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Refractor Star Test In Less Than Good Seeing

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#1 Orion68

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Posted 23 July 2021 - 06:00 PM

IMG 20210720 231224283

My new FC-100DF is a beauty for sure. The weather hasn't been good for testing her but last night the seeing was average so outside she went.

 

Even with a nearly full moon, the wonderful contrast came through. Can't wait to see her perform on a dark night.

 

However, the star test was disappointing. I could not get an evenly illuminated first diffraction ring, even when the air steadied for a brief second or two. In fact, at times the first diffraction ring disappeared from the top edge of the airy disk, leaving only the bottom of the airy disk pattern showing a diffraction ring or rings. 

 

I should mention that I was checking the star pattern at focus. The pattern inside and outside of focus looked the same to me. Stars used for testing were Vega, Deneb and Eltanin.

 

Occasionally, the first diffraction ring would appear whole and complete for a split second and then the diffraction pattern would dance around again, leaving the top side of the pattern without a diffraction ring while the bottom portion would show the moving diffraction ring or rings.

 

To me, it seems that in less than good seeing when the diffraction pattern is fading in and out, it should do so evenly, so that diffraction rings are fading in and out equally on all sides of the pattern. But, I may be totally wrong about that!

 

Hoping the many experts on CN can provide some guidance and comment on their experience.

 

Thanks in advance everyone.


Edited by Orion68, 23 July 2021 - 06:11 PM.

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#2 Junoscope

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Posted 23 July 2021 - 06:53 PM

All of the Takahashi’s I’ve looked through have had wonderful optics. I’d repeat the star test on an evening when the seeing is good. 
 

A model for an unsteady night is looking over a hot BBQ grill.  The distortions in seeing aren’t symmetric. 
 

You may consider using an artificial star.  They aren’t very expensive. The sky is rarely steady in New England and I’ve collimated a C6 and C11 using an artificial star.  


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#3 ButterFly

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Posted 23 July 2021 - 06:58 PM

Unless you are looking over the course of many nights of good to great seeing, with a properly cooled scope, you are not star testing optics.

 

What you are seeing is called: seeing.


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#4 Stellar1

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Posted 23 July 2021 - 07:03 PM

Don’t get hung up on star tests too much, too many variables involved which have to be ideal in order to produce the star text one expects. I have never heard of a bad Tak, has anyone on this forum seen one? it kind of like buying a Ferrari and worrying about it not hitting 200mph like they said, only getting 191mph. Since I got my TSA-102 I have seen my share of awful star tests, and a couple of phenomenal ones. They’re few and far apart but then again I am not going out to look at out of focus stars. Give your scope some time, one night you’ll see a star test that will knock you off your chair.


Edited by Stellar1, 23 July 2021 - 07:08 PM.

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#5 Mirzam

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Posted 23 July 2021 - 07:59 PM

Well, no one has given you any help with your problem.  I agree with the suggestion to try again with better seeing and be sure the scope is fully equilibrated. However, you are correct that the diffraction ring should not show any consistent bias to one side or the other.  It may be helpful to examine a brighter star, which will tend to exaggerate any errors due to miscollimation.  Also be sure to choose a star that is high enough in elevation to avoid atmospheric dispersion effects.

 

The effect you describe can be due to a slight decentering of a lens element.  If very minor it may not be worth worrying about.  However, it does not take much to impair the ability to discern difficult double stars.

 

JimC


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#6 MisterDan

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Posted 23 July 2021 - 08:09 PM

 

 

My new FC-100DF is a beauty for sure. The weather hasn't been good for testing her but last night the seeing was average so outside she went.

 

Even with a nearly full moon, the wonderful contrast came through. Can't wait to see her perform on a dark night.

 

However, the star test was disappointing. I could not get an evenly illuminated first diffraction ring, even when the air steadied for a brief second or two. In fact, at times the first diffraction ring disappeared from the top edge of the airy disk, leaving only the bottom of the airy disk pattern showing a diffraction ring or rings. 

