An oil flat can be used to test the quality of optics in a finished telescope. It can be used for Newtonians, Cassegrains, refractors, photographic lenses, etc.
From a practical point of view, liquid flats are "perfect" reference flats since they assume the curvature equal to the averega earth's radius, which is about 6,400 km or roughly 4,000 miles (distance from New York City to Berlin, Germany).
To make and use an oil flat is simple and cheap.
You will need:
(2) a means to cut a hole in it, and attach three leveling screws
(3) a pizza pan and
(4) motor oil. If your place is subject to vibrations, pick heavy oil such as SAE 85W 140, or chain saw oil.
Setting up the test
1. Clean the pizza pan and make sure it's dust and lint free. Pour the oil slowly into the pan and stir it gently with a plastic stick to remove as many air bubbles as possible. Scoop up any remaining air bubbles form the surface with a plastic spoon.
2. Place the plywood stand over the oil pan and place your telescope on it, the sky end facing the oil flat.
3. In the eyepiece holder inset a DIY eyepiece that has an LED and a Ronchi grating. Adjust your focuser so so that the Ronchi grating is close to the telescope's infinity focal plane.
4. Turn on the LED and look down the eyepiece. You will most likely initially see some strange pattern but don't panic!
1. Using leveling screws (the finer the better, but 1/4-28 or even 1/4-20, with large knobs made of toy wheels, is more than sufficient), tilt and tip the platform until you see the reflection of the aperture move into the view. You should see numerous Ronchi bands fill the POV at this point.
2. Using your focuser knob, bring the number of viisble bands to 3-4 per field and observe the pattern.
This oil autocllimation test is a double-pass test, which is to say it's twice as sensitive as a single-pass collimation tests.
Closing comments and reminders
1. The biggest problem you'll encounter will be the air bubbles in your oil flat.
2. The biggest advantage of of the oil flat test is that the flat size is limited only by the size of your oil pan, and the cost is pretty much negligible.
Edited by MKV, 06 March 2015 - 10:40 AM.