All you need is a simple plastic filter. I have used clear green or red candy wrappers or cover sheets from " book report" binders from Staples. Also you don't have to filter the light source you can just look thru a filter like an eyepiece filter to improve the contrast. Even if you don't use a filter the fringes are pretty easy to see with a CFL bulb.
Here is a picture of the fringes from the CFL bulb in my desk lamp that I use all the time to check small flats and the centering of the elements in lenses.
The test plate doesn't need a full polish. Just enough to test to be sure it a sphere. If you used the tool that you used to grind the convex secondary, the problem you might have is getting the radius to match close enough so you don't see 20 or more fringes when you test. That is too many to have the needed sensitivity. You need around 5 fringes. Even though you ground the pieces together there is still a difference in the radii between the pieces and fully polishing the convex surface may increase the difference. So I would flash polish both surfaces and test them by interference and see how close they match.
Edited by DAVIDG, 28 May 2019 - 12:41 PM.