 

I should mention that I was checking the star pattern at focus. The pattern inside and outside of focus looked the same to me. Stars used for testing were Vega, Deneb and Eltanin.

 

Occasionally, the first diffraction ring would appear whole and complete for a split second and then the diffraction pattern would dance around again, leaving the top side of the pattern without a diffraction ring while the bottom portion would show the moving diffraction ring or rings.

 

To me, it seems that in less than good seeing when the diffraction pattern is fading in and out, it should do so evenly, so that diffraction rings are fading in and out equally on all sides of the pattern. But, I may be totally wrong about that!

 

Hoping the many experts on CN can provide some guidance and comment on their experience.

 

Thanks in advance everyone.

 

If the pattern changes, it is due to either atmospheric turbulence (including the air very near your telescope) or thermal gradients within your telescope - not your Takahashi's optics.  "Local" seeing includes your immediate vicinity.  Your descriptions remind me of the effects of either intra-scope "thermals" or nearby air masses interacting -- one cooler, the other warmer. This is a common phenomenon in locales that include two or more thermal "islands," such as a grassy park near a pond or creek, or a front yard bordered by a concrete driveway.  If a property includes a big driveway and a pond or a creek, you might encounter all sorts of crazy local seeing.

 

Most importantly:  they do not always evenly smear the diffraction or Fresnel pattern, because their interaction can occur on a very small scale.

 

If anyone says refractors don't require "cool down," they're either generalizing (poorly) or simply incorrect.  Here's another thing that may need "cool down:"  a nice, beefy equatorial mount, especially if it sat in the sun for a while before one's nightly observations.  A warm, massive counterweight can radiate for quite a while.  If it ends up aligned with (and "beneath") your scope's field of view, it can cause very local seeing issues.

 

Give it another go.  If you need to wait a while longer before star testing, then use that time actually observing stuff.

 

Best wishes.

Dan


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#7 MisterDan

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Posted 23 July 2021 - 08:23 PM

Well, no one has given you any help with your problem.  I agree with the suggestion to try again with better seeing and be sure the scope is fully equilibrated. However, you are correct that the diffraction ring should not show any consistent bias to one side or the other.  It may be helpful to examine a brighter star, which will tend to exaggerate any errors due to miscollimation.  Also be sure to choose a star that is high enough in elevation to avoid atmospheric dispersion effects.

 

The effect you describe can be due to a slight decentering of a lens element.  If very minor it may not be worth worrying about.  However, it does not take much to impair the ability to discern difficult double stars.

 

JimC

Good points, all.  However, the effects he described were transient:  "...a brief second or two..." "...at times..." "...split second..."  Diffraction and Fresnel patterns were "moving" and "dancing" and "disappearing."  I would not be confident that a two-second apparition of a diffraction pattern - even if it appeared to be "stable" - was not distorted.  Distortions can be quite stable, especially when caused by "intra-scope" thermals.

 

Best wishes.

Dan


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#8 Orion68

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Posted 23 July 2021 - 08:41 PM

Thanks all for the fast replies.

I forgot to mention that the scope had at least an hour of cool down time.

However, I am viewing from my driveway so that could be my issue.

I am going to look into an artificial star, it can be mounted it at my neighbors across the street, probably 100 feet away.

Definitely had planned on testing and re-testing before making any kind of judgement. I'll be out again tonight and will wait until early morning hours to do another star test.

Thanks to everyone who responded!

#9 ButterFly

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Posted 23 July 2021 - 08:45 PM


To me, it seems that in less than good seeing when the diffraction pattern is fading in and out, it should do so evenly, so that diffraction rings are fading in and out equally on all sides of the pattern. But, I may be totally wrong about that!

Consider two things to help sort this out; smaller and smaller rocks at the bottom of a river; and, why stars bloat with poor seeing.

 

A house can easily cause poor seeing.  Most of the badness of seeing comes from the jet stream, and the lowest few tens to hundreds of feet.

