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Equipment Discussions >> Reflectors

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garyp1936
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Reged: 06/30/10

Mirror Star Test
      #5679203 - 02/14/13 11:10 AM

I have a new 10" Orion XT collimated with Catseye passive tools, but there seems to be a serious problem which, being a reflector newbie, I can't identify. During a star test the outside-of-focus pattern appears quite good, BUT the inside-of-focus is a white blob with uneven edges. My understanding is tht both outside and inside should be identical, or close to it. What is the problem and what needs to be done? I have had an 8SE for several years, and the DSO images of the 10" are clearly superior to those of the 8". The star test, however, results have me worried. Gary

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dpwoos
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Re: Mirror Star Test new [Re: garyp1936]
      #5679304 - 02/14/13 12:01 PM

How are your high-power views of Jupiter? If they are good then you can't have a serious optical problem.

The best way to understand the star test is to do it with folks who know what they are looking at. Your local astro club will have folks who can help you with this.


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stratocaster
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Reged: 10/27/11

Re: Mirror Star Test new [Re: garyp1936]
      #5679366 - 02/14/13 12:31 PM

Quote:

During a star test the outside-of-focus pattern appears quite good, BUT the inside-of-focus is a white blob with uneven edges.




Interesting. I had a similar experience with my stock 10" GSO mirror. I'm not a star testing authority by any means, so I don't know the technical implications of such a view. However, while my DSO views seemed quite impressive to me, my planetary and lunar views seemed soft - even under good skies.

It would be interesting to see your planetary/lunar impressions.


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nirvanix
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Reged: 06/07/07

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Re: Mirror Star Test new [Re: garyp1936]
      #5679387 - 02/14/13 12:44 PM

Make sure scope is very well cooled, properly collimated, and you've got good seeing and then try. This time of year going from warm house to cold exterior I give the mirror 2 or more hours cool down. As someone suggested try some real world targets too. Detail on Jupiter, moon, splitting some doubles are indications of performance.

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garyp1936
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Reged: 06/30/10

Re: Mirror Star Test new [Re: nirvanix]
      #5679424 - 02/14/13 01:05 PM

I did observe the transit of Io across the face of Jupiter last Thursday night. Didn't stay up for the shadow's crossing, but the image of Io itself was quite clear. Gary

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dan_h
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Reged: 12/10/07

Re: Mirror Star Test new [Re: garyp1936]
      #5679435 - 02/14/13 01:10 PM

What powers were you testing at? And how much defocus were you using?

The star test has to be done at high power and the defocus should only show 5 or 6 rings.

dan


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Jason D
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Re: Mirror Star Test new [Re: garyp1936]
      #5679547 - 02/14/13 01:53 PM

Quote:

BUT the inside-of-focus is a white blob with uneven edges



That sounds like a spherical aberration (overcorrection). Spherical aberration is common among mass produced mirrors. Getting both sides to focus to look identical requires high-quality mirrors such as premium optics. Star testing requires some experience. Many would defocus too much on both sides and convince themselves that the views are identical – it does not work this way.

Having some degree of spherical aberration does NOT mean your mirror is of low quality. Some degree of spherical aberration will still provide pleasing views.

I have an XT10 and my original stock mirror had some spherical aberration but it provided pleasing views. I have upgraded to premium optics years ago. At low/medium magnification the difference is subtle.

Jason

Edited by Jason D (02/14/13 02:47 PM)


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Pinbout
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Re: Mirror Star Test new [Re: garyp1936]
      #5679618 - 02/14/13 02:26 PM

Quote:

During a star test the outside-of-focus pattern appears quite good, BUT the inside-of-focus is a white blob with uneven edges.




the inside focus big white blob thing is either overcorrection or tde, they're both similar in the way they reflect light back toward focus.

but you should look at the star just barely out of focus with high power mag to better determine the issue.


