Mirror lens guide for budget AP

[Lokifish]

Questions like "600mm Mirror Lens for Astro?" pop up from time to time and feedback is often no more than "I like lens X". To help folks narrow things down I've put the Modern Photography mirror lens tests into a sortable list. While somewhat out of date, it covers the bulk of older lenses one would find on the internet. What's missing is most LZOS Rubinars,  Opteka/off brand 800, 900 and 500/6.3 lenses, and some more obscure lenses like the Contax/Yashica 1000/10. 

While not the exact same chart, Bob Atkin's lens testing guide can be used for those wanting to submit their own tests for inclusion into the list. I have created a resolution/contrast variation test target which includes 100/80/60/40/20 percent contrast targets and cleaner lpm targets down to group 9. This should give reasonably accurate lpm results but contrast will be a more general 20% grouping. I may update the chart later with a finer contrast scale and inverted values.

I will also add that this is where tiny pixels shine if doing photographic tests. A prime example is while testing alterations to my Sigma Ultratel (late model) 500/8. It took using a 1.12um sensor to even come close to visual tests with a 10mm Plossl without a lot of stacking and editing in post. 

[Lokifish]

I've added a pinpoint positive and negative chart (example below). It adds a contrast resolution chart in 10% steps and is far less forgiving than a bar chart.

[Stub Mandrel]

I Have a Chinon 500m f6.3

What do you want me to do????

[Lokifish]

If you want to test your lens; 

1. Print the targets 
2. Place them on a flat surface at least 25ft per 500mm EFL away (the further the better)
3. Space them so the fit edge to edge in the camera frame (Bob's article shows examples) 
4. Make sure everything is aligned (the closer the camera is to being perfectly face on, the more accurate the results)
5. Make sure you are in focus and take some shots

From here you can figure the resolution as follows;

1. Divide image pixel width by the 100mm line's width in pixels in the image
2. Multiply the above result by the sensor width in mm
3. Multiply the the above result by the lowest resolved group
4. If possible, confirm the lowest resolved group by using an eyepiece on the lens

The result is the lp/mm.  If using the updated chart from the second post, find the lowest visible contrast for the same resolved group. This gives the single shot lp/mm and contrast for your lens. Share your results, and the image if possible. I'll periodically update the list and have the entries link back to the appropriate posts.

When I get the chance I'll do a "baseline" using a common telescope, or two, and add it to the list. This should help folks get a better feel for what to expect when using a mirror lens.

Thanks and clear skies