 

The eyepiece is also a little refracting telescope, and that should be cool also, and swapped out during testing.  Which element are you testing, otherwise?  Multiple, big elements take longer to cool, especially when encased.


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#10 Orion68

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Posted 23 July 2021 - 08:51 PM

Deneb will be near the zenith at 2 am. By then the driveway should have cooled off and I can try again. Fingers crossed.

Just like Junoscope said for New England, skies here are rarely steady either so an artificial star may be the way to go.

Clear skies all.

#11 Ben the Ignorant

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Posted 24 July 2021 - 03:05 AM

Don't star test at the zenith, heat from your own body rises up and causes turbulence, try with bright stars 30° to 45° above the horizon, that will keep the objective end of the scope far from your body. If you're not sure how much body heat affects local seeing, just put your hand under the last centimeters of the light path and watch the seeing explode. Also, try different parts of the sky, you're looking through many kilometers of air and these masses of air are not the same in all directions.


Edited by Ben the Ignorant, 24 July 2021 - 05:47 AM.

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#12 Rutilus

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Posted 24 July 2021 - 03:49 AM

Unless you are looking over the course of many nights of good to great seeing, with a properly cooled scope, you are not star testing optics.

 

What you are seeing is called: seeing.

Totally agree.  

I have seen what the OP has described many times with my TSA102 and FS-102,

when the seeing conditions have not been so good.

On nights of good seeing both scopes gave perfect text book ring patterns.


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#13 Stellar1

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Posted 24 July 2021 - 08:08 AM

Totally agree.  

I have seen what the OP has described many times with my TSA102 and FS-102,

when the seeing conditions have not been so good.

On nights of good seeing both scopes gave perfect text book ring patterns.

Yup, same here, as I said earlier give the scope time and don’t make it a point to pass judgment because if you approach your scope from that POV you’ll be disappointed till that one great night. 


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#14 daquad

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Posted 24 July 2021 - 01:06 PM

Observing from a paved driveway, especially macadam, is not a good idea, unless you are staying at low to medium power.  Following a warm summer day, my driveway feels warm to the palm of my hand even several hours after sunset.

 

My observing deck is on the south side of my house and the driveway is to the north.  I like to use Polaris to tweak the finder alignment at high power and find that its diffraction pattern is usually very unsteady compared to stars in other parts of the sky.  I'm sure that part of the reason is the thermals from the driveway and the garage roof. 

 

When the seeing is fairly steady the Airy diffraction pattern can become textbook perfect for a few seconds at a time, while most of the time the rings are scintillating and broken, although clearly symmetric.  As has been mentioned (post #11) try other parts of the sky: also, if you can, avoid observing from the driveway and neighbors' roofs.

 

Dom Q.


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#15 DeanD

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Posted 24 July 2021 - 07:20 PM

Yes to most of the above! The fact that you can't discern any difference between the images inside and outside of focus indicates to me that you have a good one. Also, you mentioned that "the first diffraction ring would appear whole and complete for a split second" is significant. These views would not occur if the optics were not good: if there was a tilted element or poor collimation or figuring.

 

I would attribute seeing part of the first diffraction ring appear and disappear as you describe to atmospheric seeing or tube currents: nothing to do with the optics as such. The diffraction ring/s will never fade in and out evenly: turbulence is random even at small scale and causes random variations in the Airy patterns.

 

Like Rutilis and Stellar1 above I have a TSA102, and I have often seen what you describe. Occasionally I have seen a near-perfect star test on a near perfect night. I have confidence that the scope is good enough to give pretty much the best views possible for a 4" aperture in any given conditions - and that is what good optics are all about. If the seeing is 1/10, then the view I get is pretty much the best 1/10 view possible. wink.gif I have had nights when Saturn appeared as a bouncy oval-shaped blob at 50x, and other nights when I pushed up to 816x just for fun without the image breaking down.

 

Personally I would relax and enjoy the views and that wonderful contrast you mentioned, and on that one night in a 100 (or more) when the seeing is near perfect have a play with the star tests just to give yourself that inner smile that you have a great scope. I am betting that you do!