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garyp1936
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Reged: 06/30/10

Re: Mirror Star Test new [Re: Pinbout]
      #5679727 - 02/14/13 03:20 PM

On the star test I tried several EPs, ranging from 50X with an ES 24mm to a barlowed 5mm at 480X--the highest power I have. The result was the same. I very slowly de-focused on both sides. Also interesting is that views of DSOx--primarily OCs--are significantly imroved over my 8SE. Gary

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Pinbout
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Re: Mirror Star Test new [Re: garyp1936]
      #5679737 - 02/14/13 03:26 PM

you outside focus are the rings seperations pronounced or do they blur between the rings.

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garyp1936
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Reged: 06/30/10

Re: Mirror Star Test new [Re: Pinbout]
      #5679991 - 02/14/13 06:13 PM

Outside of focus the rings are clear and separated; outside focus seems quit good. Gary

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Scanning4Comets
Markus
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Reged: 12/26/04

Re: Mirror Star Test new [Re: garyp1936]
      #5680314 - 02/14/13 10:12 PM

Many mirrors fail the star test. IMO, the ultimate test of a mirror is the focus "snap" test, fine detail on planets, and fine detail on globular clusters.

I have owned three reflectors that showed a really nicely defined outside of focus diffraction pattern and the inside is blurry, but the tests i just mentioned all passed with flying colors.

Sounds to me like Spherical Aberration as well...and over correction. All of this would of course be after a good collimation and proper cool down and leaving the fan on the back of your mirror running while observing to keep it at ambient temp.

Cheers,


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David Knisely
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Reged: 04/19/04

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Re: Mirror Star Test new [Re: nirvanix]
      #5680570 - 02/15/13 02:01 AM

Quote:

Make sure scope is very well cooled, properly collimated, and you've got good seeing and then try. This time of year going from warm house to cold exterior I give the mirror 2 or more hours cool down. As someone suggested try some real world targets too. Detail on Jupiter, moon, splitting some double, are indications of performance.




This is indeed quite true. My custom-made 14 inch mirror shows a little spherical aberration during cool-down, but once down to temperature passes the star test with flying colors. Let things get good and cooled-off before you try star testing. Clear skies to you.


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dpwoos
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Re: Mirror Star Test new [Re: garyp1936]
      #5680822 - 02/15/13 08:23 AM

Quote:

On the star test I tried several EPs, ranging from 50X with an ES 24mm to a barlowed 5mm at 480X--the highest power I have. The result was the same. I very slowly de-focused on both sides. Also interesting is that views of DSOx--primarily OCs--are significantly imroved over my 8SE. Gary




If the results were the same at 50x and at 480x then I don't think you are doing the test correctly. However, why do the the test at all? If you are happy with your high-power views then you are good to go - enjoy! On the other hand, if you are interested in evaluating your mirror besides diagnosing a problem (which it sounds like you don't have) then you will have to learn more about the star test. A google search will provide you with a lot of info, but the best way to learn about this is to find folks who know what they are doing, e.g. your local astro club.


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Binojunky
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Reged: 12/25/10

Re: Mirror Star Test new [Re: dpwoos]
      #5681101 - 02/15/13 10:35 AM

People need to except the fact that the popular low cost dobs and newts offer oustanding value for money, what they don,t offer is premium optics, in the Zambuto class for example.
Once in a while you hear of a owner thats got a better than average example but by the very nature of the beast(low cost) most are average to good, stop worrying and just enjoy the scope for what it is, DA.


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KerryR
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Reged: 12/05/07

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Re: Mirror Star Test new [Re: Binojunky]
      #5681140 - 02/15/13 10:50 AM

If you were resolving the disk of Io across the disk of Jupiter, it's unlikely you have any serious issues with your optics-- this requires precise focus, resolution, and contrast.

