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Crack in Secondary Mirror!

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#1 sdeming

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Posted 25 January 2021 - 07:50 AM

Hello,

 

Yesterday I installed the Kendrick's Dew Heater to my secondary mirror. I caught myself grabbing the edge of the mirror with my index finger as I squeezed the dew heater to the back of the mirror to set the bond. I quickly stopped. When I was looking through the empty focuser tube, I saw where my finger was on the mirror, as there was a pretty significant finger print. I gently wiped it and noticed what looks like a bubble or small chip. At first I thought perhaps some of the glue got on the mirror, so I took a cotton wipe and put a little distilled water on it and gave it a very gentle wipe. This crack, chip or whatever it is remained. It's very, very small on the lower left edge of the mirror as I'm looking at it through the focuser. I'm now wondering - A. Will this effect the image? B. Will the crack continue to grow? And finally C. Can I get a replacement mirror for my telescope?

 

I've been putting off installing the dew heater for weeks. I am now regretting putting it on!

 

Scott


Edited by sdeming, 25 January 2021 - 07:51 AM.


#2 wrvond

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Posted 25 January 2021 - 08:03 AM

The answer for 'C.'

 

https://www.antareso...m/SEMirrors.php



#3 sdeming

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Posted 25 January 2021 - 08:05 AM

Thank you! How do I figure out my minor mirror axis/size?


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#4 imtl

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Posted 25 January 2021 - 08:13 AM

Moved from BDI. Good luck!


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#5 wrvond

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Posted 25 January 2021 - 08:21 AM

IMG_4210.JPG

 

Measure from one side to the other then look at the sizes offered by the vender/manufacturer (in this case Antares Optics) and pick the same size (or one up) from what you measure. In this example, I would choose a 2.14 inch 1/30th wave mirror because I simply must have the best! ;)

 

I'll tell you this, I once accidentally ordered a 1/14th wave from Antares and didn't even know it until I happened to look at the test sheet included with the mirror! 


Edited by wrvond, 25 January 2021 - 08:32 AM.

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#6 scotsman328i

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Posted 25 January 2021 - 08:22 AM

Thank you! How do I figure out my minor mirror axis/size?

Yes, I’d like to know that also.



#7 scotsman328i

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Posted 25 January 2021 - 08:26 AM

Ahh, I see. Thanks wrvond. So since your elliptical is 2.125, you would purchase the 2.6 size. I’m sure that 1/30 wave elliptical has made a big difference in sharpness and resolution also. Might have to look into one of those. waytogo.gif


Edited by scotsman328i, 25 January 2021 - 08:26 AM.


#8 happylimpet

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Posted 25 January 2021 - 09:05 AM

Hello,

 

Yesterday I installed the Kendrick's Dew Heater to my secondary mirror. I caught myself grabbing the edge of the mirror with my index finger as I squeezed the dew heater to the back of the mirror to set the bond. I quickly stopped. When I was looking through the empty focuser tube, I saw where my finger was on the mirror, as there was a pretty significant finger print. I gently wiped it and noticed what looks like a bubble or small chip. At first I thought perhaps some of the glue got on the mirror, so I took a cotton wipe and put a little distilled water on it and gave it a very gentle wipe. This crack, chip or whatever it is remained. It's very, very small on the lower left edge of the mirror as I'm looking at it through the focuser. I'm now wondering - A. Will this effect the image? B. Will the crack continue to grow? And finally C. Can I get a replacement mirror for my telescope?

 

I've been putting off installing the dew heater for weeks. I am now regretting putting it on!

 

Scott

Assuming this is restricted to being near the edge I cant imagine it'll make any difference to your image. What WILL make a difference to your image is having one of the superb kendrick dew heaters on it, and not getting dew!

 

Be great if we could see it, but I strongly suspect this isnt a big deal....how far into the mirror does it extend? The uncertainty over whether its a chip or crack could do with resolving too!


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#9 sdeming

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Posted 25 January 2021 - 09:12 AM

Assuming this is restricted to being near the edge I cant imagine it'll make any difference to your image. What WILL make a difference to your image is having one of the superb kendrick dew heaters on it, and not getting dew!

 

Be great if we could see it, but I strongly suspect this isnt a big deal....how far into the mirror does it extend? The uncertainty over whether its a chip or crack could do with resolving too!

I will take a photo of it today and post it. Thank you!



#10 scotsman328i

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Posted 25 January 2021 - 09:19 AM

Just measured my elliptical mirror for my ES Gen II 12” Dob. Got 3.015. So I gather I’d order a 3.1 elliptical from Antares Optics if I so choose to upgrade my current secondary?


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#11 scotsman328i

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Posted 25 January 2021 - 09:22 AM

Those things are expensive, but I’m sure we’ll worth it. If I get a 1/20 wave, it’s $259 and the 1/30 wave is $299. 



#12 wrvond

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Posted 25 January 2021 - 09:22 AM

Just measured my elliptical mirror for my ES Gen II 12” Dob. Got 3.015. So I gather I’d order a 3.1 elliptical from Antares Optics if I so choose to upgrade my current secondary?

