the digicam lens, and fooling the autofocus by locking the autofocus at the halfway position of the shutter button, then readjusting the fine focus on the scope, it's possible to get close to what can be seen through the eyepieces. Inset below:
It took a lot of trial and error to find out how much and in which direction the fine focus needed to be adjusted after locking the camera autofocus, before taking the picture.
I took the plunge and ordered a 2 megapixel eyepiece camera from China on Ebay, only about 25 USD and an extra 32 cents to cut the shipping time from a month and half down to about 3 weeks. I've bought a few inexpensive items like this on Ebay from China, and while shipping often took close to a month, everything I've ordered so far has been delivered.
The tiny image chip will cover only a small fraction of the image field, but this will give me a chance to see if I want to get something similar but more expensive. There is way to create panoramas in GIMP by stitching together multiple images, but I suspect this will be time consuming. Will give it a try and see what happens.
Viewing this slide with 10x objectives gives a very enjoyable image, but doesn't seem to be a good way to critically test the objectives. I tried several antique brass 10x achromats, and was unable to see any real difference in resolution or sharpness at the center. Some of the antiques had poorer image sharpness near the edges of the circle.
A Darkfield Stop and a Diatom Slide
A Darkfield Stop and a Diatom Slide
Rick
A/O 10 Series Microstar
A/O 4 Series Microstar
A/O 4 Series Phasestar
A/O 4 Series Apostar
A/O Cycloptic Stereo
Several old monocular scopes in more or less decrepit but usable condition
A/O 10 Series Microstar
A/O 4 Series Microstar
A/O 4 Series Phasestar
A/O 4 Series Apostar
A/O Cycloptic Stereo
Several old monocular scopes in more or less decrepit but usable condition
Re: A Darkfield Stop and a Diatom Slide
Excellent - love to see this kind of experimentation.
I had good luck with stops with 10 and 20x objectives, at 40x it got to be a real trick getting the precise alignment and the stop size right.
Great job, keep posting your results.
Rod
I had good luck with stops with 10 and 20x objectives, at 40x it got to be a real trick getting the precise alignment and the stop size right.
Great job, keep posting your results.
Rod
-
- Posts: 6327
- Joined: Fri May 15, 2015 12:15 am
Re: A Darkfield Stop and a Diatom Slide
The 8 form test slide is a better one for objective assessment.