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8/17/2010

XX14i Update, info and Observing

Well, I am processing my sketches digitally and the originals so I'll have those up soon. I also want to talk about testing a mirror, and what that all means. I spent five days last week observing, from home, from Pit n Pole in Rush Valley in the Utah West Desert and up at Wolf Creek Pass. Wolf Creek Pass is at 9879 feet as measured by a GPS unit and the SQM gave me a reading of 22.2 that night up there. Nice and dark! Pit n Pole gave me a SQM of 21.7 that same week so it was a good week of observing. Skies were average at best in terms of seeing, but it was clear and just after a new moon so it was good to be observing!

I want to talk about mirrors and why its important to try to get the best mirror possible for the instrument that you use. A lot of my information is going to be taken from Royce Optical Please check out their site as it is quite informational. I recently had my mirror tested to determine the quality of the mirror and its flaws. My mirror was tested by two people who have complete trust in their ability to determine the quality of the mirror. I do not have permission to share their names though at this time. Each tested the mirror separately. The mirror according to them tested out quite nicely, in the low to mid 90's on the Strehl ratio. To understand more of that I recommend the Royce's article called A Better Way for Measuring Optical Performance. A few quotes from the article may entice you to read the article:

"As a final note, the Strehl ratio derived from an interferometric measurement of the RMS wavefront error represents a ceiling on the attainable performance. In use, a mirror will never attain this performance, mostly because of seeing, but also because of collimation errors and errors from the other optical elements in the path. Even the steadiest nights at the best sites do not allow a system to achieve this performance, but it can come quite close."

What does it mean? Though I got lucky (because with a mass produce scope I believe it is luck) and got an excellent mirror in my XX14i, it doesn't mean that I will be able to always maximize my mirror to the fullest of its capabilites because of collimation errors, the sky/atmosphere conditions, an average secondary etc.

However, since SO many are visual learners, I wanted to share from the Royce site what the impact on the wave error is to viewing. A computer simulation of Jupiter was used to show the impact. You can see it frame by frame here or see all the images at one time here; this surely shows the impact of wavelength error or Strehl ratios on the views your telescope will provide to you.

What is the take away from this? First, I realize not everyone is going to have their mirrors tested on mass marketed scopes. I did for the sake of knowing what the quality of the mirror is in the XX14i. In the end, I know what my star tests shows me, a very good to excellent mirror and one that does quite well in both light polluted and dark skies. If you want to do a basic star test to gauge the quality of your mirror use this link from Starizona's site. If your interested in conducting a test of your mirror see this link on the knife edge method. One of the testers has his own setup and the other had this one from Edmund's Scientific that was used to test my mirror.

Now, after all of this, I want to share an article and some quotes from an article on mirror quality that I really enjoyed reading. It is called
What's Important in Optical Quality to Amateur Observers (And what's not) by John Lightholder. Mr Lightholder states in the article:

"Actually, the user of the scope has the most influence over how his mirror performs, more so than the mirror's limitations expressed by its absolute error. I'd even go so far as to state that a well-collimated 1/2-wave peak-to-valley wavefront (pvwf) mirror/system will perform better than a casually collimated one with a 1/20-wave peak-to-valley-wavefront (pvwf) primary mirror. I've known people who were absolutely amazed at how well their system performed when they got serious about their collimation. If you consider how slight a component misplacement or optical path difference results in a 1-wave error (which is only about .2 ten-thousandths of an inch), then you realize how quickly poor collimation can ruin your fine mirror's images. And if you have a lesser quality mirror, it suffers even more from de-collimation. Having an good-to-excellent mirror gives a slight but significant advantage when seeing is marginal but not quite "over the edge."

He actually says a lot here I believe. You may have a poor or only good mirror, but much of what you see is in your control. A great mirror poorly collimated will show less than a so so mirror that is greatly collimated by its owner. His point on 1-wave error in collimation is significant too. So being able to be fully collimated is critical to improving the image of the objects your viewing. Of course, an excellent mirror in poor seeing conditions will show more and perform better, if collimated, but in the end it is the user that determines who well the mirror performs on the object they see. To summarize Mr. Lightholder:

"The most fundamental aspect of observing, the FUN of it, is an observer looking through the telescope at an object. (And, hopefully, the telescope won't get in the way!) A telescope that gets used provides limitless hours of in-depth, four-dimensional observing experience."

Here's hoping the quality of your telescope is good, that it doesn't get in your way, and that it provides you many hours of in depth enjoyment. I'll get those observations up shortly.