 

Happy viewing, and all the best!

 

Dean


Edited by DeanD, 24 July 2021 - 07:21 PM.

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#16 Orion68

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Posted 24 July 2021 - 07:21 PM

Don't star test at the zenith, heat from your own body rises up and causes turbulence, try with bright stars 30° to 45° above the horizon, that will keep the objective end of the scope far from your body. If you're not sure how much body heat affects local seeing, just put your hand under the last centimeters of the light path and watch the seeing explode. Also, try different parts of the sky, you're looking through many kilometers of air and these masses of air are not the same in all directions.

I had no idea body heat could cause this! Thank you for sharing your knowledge and experience.

.



#17 Orion68

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Posted 24 July 2021 - 07:23 PM

Yes to most of the above! The fact that you can't discern any difference between the images inside and outside of focus indicates to me that you have a good one. Also, you mentioned that "the first diffraction ring would appear whole and complete for a split second" is significant. These views would not occur if the optics were not good: if there was a tilted element or poor collimation or figuring.

 

I would attribute seeing part of the first diffraction ring appear and disappear as you describe to atmospheric seeing or tube currents: nothing to do with the optics as such. The diffraction ring/s will never fade in and out evenly: turbulence is random even at small scale and causes random variations in the Airy patterns.

 

Like Rutilis and Stellar1 above I have a TSA102, and I have often seen what you describe. Occasionally I have seen a near-perfect star test on a near perfect night. I have confidence that the scope is good enough to give pretty much the best views possible for a 4" aperture in any given conditions - and that is what good optics are all about. If the seeing is 1/10, then the view I get is pretty much the best 1/10 view possible. wink.gif I have had nights when Saturn appeared as a bouncy oval-shaped blob at 50x, and other nights when I pushed up to 816x just for fun without the image breaking down.

 

Personally I would relax and enjoy the views and that wonderful contrast you mentioned, and on that one night in a 100 (or more) when the seeing is near perfect have a play with the star tests just to give yourself that inner smile that you have a great scope. I am betting that you do!

 

Happy viewing, and all the best!

 

Dean

Fantastic advice Dean, from you and the others who responded. I am going to take it to heart!


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#18 Orion68

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Posted 24 July 2021 - 07:35 PM

I was able to view last night and the seeing was better. Using the advice you've all offered the result was a vastly improved star test and some crisp, clear images of star clusters better than I have ever seen before.

 

Now I understand "Takitis"!

 

My gosh, breathtaking views and the seeing was not even excellent. Can't imagine how good it will be on a still night. Now I want a bigger one (TOA 130?)!!

 

Thank you all for sharing your experience and point of view. Very much appreciated.

CS


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#19 Jeff B

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Posted 25 July 2021 - 10:38 AM

In my experiences the only useful knowledge I can learn about a scope's optical train during star testing in rough-ish seeing is some knowledge of astigmatism, but only if the scope is thermally stable.

 

Jeff


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#20 Wildetelescope

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Posted 25 July 2021 - 12:03 PM

 

 

My new FC-100DF is a beauty for sure. The weather hasn't been good for testing her but last night the seeing was average so outside she went.

 

Even with a nearly full moon, the wonderful contrast came through. Can't wait to see her perform on a dark night.

 

However, the star test was disappointing. I could not get an evenly illuminated first diffraction ring, even when the air steadied for a brief second or two. In fact, at times the first diffraction ring disappeared from the top edge of the airy disk, leaving only the bottom of the airy disk pattern showing a diffraction ring or rings. 

 

I should mention that I was checking the star pattern at focus. The pattern inside and outside of focus looked the same to me. Stars used for testing were Vega, Deneb and Eltanin.

 

Occasionally, the first diffraction ring would appear whole and complete for a split second and then the diffraction pattern would dance around again, leaving the top side of the pattern without a diffraction ring while the bottom portion would show the moving diffraction ring or rings.