That will work.

Just to be sure the OEM secondary is providing full field illumination, I'd run the formula to determine the required secondary size and compare that, as well.


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#13 wrvond

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Posted 25 January 2021 - 09:23 AM

Those things are expensive, but I’m sure we’ll worth it. If I get a 1/20 wave, it’s $259 and the 1/30 wave is $299. 

Compared to the price of a quality primary mirror, that's a steal! :D


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#14 scotsman328i

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Posted 25 January 2021 - 09:26 AM

One last question. My stock elliptical does not have a casing around it. It seems to be adhered onto the angled secondary holder. How to remove? Is it held on by a strong 3M type of adhesive or is it cemented on? Just trying to figure out how to remove it AND how to attach a new elliptical if and when I was to upgrade my current stock secondary. Where would I purchase the adhesive/cement to attach a new elliptical? Thanks for any advice! 



#15 happylimpet

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Posted 25 January 2021 - 09:28 AM

I will take a photo of it today and post it. Thank you!

Cool. I know everyone's really excited to get you to buy a new secondary but you describe it as very, very small and i suspect it wont make any difference in practise.



#16 scotsman328i

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Posted 25 January 2021 - 09:28 AM

That will work.

Just to be sure the OEM secondary is providing full field illumination, I'd run the formula to determine the required secondary size and compare that, as well.

Thanks! I’ll probably call ES tech support and ask them about the equation for the full field illumination when everything settles down in the summer. Nothing that needs to be done right now. 



#17 scotsman328i

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Posted 25 January 2021 - 09:31 AM

Compared to the price of a quality primary mirror, that's a steal! laugh.gif

No doubt! However I have a stock 12” Explore Scientific 12” primary. It is very good, however not a Pegasus or Zambuto of course, so no matter how good the elliptical is, it’s limited by the performance of the primary also I would think, so...would I truly need a 1/20 or 1/30 wave elliptical? I would think the stock elliptical is more than probably a 1/10 wave. I’m sure going with a 1/14 or 1/15 wave Antares elliptical would make a decent degree of difference.

    I am by no means a purist, I don’t need to have the best, I’d like to simply have better. 


Edited by scotsman328i, 25 January 2021 - 09:37 AM.


#18 wrvond

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Posted 25 January 2021 - 09:39 AM

One last question. My stock elliptical does not have a casing around it. It seems to be adhered onto the angled secondary holder. How to remove? Is it held on by a strong 3M type of adhesive or is it cemented on? Just trying to figure out how to remove it AND how to attach a new elliptical if and when I was to upgrade my current stock secondary. Where would I purchase the adhesive/cement to attach a new elliptical? Thanks for any advice! 

Yes, it's held on to a solid aluminum cylinder (threaded to accept the center screw) with double sided tape. Do not use double sided tape to install a new mirror. I use three dabs of GE silicone:

https://smile.amazon...,aps,183&sr=8-8

 

Truly need? Probably not. I wouldn't go less than 1/18th wave, though. Coming from Antares, even their "economy" mirrors are quite good, IMHO.


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#19 wrvond

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Posted 25 January 2021 - 09:41 AM

Thanks! I’ll probably call ES tech support and ask them about the equation for the full field illumination when everything settles down in the summer. Nothing that needs to be done right now. 

https://garyseronik....uminated field.


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#20 scotsman328i

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Posted 25 January 2021 - 09:56 AM

Thank You for this! waytogo.gif


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#21 scotsman328i

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Posted 25 January 2021 - 09:59 AM

Yes, it's held on to a solid aluminum cylinder (threaded to accept the center screw) with double sided tape. Do not use double sided tape to install a new mirror. I use three dabs of GE silicone:

https://smile.amazon...,aps,183&sr=8-8

 

Truly need? Probably not. I wouldn't go less than 1/18th wave, though. Coming from Antares, even their "economy" mirrors are quite good, IMHO.

Yes, I actually just got off the phone with ES. The tech support told me the stock elliptical is cemented on. Good to know as I will need to remove all the remaining cement off the base of the holder when I remove the stock one. I’ll keep in mind the GE silicone also. Thanks much!


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#22 sdeming

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Posted 25 January 2021 - 04:30 PM

Cool. I know everyone's really excited to get you to buy a new secondary but you describe it as very, very small and i suspect it wont make any difference in practise.

I took several photos with my phone, but the small aberration won't show up in the pictures. I literally cannot tell if it's a bubble, crack, or what it is. It's tiny and on the very edge, so I'm leaving well enough alone. The mirror is clean and the dew heater is installed. I'll see what I get next time out. Thanks again!


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#23 scotsman328i

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Posted 25 January 2021 - 04:35 PM

I took several photos with my phone, but the small aberration won't show up in the pictures. I literally cannot tell if it's a bubble, crack, or what it is. It's tiny and on the very edge, so I'm leaving well enough alone. The mirror is clean and the dew heater is installed. I'll see what I get next time out. Thanks again!

Definitely! Try it out first and give us a report. See if there is no difference in viewing. If not, hey...’don’t go fixing something that ain’t broke’. waytogo.gif


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