Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_2019_sp_banner.thumb.jpg.a0ff260c05b90dead5c594e9b4ee9fd0.jpg

loron91423

TESTING THE OPTICS OF YOUR TELESCOPE

Recommended Posts

My own view is that any mirror with a specified error is worth paying for. The fact that they are checked is why they cost more. Otherwise the scope has what ever comes off the machine. I do think that people would notice the difference between a mirror that has errors of a 1/4 wave on it's surface at high magnification. While the numbers sound stupidly small that 200mm newton of yours is trying to get all of the light in a star into a circle that is 0.0068 mms dia, that's 0.27 thousandths of inch in old money. A slight change in the slope in the shape of the mirror from a sphere is needed to do that. The change in the shape on the mirror is a lot lot smaller realistically measured in millionths of an inch. I wave length of light is 22 millionths of an inch.

Orion make things difficult. They specify PV which from what I can gather is exactly the same as rms (which it may not be) and also a wavefront error rather than a mirror error. That's actually a better way to do it but a true 1/4 wave front error is nigh on perfect. Without knowing how the calculation is done it's not possible to say if their 1/4 wave mirrors are "perfect". The catch is that to be good a mirror must be smooth. RMS can hide bad zones in the mirror and that can affect the image. That's why some makers say a maximum surface error of 1/10 wave. That gives a 1/5 wave front error. Slightly better than the goal but it costs.

The other factor is seeing - that has a wave front error too - they add to what the telescope has already.

My own view is that the difference will show up when magnification is at it's higher levels. There are other complications. The size of the central obstruction has an effect too. A rather famous american is arguing that this can degrade a scope to 1/5 wave at the usual sizes. I don't see it as the effects are totally different. It definitely does effect image contrast which in turn will effect what can be seen.

This whole area is confusing as the specifications many manufacturers use aren't clearly stated. eg is Orions maximum 1/4 wave wavefront a true max or is it an rms and how does it relate to surface error. Strehl ratio's are also often stated now. At 1st sight it seems to relate to diffraction limited optics. Turns out that it's calculated from the measured rms errors and is actually the ratio between the theoretical light intensity in a diffraction spot and the actual one. That doesn't really have any baring on diffraction limited optics unless it's a very high figure approaching 100%.

John

Share this post


Link to post
Share on other sites

Thanks again for your reply there John (rms = Root Mean Square (found it back on your first reply)).

Your replies all make sense, with regard to explaining how manufacturers measure (or try to) the quality of their optics. However, I still wonder what relevance all this has to an amateur (especially one not so well versed in mathematics!).

When I eventually come to buy an Orion Optics scope I will at the end of the day, have to accept that there "interferometer" test results are a true indication of the scopes optical perfection! I can then only make a practical "MK1 eyeball" test on a clear night against one of my other scopes (albeit they are different sizes, and different types of scope). If the view is clearer, crisper and brighter, then I will be impressed.

Your replies have been very enlightening though!

Regards,

philsail1

Share this post


Link to post
Share on other sites

Actually Phil rms has been known to be the sum as mentioned with all of the errors divided by 2. What I'm trying to point out is that manufactures seem to be inclined to hide what they are actually selling - no spec, a rumored spec, or a spec that may not mean what it suggests. Bit like 2ndry obstructions. Once many people became aware that 20% diameter is a good idea some if not all started specifying % area. Much smaller figure so it looks better. However if some one wants a small compact scope eg cassegrains etc they have little alternative other than to put up with it. Telescopes are a compromise. Gain one thing and loose another.

John

Share this post


Link to post
Share on other sites

Yes, I see John. Are you saying that the smaller a Shmitt Cassegrain (or Maksutov, or Reflector) telescope one buys, the larger is the obstruction ("secondary" mirror, or reflecting spot) one has to put up with (in relation to the size of main mirror). Conversely, the percentage of obstruction from the secondary mirror of a much larger scope would be much smaller in relation to the size of the main mirror?

I do think manufacturers should be totally honest in advertising the specifications of their scopes - especially when one considers the cost of some scopes. We all expect the "best" for our hard earned cash!

many thanks,

philsail1

Share this post


Link to post
Share on other sites

Hi, well I'm very much a newbie but I can say that I have read all of this thread - God it's frightening what you guys talk about. For me I'll just look through the thing and marvel at what I see if I'm lucky enough.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.