Use a high power ep, 5-10mm on a 4th-ish magnitude star. Defocus only enough to break the Airy disk into 4-7 rings. Defocus the other direction the same distance. Compare. It's not reasonable to expect to see identical images in anything but premium optics, and not likely. The easiest thing to diagnose is general under or over correction, and turned down edge. Yours is likely over corrected, and/or has some TDE. Diagnosing other issues and their extent requires some significant precision and understanding, and you'd do well to spend some time with Suiter's book.

It's my (possibly erroneous) understanding that it's very rare for larger mirrors without active venting to attain thermal equilibrium because it's common for night time temperatures to continuously drop. The outside edge of the mirror cools fastest, so it contracts a bit, squeezing the warmer center into a slightly deeper curve, usually yielding over correction. The shorter the cool down time or the faster the temps are dropping, the more over correction you'd expect to see.


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Pinbout
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Reged: 02/22/10

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Re: Mirror Star Test new [Re: garyp1936]
      #5682165 - 02/15/13 06:54 PM

Hee's another star test you can do. Get a bright star in the eyepiece, remove the eyepiece and see if you can see it on the secondary. Lay a piece of electrical tape across half the focuser place your eye so the tape cuts half the star image off. As you go thru focus you should see it null. If there is SA it will start to look like foucault parabloid donut.

Also with your high power eyepiece in you can mask off the front of your aperature so its smaller then the 10in mirror. Say 9in in dia. To see if the inside focus is less blury cause you masked off any tde.


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Scanning4Comets
Markus
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Reged: 12/26/04

Re: Mirror Star Test new [Re: Pinbout]
      #5682519 - 02/15/13 10:50 PM

Quote:

Here's another star test you can do. Get a bright star in the eyepiece, remove the eyepiece and see if you can see it on the secondary. Lay a piece of electrical tape across half the focuser place your eye so the tape cuts half the star image off. As you go thru focus you should see it null. If there is SA it will start to look like foucault paraboloid donut.

Also with your high power eyepiece in you can mask off the front of your aperture so its smaller then the 10in mirror. Say 9in in dia. To see if the inside focus is less blurry cause you masked off any tde.

--------------------
-Danny




Great idea with the tape...I'm going to try that sometime!


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Pinbout
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Reged: 02/22/10

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Re: Mirror Star Test new [Re: Scanning4Comets]
      #5682589 - 02/15/13 11:27 PM

Quote:

Great idea with the tape...




if you have stig the shadow will clock around as you go thru focus.


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azure1961p
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Reged: 01/17/09

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Re: Mirror Star Test new [Re: Pinbout]
      #5684291 - 02/16/13 09:32 PM

Sounds like a thermal issue to me, ie; cool down. A reflector - particularly if it isn't fan cooled is warmer at one end than the other due to the heat radiating off the primary. In practice - even with multiple fans it'll never be like having optics housed in a vacuum - there's always something radiating off the glass. The good news is that beyond a certain point it doesn't matter. What Id high recommend is sealing the mirror end of the tube and blowing air on the back of the mirror. This will go a long way to defuzzing the blurry blob on that troublesome side of focus. You'll still see some heat signature in out of focus stars but its lessened nicely. Another thing, try to keep your hand away from the opening (secondary) end of the scope as the heat coming off your hand does blur the image noticeably. If u can't avoid this just kno it'll take ten seconds or so for it to dissipate.

I think u probably have common reflector thermal problems. It's an easy treatment . Your observation of Io s disc is no small feat and it sounds like u have a mirror that's at least reasonably good.

Please see the reflector forum for a thread that I started (but now has the momentum of other successful contributors) called thermal Issues and Fan Success - it might be a page deep. A lot of folks have great advice on what worked for their own scopes. A general wisdom arguably is open truss scopes are better when the air is sucked from behind while tube scopes like yours are better when the air blows up the tube.

I'm betting you just need a fan. Trust me, if you don't now, u will after you see the results. It'll never be perfect but it'll go a long way to reducing flare and bloating caused by too too much heat off the glass .

Good luck.

Pete


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