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Equipment Discussions >> Reflectors

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Atl
professor emeritus
*****

Reged: 04/13/12

Star Test Results
      #5555448 - 12/05/12 10:43 AM

Last night I star tested my Dob on Polaris. I did read Star Testing Astronomical Telescopes, but I am wanting a double check on my results sort of. Viewing conditions were according to Clear Sky "good". There was a little wind. Outside of focus at 500X I got the classic diffraction pattern (a little in motion from the atmosphere) showing a little coma from very slight misalignment which I confirmed with the cheshire and corrected. My question concerns inside of focus. Inside of focus I got some wavy spikes. Am I correct in saying these moving spikes are the atmosphere? The pictures in my book of inside of focus are mostly simulated images so the atmosphere isn't much accounted for.

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AhBok
professor emeritus


Reged: 12/02/10

Re: Star Test Results new [Re: Atl]
      #5555512 - 12/05/12 11:18 AM

Either the atmosphere or tube currents. 500X will show the slightest of temperature distortions. I suggest you reduce power just until the spikes settle down. Most scopes will show all you need for star testing at 250X-300X at the in-focus airy disc.

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Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Star Test Results new [Re: Atl]
      #5555526 - 12/05/12 11:32 AM

You got spikes inside but not outside, and inside showed a little motion - but no spikes. So, just wavy spikes outside.

That sounds like /possibly/ a turned edge or a little spherical over correction. Hard to say because I don't know what you mean by wavy spikes or a classic pattern. Nor how far in or out of focus. I mean, I think I can visualize them, if they are the same as I have seen. The atmosphere does seem to stir them up a bit, under corrected in my case. (Outside is a little soft and low contrast, any atmospheric turbulence gives it a spiky appearance.)

Really, it's so hard to do a star test second hand without a visual, but that would be a first stab at a diagnosis...and IMO. As for classic (I assume perfect textbook diffraction) patterns, don't expect them to be the same both sides. That's a myth for anything but a absolutely perfect optic which no one has. It's completely normal to be different either side.

So, maybe a little waviness inside but not outside could be nothing to worry about. A lot depends on the severity which is also difficult to quantify. Those by themselves by no means your scope is not performing as advertised. And remember, the star test is the sum of all aberrations.

How do your images look in focus? Does focus happen fairly quickly or does it seem to be mushy as you approach focus? A good optic will be more snappy at focus.

I'd recommend comparing with Suiter's images as best you can. Or grab a copy of Aberrator and plug in some values. See if you can replicate your patterns closely and read the results. Others will probably chime in and offer an opinion, too.

Here are some images. Often they aren't exactly what you see, but the idea is there.
http://www.telescope-optics.net/diffraction_pattern_and_aberrations.htm
http://www.willbell.com/tm/tm5.htm
http://www.astrosurf.com/altaz/startest_e.htm


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Atl
professor emeritus
*****

Reged: 04/13/12

Re: Star Test Results new [Re: AhBok]
      #5555645 - 12/05/12 01:02 PM

Quote:

Either the atmosphere or tube currents. 500X will show the slightest of temperature distortions. I suggest you reduce power just until the spikes settle down. Most scopes will show all you need for star testing at 250X-300X at the in-focus airy disc.



Awesome even keeping it on Polaris at 500X is a chore with wind blowing. I thought these really high powers were required. At 250X it was a lot more tame looking inside focus. There was still evidence of waviness but not like on 500X


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Atl
professor emeritus
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Reged: 04/13/12

Re: Star Test Results new [Re: Asbytec]
      #5555655 - 12/05/12 01:05 PM

According to the links you posted it looks like roughness, but it is in motion. Apparently still skies are needed to tell the difference according to the page.

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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Star Test Results new [Re: Atl]
      #5556473 - 12/05/12 09:35 PM

Yes, could be roughness, you're correct calm skies help a lot.

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ausastronomerModerator
Carpal Tunnel
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Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: Star Test Results new [Re: Asbytec]
      #5556629 - 12/05/12 10:59 PM

I think it is probably that you still had some tube currents and the mirror was not 100% cooled. "ALL" mirrors can show less sharp diffraction rings and a less defined secondary shadow breakout inside focus whilst they are still cooling. This can also indicate slight overcorrection but that is intrinsic to an optic that is not properly cooled in any case.

Before you even think about star testing, the telescope needs a good couple of hours to cool and settle.

Cheers,


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Scanning4Comets
Markus
*****

Reged: 12/26/04

Loc: Ontario, Canada
Re: Star Test Results new [Re: ausastronomer]
      #5556899 - 12/06/12 05:11 AM

I've done the same test with about 3 different mirrors and got the exact same results even after 3-4 hours of cooling. TDE looks a lot like Spherical Aberration, so it is hard to tell. One way to detect a turned down edge is to try and mask off the outer 1/8" of your mirror.

