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Replaced my secondary

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#1 Tonimus

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Posted 13 June 2021 - 01:55 AM

The secondary mirror on my Meade 826 was really hazy after multiple cleanings and scrubbings. I replaced it with a 46.5mm GSO from Agena. That’ll probably help a little. 

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#2 Jim Waters

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Posted 13 June 2021 - 02:04 AM

"scrubbings"?

 

https://www.astro-ba...cal cotton wool.


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#3 Tonimus

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Posted 13 June 2021 - 09:03 AM

Dish soap and fingertip was as aggressive as I got before giving up. Tried it 3 times without improvement. 



#4 DAVIDG

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Posted 13 June 2021 - 09:39 AM

 While having coating damaged can scatter light what is more important is how optically flat the diagonal is. Replacing one with perfect coating but that isn't optically flat will cause more harm to the image quality then one with coatings that have some hazy.  A good diagonal can always be recoated. 

    I have  tested many many diagonals and around 25% are not as flat as what the manufacture states.  Here  is  a typical example of one that is around 1 wave from flat when it was  stated to be 1/8 wave. 

 

                  - Dave 

 

bad diagonal.jpg


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#5 Tonimus

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Posted 13 June 2021 - 10:52 AM

How would I verify that the mirror is as flat as they claim? Or good enough, anyway. 



#6 DAVIDG

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Posted 13 June 2021 - 07:44 PM

How would I verify that the mirror is as flat as they claim? Or good enough, anyway. 

 There a couple of methods to test the flatness of optical surface. What I do and most do is use a known reference flat and place it in contact with the surface and shine  monochrome light on them. You'll see interference fringes and the straightness of fringes is a direct indication of the flatness. If a flat is claimed to be 1/10 wave then the fringes should be very straight when tested against a master surface that has the same flatness. 

   Here is picture of 4" minor axis flat that I tested. It show the fringes are very curved. The second picture is after I refigured it and now the fringes are straight indicating the surface of the diagonal matches that of the master surface placed on top of it. 

 

Coulter Diagonal start.jpg

 

coulter diagonal.jpg

 

 

 

 

   If you watch places like Ebay you can find master flats for not much money. Scratches and chips or a bad coating on the Master surface won't matter. You need to see thru the Master surface so if you find one with bad coatings that is not a problem back you'll need to remove it anyway. Since it looks ugly they usually sell inexpensively. 

    I got 8" flat that had bad coating and a couple of small scratches but when I tested it  was flat across the whole surface to 1/10 wave.  A new  flat of that size cost easily over $1000 if not more. I paid $35 of Ebay for it. 

 

          - Dave 


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#7 moonrakercat

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Posted 14 June 2021 - 12:02 PM

 While having coating damaged can scatter light what is more important is how optically flat the diagonal is. Replacing one with perfect coating but that isn't optically flat will cause more harm to the image quality then one with coatings that have some hazy.  A good diagonal can always be recoated. 

    I have  tested many many diagonals and around 25% are not as flat as what the manufacture states.  Here  is  a typical example of one that is around 1 wave from flat when it was  stated to be 1/8 wave. 

 

                  - Dave 

 

Dave -what is the visual effect of replacing a stock secondary mirror (ie one that comes with an orion or meade 8" -16" dob lets say) with something of high quality - ie maybe something of a 1/20 or 1/30 wave pv ?   I don't know much about this - but I've read on a few threads that this can make a major improvement on a scope - I'm wondering how so ? what would I see differently ?
 



#8 Frenchy

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Posted 14 June 2021 - 12:29 PM

 That’ll probably help a little. 

I got a meade 4504 not too long ago from a hoarders house down the street. After cleaning, the secondary was pretty close, if not slightly worse off than yours. The moon still looked good through it, but definitely sucked on just about any other observable target. M13 looked better through 10x50 bino's lol. Replacing/recoating mirrors that far gone are always worth it waytogo.gif



#9 Mirzam

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Posted 14 June 2021 - 12:30 PM

I’m not Dave, but the typical effect of a non flat secondary is astigmatism.  If you don’t detect any astigmatism at the eyepiece, chances are the secondary is ok even if it is not great.  Absence of astigmatism and good “snap to focus” tells you that the optics are working pretty well.  A mushy focus that you constantly have to chase suggests poor primary mirror quality. This assumes good collimation and thermal equilibrium.  When building a scope I pay a little more for a known good secondary of 1/10th wave flatness or better (Antares).  
 

JimC


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#10 peleuba

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Posted 14 June 2021 - 12:52 PM

 There a couple of methods to test the flatness of optical surface. What I do and most do is use a known reference flat and place it in contact with the surface and shine  monochrome light on them. You'll see interference fringes and the straightness of fringes is a direct indication of the flatness. If a flat is claimed to be 1/10 wave then the fringes should be very straight when tested against a master surface that has the same flatness. 

   Here is picture of 4" minor axis flat that I tested. It show the fringes are very curved. The second picture is after I refigured it and now the fringes are straight indicating the surface of the diagonal matches that of the master surface placed on top of it. 

