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Upgrading secondary mirror ?

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18 replies to this topic

#1 Romain V

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Posted 16 August 2019 - 12:54 AM

I. I'm new to the forum and have this question.
Is getting a high end secondary mirror a valuable upgrade for a synta or gso dobsonian ?
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#2 Allan Wade

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Posted 16 August 2019 - 01:21 AM

I am a big fan of the Antares premium secondary mirrors. I've said it many times on here, but when I swapped out the original secondary mirror in my Orion XT12g and replaced it with a 1/30 wave Antares secondary, it completely transformed that telescope. It made the single greatest improvement I made to that scope, which also included upgrading later to a Zambuto primary.

 

Make sure to also research how to properly fit the new secondary into your dob. If you get that a bit wrong it can introduce other problems.


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#3 happylimpet

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Posted 16 August 2019 - 04:08 AM

Only if you have reason to suspect the original is inferior. I recently replaced a stock mirror with a GSO (yup!) and it has hugely improved the scope.  I also fitted an antares to a different scope. In both cases I knew the secondaries were low quality, though one was from a manufacturer supposedly offering quality products.



#4 ngc7319_20

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Posted 16 August 2019 - 05:08 AM

If you do a so-called "star test" on the telescope and find astigmatism, you may have an issue with the secondary mirror -- especially if the astigmatism axes are aligned with the axis of the telescope tube.  It can be further confirmed by rotating the primary mirror and seeing whether or not the astigmatism rotates.

 

I have had good luck with both Antares and Protostar secondary mirrors.

http://www.antaresop...m/SEMirrors.php

https://www.fpi-prot...om/products.htm


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#5 havasman

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Posted 16 August 2019 - 06:14 AM

I was trying to cure a problem with dew at the club's dark site when I replaced the entire upper works of my XT10i with Astrosystems spider, secondary holder and secondary so i could add their really good dew heater. Not only did it cure the dew problem but it made the greatest optical improvement of the many mods I've made to that scope. Yes it is very much worthwhile and recommended.

 

I had the primary tested and it is actually fine as-is.

 

And welcome to the forums.


Edited by havasman, 16 August 2019 - 06:15 AM.

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#6 Slartibartfast

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Posted 16 August 2019 - 10:18 AM

Are replacement spider assemblies adjustable enough so that you can replace them on a GSO or Synta Dob without needing to drill new holes?



#7 SteveG

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Posted 16 August 2019 - 04:59 PM

Short answer is no. Replace your secondary only if it needs replacing. There are 4 people here that responded, that all needed a new secondary. My GSO stock secondary was measured at 1/10 wave or better, so I left it in place.



#8 havasman

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Posted 16 August 2019 - 07:57 PM

Are replacement spider assemblies adjustable enough so that you can replace them on a GSO or Synta Dob without needing to drill new holes?

Randy at Astrosystems told me their system would not fit an XT10i but the prints on their website indicated that all I had to do was drill new holes to mount the spider and it would fit fine. There was plenty of room. I measured very carefully and drilled the new holes with a manual hand drill. It was really easy. ( https://www.amazon.c...s/dp/B07MQN46TH )  Adjustability wasn't the issue. The new secondary holder was longer than the stock stalk. But the prints were exactly spot-on. The most rigid configuration was with everything jammed all the way up and tightened down. That's the way it still sits.

 

BTW, there's an old school machinists' trick for keeping shavings from falling away from a hole drilled in thin sheet metal like a Dob tube. Put a dollop of shaving cream on the inside wall and it will catch everything. Works like a champ.

________________________________

EDIT

I can't believe those drills cost > $20. I've had mine @ 40 years and probably paid $3 or 4 for it.  lol.gif


Edited by havasman, 16 August 2019 - 08:00 PM.

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#9 Pinbout

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Posted 16 August 2019 - 08:57 PM

I. I'm new to the forum and have this question.
Is getting a high end secondary mirror a valuable upgrade for a synta or gso dobsonian ?

I’ve seen a lot of bad secondaries, but that doesn’t mean you have one.

 

Sometimes it’s how they’re mounted that’s a problem, even in large custom dobs $$$

 

But for around $100 a lot of people shoot their shot anyway.

 

some do up grade the secondary even after a refiguring of their primary, and have said that a new secondary was the biggest performance upgrade.


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#10 Astro-Master

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Posted 16 August 2019 - 09:51 PM

 Why not get it tested before you spend your money.



#11 Pinbout

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Posted 16 August 2019 - 10:35 PM

 Why not get it tested before you spend your money.

Where, who, how much...easier to just spend $100 for most.

 

https://youtu.be/XuE70iyWJM0 

 

heres a gso that isn’t bad 

 

https://youtu.be/fI3MNnUOWvc

 

how I test a flat

https://youtu.be/BG5tzwAxroc

 


Edited by Pinbout, 16 August 2019 - 11:03 PM.

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#12 Romain V

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Posted 17 August 2019 - 05:09 AM

Thanks for your answers. Overall, it seems to be a nice upgrade.
Idon't have my dob yet, but as I'm enquiring for my purchase, I realised that there was a huge gap on price between mass products like synta/gso and higher end dobs.