 

To me, it seems that in less than good seeing when the diffraction pattern is fading in and out, it should do so evenly, so that diffraction rings are fading in and out equally on all sides of the pattern. But, I may be totally wrong about that!

 

Hoping the many experts on CN can provide some guidance and comment on their experience.

 

Thanks in advance everyone.

 

http://www.damianpea...m/pickering.htm

 

You might find this helpful.   Based on your post, it sounds like things were symmetric, identical in and out of focus, and you did not mention color.  All good things regarding the optics!  Your comments about start clusters are another excellent indicator that your optics are what one would expect from a Tak.  Point it at Jupiter and see how things look.  Try to pick a good night.  I find that the hotter and more humid the air is outside, the more stable the atmosphere.   Usually a night or two before a front comes through.  I suspect you will be pleased :-). 

 

Have fun!

 

JMD


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#21 Rich V.

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Posted 25 July 2021 - 02:06 PM

http://www.damianpea...m/pickering.htm

 

You might find this helpful.  

waytogo.gif 

 

I'd expect that "star testing" would be difficult for most if the image appeared worse than #8 on this scale.  Waiting for a better night (or even time of night) would very likely produce a much more useful result.  In my area, after 2 AM generally gives me the best chance of better atmospheric turbulence, but it sure doesn't happen on most nights.  Be patient.  

 

Rich


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#22 Ben the Ignorant

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Posted 25 July 2021 - 05:42 PM

The guy who uses the club's observatory the most has noticed that one half of the sky there always has finer seeing than the other half. I don't remember if it's the eastern or western part because I haven't been there for many months due to the covid mess.

 

But he gets better results in high resolution observation and photo in that half. The split could be caused by terrain tens of meters, hundreds of meters or kilometers around the building, I don't know but I wish I understood the reasons. Anyway, it's worth looking for long term trends like that to help you plan your high resolution work.


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#23 ButterFly

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Posted 25 July 2021 - 07:44 PM

I was able to view last night and the seeing was better. Using the advice you've all offered the result was a vastly improved star test and some crisp, clear images of star clusters better than I have ever seen before.

 

Now I understand "Takitis"!

 

My gosh, breathtaking views and the seeing was not even excellent. Can't imagine how good it will be on a still night. Now I want a bigger one (TOA 130?)!!

 

Thank you all for sharing your experience and point of view. Very much appreciated.

CS

Now do it with a few narrowband filters on brighter stars.  Air is always a refractive medium.  Dispersion only means that air reacts differently to different wavelengths, whether steady or not.  Omega Centauri has been fun this year.  When the seeing was poor, I would pop the PVS14 onto the eyepiece.  And poof!  At twice the wavelength, it takes twice the poop factor for seeing to be distruptive - the Airy disk, and thus PSF, are twice as big.

 

I've also been using an ADC with my dob.  Sometimes, when on a mesa with a settled inversion layer, the fringes can reverse due to the bending of the path as a function of wavelength.


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#24 teashea

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Posted 25 July 2021 - 08:01 PM

It only takes slight problems to create problems with start tests.  Remember that you are dealing with very small differences optically.  It is not surprising that you are not getting optimal results.  

 

Most of all ----- enjoy that excellent telescope.


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#25 Orion68

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Posted 25 July 2021 - 11:22 PM

The guy who uses the club's observatory the most has noticed that one half of the sky there always has finer seeing than the other half. I don't remember if it's the eastern or western part because I haven't been there for many months due to the covid mess.

 

But he gets better results in high resolution observation and photo in that half. The split could be caused by terrain tens of meters, hundreds of meters or kilometers around the building, I don't know but I wish I understood the reasons. Anyway, it's worth looking for long term trends like that to help you plan your high resolution work.

Totally agree, I have also noticed that sometimes when I switch stars the image improves. There doesn't seem to be a pattern that I can figure out. What I need to do is start recording my viewing by areas of the sky. Then, over time maybe a pattern will show itself.

 

Good tip!




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