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what effect did changing your secondary have

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#1 Itz marcus

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Posted 13 September 2019 - 03:10 PM

Hi,

Just ordered a 1/20 Antares secondary.. My rational behind it was that since my secondary tested pretty horribly I would replace that weak link and see what effect it had on my views. The primary was tested by the same company (Nova) and was estimated at about 1/6 wavefront so why refigure the primary when the secondary was all that may be needed to be replaced and the secondary for sure needed to be replaced.

 

How many have replaced a secondary with a premium and have seen much improvement on a mass produced produced mirror. I am fully aware that it will not make the primary mirror better. It is what it is but it can make the system better.

 

 

Clear Skies

Itz



#2 wrvond

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Posted 13 September 2019 - 04:05 PM

When we spend money on stuff it's natural that we want to see an improvement. It's hard to separate subjective from objective observations.

Nonetheless, I feel that my XT10g view was cleaner, sharper and brighter after I replaced the secondary with an Antares mirror.



#3 TOMDEY

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Posted 13 September 2019 - 04:41 PM

We tested a lot of commercial 90-deg folding flats at work in the as-used orientation, double-pass. So-configured, it is allowable to analytically remove both power and coma, because focusing and aligning/collimating on a star drives those out anyway. The more costly ones were generally better, but still a lot of variations, and an occasional clinker. I recall that the Edmund Optics ones were surprisingly good, for the bargain price. We found about 80% were fine for use. I don't know it they farm them out or build in-house... but it was clear that they must do conscientious pre-ship interferometry before releasing them to customers.

 

The sizes we had in the bins ranged from piles of dinky ones (2mm-12mm), lots of mid-sized ones (to 5in), handful of biggish ones (to 12in) and a very few very big ones (to ~2ft wide by 3ft long!) The huge ones we made in-house... custom for our own use or contracted customers.

 

I also tested a bunch of commercial Star Diagonals, with widely varying results. Again, the premium brands (e.g. Astrophysics, Televue, Denkmeier, Lumicon) were wonderful.

 

The weak link is often not the glass itself, but how it is mounted in the cell/spider. Sometimes just re-mounting with less stress on it can improve it a lot! Also, where they are in the imaging-chain. Close to focus (Star Diag) is more forgiving, because each field point sees only a small footprint on the fold. Farther (Newt Fold) is more demanding, because each field point sees most of the fold.  Tom

 

Example test results on one. [I designed and built the test sets myself.]  >>>  Tom

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#4 Starman1

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Posted 13 September 2019 - 05:07 PM

Hi,

Just ordered a 1/20 Antares secondary.. My rational behind it was that since my secondary tested pretty horribly I would replace that weak link and see what effect it had on my views. The primary was tested by the same company (Nova) and was estimated at about 1/6 wavefront so why refigure the primary when the secondary was all that may be needed to be replaced and the secondary for sure needed to be replaced.

 

How many have replaced a secondary with a premium and have seen much improvement on a mass produced produced mirror. I am fully aware that it will not make the primary mirror better. It is what it is but it can make the system better.

 

 

Clear Skies

Itz

I did.

I had a 6" f/5 with standard Celestron optics in the early '80s.

I completely rebuilt the scope from the ground up and changed the 2" stock secondary to a 1.83" because I had gone from a 4" tall focuser to one 1.5" tall.

It was a profound difference.  Star images became textbook after that.  And a Ronchi eyepiece showed textbook-straight lines.  Prior to that, they were somewhat, uh, wavy.

Suddenly, a 3.8mm eyepiece not only became usable, but even the 3.8mm with a 2X Barlow.


Edited by Starman1, 13 September 2019 - 05:10 PM.

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#5 25585

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Posted 14 September 2019 - 02:03 AM

I did.

I had a 6" f/5 with standard Celestron optics in the early '80s.

I completely rebuilt the scope from the ground up and changed the 2" stock secondary to a 1.83" because I had gone from a 4" tall focuser to one 1.5" tall.

It was a profound difference.  Star images became textbook after that.  And a Ronchi eyepiece showed textbook-straight lines.  Prior to that, they were somewhat, uh, wavy.

Suddenly, a 3.8mm eyepiece not only became usable, but even the 3.8mm with a 2X Barlow.

Don, how does focuser height affect mirror size?



#6 howardcano

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Posted 14 September 2019 - 06:21 AM

A lower-profile focuser permits moving the focal plane closer to the secondary mirror (by moving the primary mirror back in the tube).  Since the light cone expands away from the focal plane, moving the focal plane closer to the secondary mirror gives a larger fully-illuminated field with the same secondary mirror, or the same fully-illuminated field size with a smaller secondary mirror.

 

I replaced the stock secondary mirror on the advice of the gentleman who refigured my primary mirror (after he tested both).  The stock mirror was 2" minor axis, the new mirror was an Antares 1/30 wave with 1.52" minor axis.  The views are vastly improved, though I haven't done an A-B comparison of just the secondary mirrors to determine how much was due to either the reduced size or improved figure of the secondary mirror.


Edited by howardcano, 14 September 2019 - 06:37 AM.

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#7 25585

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Posted 14 September 2019 - 07:18 AM

If I ever ordered a personalised Dob, I will get such specifications from the start.



#8 havasman

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Posted 14 September 2019 - 11:45 AM

How many have replaced a secondary with a premium and have seen much improvement on a mass produced produced mirror. I am fully aware that it will not make the primary mirror better. It is what it is but it can make the system better.

I replaced the secondary on my XT10i after the primary was found good by testing. The 1/16th wave secondary from Astrosystems improved the system performance more than any other single modification I have made to that scope.



#9 Itz marcus

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Posted 14 September 2019 - 07:51 PM

I replaced the secondary on my XT10i after the primary was found good by testing. The 1/16th wave secondary from Astrosystems improved the system performance more than any other single modification I have made to that scope.

Hi,

What did your mirror test at?

Clear skies

Itz



#10 havasman

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Posted 15 September 2019 - 03:21 AM

Hi,

What did your mirror test at?

Clear skies

Itz

I really don't remember. It wasn't anything gaudy or memorable. But I remember the conversation about it with John Hall. He said he could probably refigure it and make it produce better test numbers but he doubted it would work any better. He recommended I put it back in the scope and go use it. I thought that excellent advice. 



#11 Itz marcus

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Posted 15 September 2019 - 06:12 AM

Hi,

Thanx that was what I was told re: my mirror. There are things that make it not perfect and that it can be made perfect but that I would probably not see it at the ep.

Clear Skies

Itz




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