There ARE mirrors that show the same patterns inside and outside of focus....you just need to find one, but it is not essential. 500x on a star test is a great way to test your mirror, as long as you don't defocus too far. I think 5-6 rings is just right for the test as long as your mirror has cooled for at least 2 hours or more.


Best night for a star test is a calm, hazy night. Asbytec is correct about the "snap test".

What I do now when I get a new telescope mirror is test it on higher powers on Globular clusters and seeing detail within Jupiter's belts and the sharpness on the rings of Saturn.

If you can do the "snap test" for these objects and see small detail, you have a great mirror!

Cheers,


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Atl
professor emeritus
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Reged: 04/13/12

Re: Star Test Results new [Re: Scanning4Comets]
      #5557041 - 12/06/12 08:23 AM

I think I am starting to understand this a bit. Last night Clear Sky was showing "good" conditions. I collimated the scope inside the house with a Cheshire. I took it out and looked at Jupiter. I saw stripes and moons with just a little detail. Then I fine collimated by star test at 250x...then again at 500x. I then observed Jupiter at 250x and saw Ganymede moving across the face on Jupiter. I could also see many details like spots and whirls. Then I increased the power to 500x. Jupiter maintained this detail even with my cheapy $19 Barlow...but was a little soft. I am guessing my collimation is now precise enough to let it perform near it's limit. I was also able to split Rigel clean at 300x...also with cheap Barlow.

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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Star Test Results new [Re: Atl]
      #5557084 - 12/06/12 08:51 AM

What size Dob are we talking about. My guess is 12"?

John is right. The scope /should/ be cooled, really, to do a star test. Heat does give a wavy appearance to the diffraction image. It's possible to look through any plume or boundary layer, though. Thermals add some unnecessary difficulty.

That your better collimated is great, both of these conditions are required to get the best views.

Markus, "TDE looks a lot like Spherical Aberration, so it is hard to tell." I am sure you know, but that's because they are essentially the same error, for over correction, anyway. Both send marginal rays to focus further from the primary. There really is no definite line between one and the other, that I am aware of. Basically, over correction is a more gentle slope over a larger portion of the primary while a turned edge is more steep with less surface area. But, anyway, if anyone wants to know why they are hard to distinguish, it's because they are the same error. Essentially. In both, the optic's edge falls away and the reflected wave begins to lag an imaginary spherical wavefront at the edges.

"...seeing detail within Jupiter's belts and the sharpness on the rings of Saturn...and see small detail, you have a great mirror!"

Agreed. Softer contrast, too. But, how do you know? I dunno, if you're thrilled with the view, I guess.

"I am guessing my collimation is now precise enough to let it perform near it's limit." Sounds like it. So, you were star testing for collimation? If so, then rock on. Well done.


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Atl
professor emeritus
*****

Reged: 04/13/12

Re: Star Test Results new [Re: Asbytec]
      #5557189 - 12/06/12 10:03 AM

Yes a 12". The scope comes in and out of focus quickly.

I am thinking the spikes inside focus were a bit of roughness. Last night there was no wind. Some very minor spikiness was present...just noticeable...but it was still and not moving. This does not surprise me. The scope is hand made and second hand as well. The mirror does have 2 really small corrosion looking spots...maybe 1mm each. Other than that it is fine to the eye. The aberrations present don't concern me as much as collimation. I am not in a financial position to get a large mirror re figured right this moment. I know the mirror is very serviceable. I just want to insure I am getting the best it can deliver and educate myself.

In the recent past I would star test then use the cheshire to adjust, but last night I just let the star test guide me and it was actually quite a bit faster than using collimation tools. Of course the scope has to be at least roughly aligned to align by star test.

Being a serious newbie I really don't know what to expect from this scope. So, when it shows me more detail than it has recently I assume I must be doing something right. How much detail it should bring out I really haven't the vaguest notion. I am interested to see what it can do on a night of excellent seeing.

Last night I let the scope sit outside for 2 hours before viewing. The temperature difference was not much. I live on the Mexican border of Arizona literally in eye shot of the border itself so temps here are relatively warm, but even so I saw tube currents until I switched on the rear fan and boundary layer fan...then they went away.


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Atl
professor emeritus
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Reged: 04/13/12

Re: Star Test Results new [Re: Atl]
      #5557212 - 12/06/12 10:18 AM

Tomorrow on 12/7 at 2200hrs Arizona time Io will pass precisely over the red spot on Jupiter. If seeing is good that should be a fine opportunity to test the scopes resolution I should think...also a cool thing to see.

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Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Star Test Results new [Re: Atl]
      #5557222 - 12/06/12 10:24 AM

Quote:

I could also see many details like spots and whirls.




Sounds like you have a winner. See if you can spot that tiny dark feature following oval BA...just ahead and south of the GRS.

Also, see if or how many small white ovals south of the GRS you can spot. They should be visible if conditions permit. If so, I think that's about right for a 10 to 12" scope. They are very difficult in a 6", and maybe a tad easier in an 8".

Check for color. But most of all, enjoy the view.


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