I'm searching for opinions from some of the optical experts out there.
I have a nice little 6" newt and for years have had a 1.3" secondary in it. This sec. is pyrex and was advertised as 1/10th wave (no documentation) and the scope has always star tested very good, if not excellent.
About 1-1/2 years ago I purchased a 1.52" sec. for better illumination when using 35mm film. It is one of those expensive 1/20th wave quartz sec. that come with full test documentation. It has been in the scope since I got it but dispite all my efforts has never star tested as well as the 1.3". I usually attributed the poor star test to seeing conditions and the fact that the primary needed recoating. I had the primary recoated and still the star test did not improve.
Tuesday night I reinstalled the 1.3" pyrex and even while tweaking the collimation I could tell the star test was going to be much improved. After the scope acclimated I did a star test and the results were a definate improvement.
I surely do not want to accuse anyone of falsely claiming the precision of thier secondaries so these 2 questions arise.
1. Would an oversized secondary further illuminate surface errors in the primary resulting in an apparently poorer star test?
2. Since I did extensive bare mirror star testing with the 1.3" while I was figuring the primary have I created a matched optical set that may not star test well with a different secondary? Note-- the bare mirror did not pass the star test untill the Foucault numbers were dead on so I sort of doubt this one.
Your opinions please.
Bob
p.s. I use Novak supports so no adhesives or stress is applied to the secondaries. Stress was my first thought but has been eliminated as a cause.
I have a nice little 6" newt and for years have had a 1.3" secondary in it. This sec. is pyrex and was advertised as 1/10th wave (no documentation) and the scope has always star tested very good, if not excellent.
About 1-1/2 years ago I purchased a 1.52" sec. for better illumination when using 35mm film. It is one of those expensive 1/20th wave quartz sec. that come with full test documentation. It has been in the scope since I got it but dispite all my efforts has never star tested as well as the 1.3". I usually attributed the poor star test to seeing conditions and the fact that the primary needed recoating. I had the primary recoated and still the star test did not improve.
Tuesday night I reinstalled the 1.3" pyrex and even while tweaking the collimation I could tell the star test was going to be much improved. After the scope acclimated I did a star test and the results were a definate improvement.
I surely do not want to accuse anyone of falsely claiming the precision of thier secondaries so these 2 questions arise.
1. Would an oversized secondary further illuminate surface errors in the primary resulting in an apparently poorer star test?
2. Since I did extensive bare mirror star testing with the 1.3" while I was figuring the primary have I created a matched optical set that may not star test well with a different secondary? Note-- the bare mirror did not pass the star test untill the Foucault numbers were dead on so I sort of doubt this one.
Your opinions please.
Bob
p.s. I use Novak supports so no adhesives or stress is applied to the secondaries. Stress was my first thought but has been eliminated as a cause.
Fujinon 10x70 FMT-SX binocs
Tak FS-78 ƒ/8.1 - 630mm fl.
6" ƒ/6.5 homeground newt. - 981mm fl.
10" ƒ/4.85 homeground dob. - 1231mm fl.
