Secondary Mirror Upgrade Required?

[bobro]

Some time ago I changed the primary mirror on my Meade 130 Polaris for a mirror from a SkyWatcher 130P as images suggested the main Polaris mirror wasn't great with obvious aberrations. The result was an immediate improvement in the quality of captured images.

However, the secondary mirror remains the original from the Meade. I wonder if there is a way to test the secondary to determine its quality, assuming it has a significant involvement in image quality? 

[JRM]

bobro,

 Not that I am an expert on secondaries, but years ago when I was making my 10" mirror I had a similar situation where as I  wanting a higher then average

secondary to go with my mirror, after a long chat with a gent whom made many high quality mirrors years ago,  ( he worked at an optical lab for space agency ) he told me unless you are using the scope for photos, which I was not, the human eye can not tell the difference between a 1/8th and a 1/20th wave secondary.

Now others may chime in and say hey wait a minute, but I ask you...

are you wanting to take images...then maybe, go for it,  but unless that secondary is totally horrible, I would stay the course, enjoy what you have with the better primary optics and get everything you can get outta that telescope.

just my two cents, take care.

 

Rick

p.s. let me know what you decide, I am interested either way

[Astrobits]

While it is quite easy to test primary mirrors with a variety of tests, it is not so easy with secondaries.

During manufacture, secondaries are tested against a reference flat when the interference fringes can be seen. Once the secondaries have been coated and in use for a while there are two problems that arise.

Firstly, they will have some material ( dust/dirt ) that has settled on the surface and has stuck so that the usual gentle cleaning does not remove it. It does not need to be large enough to be visible, a few wavelengths in size is enough to prevent  a glass reference surface getting properly parallel to get a good set of interference fringes. This can be overcome by using a liquid reference surface.

Secondly the secondary is very reflective while the reference surface is not. The difference causes the fringes to be very faint because of the imbalance in the reflectance of the two surfaces. Using a liquid reference surface where the mirror is under the liquid, usually water, this can be partially overcome by adding some water soluble dye which attenuates the reflectance of the mirror. It needs rather a lot of dye as the layer of water over the mirror needs to be very thin in order to get stable fringes. I have tried this and it works to a limited extent with the fringes a bit easier to see.

Nigel

 

[bobro]

Thanks @JRM and @Astrobits for the responses. The application is imaging. Whilst I wouldn't go to the expense of purchasing a quality secondary mirror for the scope in question, I do have doubts about the current mirror. Of course the way to find out is to replace the mirror - as I enjoy a bit of diy and tinkering I'll look out for a SkyWatcher mirror on Astroboot and do a comparison.

Bob

[Adam J]

38 minutes ago, bobro said:

Thanks @JRM and @Astrobits for the responses. The application is imaging. Whilst I wouldn't go to the expense of purchasing a quality secondary mirror for the scope in question, I do have doubts about the current mirror. Of course the way to find out is to replace the mirror - as I enjoy a bit of diy and tinkering I'll look out for a SkyWatcher mirror on Astroboot and do a comparison.

Bob

I am still going to suggest a 130PDS as an alternative to spending money on your current scope, the 1.25 inch focuser is very limiting for imaging.