So my reflexion was around getting one of those mass product dobs (but haven't find my way through the synta/gso battle yet) and tweak it.
Then I should have this halfway between lower and higher end scopes eventually. That's the Idea.

Thanks again to all.

Edited by Romain V, 17 August 2019 - 05:11 AM.


#13 Pinbout

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Posted 17 August 2019 - 07:05 AM

So you haven’t bought it yet...

 

umh...

 

 

i could hook ya up - pm me if you don’t mind glue stuff together.


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#14 Romain V

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Posted 17 August 2019 - 07:31 AM

Pinbout. I'm probably living in a galaxy far away. Shipping will cost as much as the scope 😄

#15 Asbytec

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Posted 17 August 2019 - 08:22 AM

So my reflexion was around getting one of those mass product dobs (but haven't find my way through the synta/gso battle yet) and tweak it.
Then I should have this halfway between lower and higher end scopes eventually. That's the Idea.

Thanks again to all.

Most commercial Dobs are a work in progress. 

 

I agree with Danny, before you buy a new diagonal flat, try removing and gluing it back to see if any observed astigmatism goes away. It may be good enough, already, but you might not know that until you try it. Or test it...but it's much cheaper and simpler just to reattach it and test it. 

 

My question has always been how does one know we might need a better diagonal flat. Part of the answer is above by observing astigmatism and ruling out (rotating) the primary mirror to see if the astigmatism remains oriented or changes. The other half of the answer is re gluing it properly. If it clears up, you're probably fine. 

 

I sometimes see astigmatism in my scope, both classic and trefoil. But not always. It comes and goes apparently with the seeing. For example, using an artificial star at some distance I see quite a bit of astigmatism frequently and it changes in severity from relatively mild to fairly strong. And it alternates between the classic 90 degree plus sign to more trefoil in appearance. But, just as frequently, I get nice circular and complete diffraction rings, too.

 

In the night sky I see astigmatism much less often than with an artificial star, but I get other pronounced seeing effects making it hard to detect in and just outside focus. I do see occasional (apparently) trefoil in focus at times, but only briefly before it disappears. One might figure optical astigmatism would be more persistent. So, really, I am not sure whether I have astigmatism or not, it may just be the seeing or some structural cooling effect. I am re-gluing the diagonal anyway to see if there are any improvements. Afterward, I might (or may not) order a better diagonal flat and take a chance on getting better performance as others have noted above. That remains to be seen. 



#16 Romain V

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Posted 17 August 2019 - 09:43 AM

I knew it wouldn't be that simple 😄
Maybe I'll see to it after having had experience with the dob.
Then I can try all these tricks. Thanks.

Regarding gso and synta, and considering the numerous post l've read about the matter, orion telescope (synta with thé xtxxi series) seems to have more consistency with quality + better thermal properties, and gso seems to have better mechanical parts (focuser and roller) Can you guys confirm this ? Or is it too irrelevant to make the decision ?
Thank you all once again for the precision of your replies.

#17 ChrisGTS

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Posted 17 August 2019 - 04:38 PM

I replaced the secondary on my XT12g almost as soon as I got the telescope. I got an Antares mirror from Randy at astrosystems, and he actually attached it to my existing secondary holder and sent it back to me already glued on. So, I was able to keep my stock spider and just screw the secondary holder back in. I also took the opportunity to add a Kendrick Dew heater at the same time, since I live in a very humid place. I don't really know how much it improved the views in my telescope, since I did it almost right away, but I can tell you I'm very happy with the way it performs now.


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#18 SteveG

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Posted 18 August 2019 - 03:12 PM

I knew it wouldn't be that simple
Maybe I'll see to it after having had experience with the dob.
Then I can try all these tricks. Thanks.

Regarding gso and synta, and considering the numerous post l've read about the matter, orion telescope (synta with thé xtxxi series) seems to have more consistency with quality + better thermal properties, and gso seems to have better mechanical parts (focuser and roller) Can you guys confirm this ? Or is it too irrelevant to make the decision ?
Thank you all once again for the precision of your replies.

No one can confirm that. It’s simply opinions you’ve read. I’ve owned many reflectors from both GSO and Synta, both can make good and bad mirrors. The best of mine so far has been a 10” f5 GSO.



#19 SteveG

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Posted 18 August 2019 - 03:14 PM

Thanks for your answers. Overall, it seems to be a nice upgrade.
Idon't have my dob yet, but as I'm enquiring for my purchase, I realised that there was a huge gap on price between mass products like synta/gso and higher end dobs.

So my reflexion was around getting one of those mass product dobs (but haven't find my way through the synta/gso battle yet) and tweak it.
Then I should have this halfway between lower and higher end scopes eventually. That's the Idea.

Thanks again to all.

Don’t think of it as an upgrade. It’s a repair. Doing so causes people to simply order the Atares replacements without even confirming their current secondaries are bad. A 1/30 wave secondary won’t fix a 1/2 wave primary.


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