 

 

Dave,  I have had the parts for a Newton Interferometer for awhile, your post has encouraged me to finish the project.

 

Is the flat under test (not the master flat) coated or uncoated?  I never get the extreme contrast of fringes that your photos illustrate.  Most of the flats I have tested are coated.



#11 firemachine69

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Posted 14 June 2021 - 12:53 PM

Non chlorinated brake cleaner or methyl hydrate. Let soak.

#12 Mike Lockwood

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Posted 14 June 2021 - 01:08 PM

You get the best contrast when the reflectivities of the test part and reference are close.  Using a good monochromatic source makes a big difference.  You will note that David is using a good source.

 

I use uncoated references to test uncoated flats.  I have a ~50% reflectivity reference to use to test coated mirrors.  Both produce superb contrast fringes.  I use a vertical test setup with folding mirror so that I view through the bottom of the reference flat, and the mirror under test sits on top.  This means the entire mirror that is being tested is being supported by the reference, and you can see the entire interferogram at the same time.

 

Also, for excellent accuracy one needs to view the fringes from a significant distance away, and ideally nearly straight through the flats, i.e. normal to the surfaces.

 

See some of my test images here:  http://www.loptics.c...llipticals.html


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#13 DAVIDG

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Posted 14 June 2021 - 01:34 PM

Dave,  I have had the parts for a Newton Interferometer for awhile, your post has encouraged me to finish the project.

 

Is the flat under test (not the master flat) coated or uncoated?  I never get the extreme contrast of fringes that your photos illustrate.  Most of the flats I have tested are coated.

 The first picture I posted of  the small but bad diagonal  which is coated is being tested with a Master with a semi reflective coating. The large diagonal is uncoated since I need to refigure it and it was tested with uncoated Master. As stated by Mile Lockwood you get the best contrast when the two surfaces have close to the semi reflectivity and using a good monochrome source.  When it comes to a monochrome source you don't need to filter the light source. You can use a source like a gas discharge bulb as in a Neon bulb which contains a number of discreet emission lines and then just look through a filter to isolate one. So a simple  orange or yellow  filter will work with a Neon source.  CFL bulbs will also work. 

   You can see fringes with a uncoated Master on a coated flat be they are faint. 

 

                  - Dave 


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#14 Pinbout

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Posted 14 June 2021 - 06:14 PM

https://youtu.be/BG5tzwAxroc

 

how I test flats 


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#15 SteveG

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Posted 14 June 2021 - 07:37 PM

Dave -what is the visual effect of replacing a stock secondary mirror (ie one that comes with an orion or meade 8" -16" dob lets say) with something of high quality - ie maybe something of a 1/20 or 1/30 wave pv ?   I don't know much about this - but I've read on a few threads that this can make a major improvement on a scope - I'm wondering how so ? what would I see differently ?
 

A lot of people read these forums and come away decided to replace their secondary as an "upgrade", when in fact there was nothing wrong with their original secondary. GSO and Synta are capable of producing 1/10 wave secondaries all day long. Yes, they throw out some stinkers, but more often than not the problem is in how it's mounted to the stalk (double-stick tape).

 

The best way for the average Joe to test a secondary is to join a local astronomy club, and have the ATM guys test it for you, IMO.


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#16 Tonimus

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Posted 14 June 2021 - 08:09 PM

I did keep my original mirror so I can send it off with the primary if/when it needs to be recoated. Once seeing out here isn’t so bad, I’ll drag it out in the yard and see what happens. 



#17 LU1AR

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Posted 17 June 2021 - 10:48 AM

Does anyone know if the surface errors are more noticeable in a mirror located in front of the objective (As in a binocular, that is pointed down through a mirror); or near the eyepiece (As in the 90º diagonal)?.
Thanks in advance.
Edgardo
 



#18 DAVIDG

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Posted 17 June 2021 - 03:43 PM

Does anyone know if the surface errors are more noticeable in a mirror located in front of the objective (As in a binocular, that is pointed down through a mirror); or near the eyepiece (As in the 90º diagonal)?.
Thanks in advance.
Edgardo
 

 The steeper the angle that the light is reflected off the surface of the flat, the flatter  the surface needs to be. Think of hole in the surface of the flat. When the surface of the flat is normal to incoming light, the goes straight into the hole and then straight back out. The error is 2x the depth of the hole. Now if you tilt the flat, the light goes into the hole on an angle and distance is now greater  so the error is now greater than 2x the depth of the hole. 

   

  So one way to test an optical flat is place it in front a refractor and  tilt at steep angle  to reflect light into the  objective of the telescope, then  examine a star at high magnification.  If  star is round the flat is good if not you will  see astigmatism. 

   Front surface mirrors used with binoculars are not very flat but why  they can work is that the binoculars have a low fixed magnification so the error is the mirror are not seen unless the mirror has large errors.

 

                - Dave 




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