Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Repair a cracked base glass

  • Please log in to reply
20 replies to this topic

#1 mandala8

mandala8

    Lift Off

  • -----
  • topic starter
  • Posts: 20
  • Joined: 27 Jun 2020

Posted 02 July 2020 - 05:57 AM

Hey, I just bought my first telescope few days ago. When I am trying to collimate the telescope I discover that the base glass have some crack frown.gif Unfortunately it's difficulty to claim warranty for this case for some reason with that shop for glass/mirror part, I did check the primary & secondary mirror before buying but never realized about the base. I wonder if there's a safe glue that I can use to prevent it from further cracking ? The glue chemical must not harm the primary mirror.

 

Please see the 2 pics below

 

https://imgur.com/OHYoJO2

https://imgur.com/LBuXbar

 

 

Thanks


Edited by mandala8, 02 July 2020 - 06:25 AM.


#2 davidc135

davidc135

    Apollo

  • -----
  • Posts: 1,121
  • Joined: 28 May 2014
  • Loc: Wales, UK

Posted 02 July 2020 - 07:29 AM

That is the primary mirror itself that is cracked, surely? If ever there was a reason to return a purchase you have it. Demand a replacement or your money back.

 

David


  • gregj888 likes this

#3 TOMDEY

TOMDEY

    Fly Me to the Moon

  • *****
  • Posts: 7,057
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 02 July 2020 - 07:41 AM

A few days should be early enough to get it replaced, at no cost to you... for ~Hidden Damage~.

 

"Hidden Damage. Damage to an asset that is not immediately apparent. For example, a roof with hidden damage may appear unharmed until it leaks during a rain storm. Insurance policies provide coverage for both hidden and obvious damage."

 

A crack in a telescope mirror is similar to one in a windshield, in the sense that it will most likely propagate with time and thermal cycling... until it falls apart. That crack has also, most likely, compromised the wavefront of the mirror. Chips are more benign, because they have already separated and stopped further incursion.    Tom

Attached Thumbnails

  • 79 crack in glass.jpg


#4 mandala8

mandala8

    Lift Off

  • -----
  • topic starter
  • Posts: 20
  • Joined: 27 Jun 2020

Posted 02 July 2020 - 07:51 AM

Hey, although it's attached to the primary mirror but it's not the primary mirror, it's just a base for the primary mirror to be placed on. The shop did mention that primary mirror / secondary mirror there won't be warrantee before I bought and that's why they asked me to check properly. Unfortunately this is bought in Indonesia, the rules here are not as easy for consumer compared to USA.



#5 davidc135

davidc135

    Apollo

  • -----
  • Posts: 1,121
  • Joined: 28 May 2014
  • Loc: Wales, UK

Posted 02 July 2020 - 08:03 AM

The three triangles in the mirror cell provide what looks like a 9 point support for the mirror.  David



#6 TOMDEY

TOMDEY

    Fly Me to the Moon

  • *****
  • Posts: 7,057
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 02 July 2020 - 08:11 AM

It's the primary mirror, really! Those guys are either entirely inexperienced or... Hmmm... Over here, we call that (colloquial) "giving someone the run-around" aka obfuscation.

 

run·a·round
/ˈrənəˌround/
noun: run-around
difficult or awkward treatment, especially in which someone is evasive or avoids a question.

 

ob·fus·cate
/ˈäbfəˌskāt/
verb
render obscure, unclear, or unintelligible, bewilder (someone).    Tom



#7 mandala8

mandala8

    Lift Off

  • -----
  • topic starter
  • Posts: 20
  • Joined: 27 Jun 2020

Posted 02 July 2020 - 08:21 AM

Unfortunately I have to take the bite, there's no function issue so far, I am going to use epoxy adhesive to cover that from further cracking, so far it seems only the outer side that crack.



#8 mandala8

mandala8

    Lift Off

  • -----
  • topic starter
  • Posts: 20
  • Joined: 27 Jun 2020

Posted 02 July 2020 - 08:56 AM

But if using the epoxy adhesive the best method to repair ? I meant any other better adhesive ?



#9 siriusandthepup

siriusandthepup

    Apollo

  • *****
  • Posts: 1,217
  • Joined: 14 Feb 2006
  • Loc: Central Texas, USA

Posted 02 July 2020 - 09:18 AM

 

But if using the epoxy adhesive the best method to repair ? I meant any other better adhesive ?

Unfortunately that's a no and a no. Glue will not help you.

 

It's an unfortunate issue to deal with. To prevent the break from propagating about the only option you have is to store the telescope in a climate controlled environment. Storing in a location that has a wide temperature variation on a daily cycle is a risk for crack propagation (ask me how I know - lol).

 

Use the scope and enjoy it as long as you can. Keep your eyes peeled for a replacement primary mirror. Yours is a common size and the mirrors come up for sale fairly often.

 

good luck!



#10 ed_turco

ed_turco

    Mercury-Atlas

  • *****
  • Posts: 2,628
  • Joined: 29 Aug 2009
  • Loc: Lincoln, RI

Posted 02 July 2020 - 09:46 AM

I've gone through a lot of glass in my time and I offer a contrary opinion.  First, that crack won't be likely to affect imagery, nor will it be likely to propagate.  There is a mighty big difference between the mirror "crack" and the broken windshield

 

I find it a little strange that the telescope wasn't tried out before the OP experienced all this angst.

 

.


Edited by ed_turco, 03 July 2020 - 09:30 AM.

  • David Castillo and Pinbout like this

#11 Lola Bruce

Lola Bruce

    Apollo

  • *****
  • Posts: 1,326
  • Joined: 22 Sep 2014
  • Loc: Southern California

Posted 02 July 2020 - 11:37 AM

Windshield repair glue may stop the crack. The glue works into the crack by capillary action but may compromise the optical quality by flowing onto the mirror surface.

 

Bruce



#12 mandala8

mandala8

    Lift Off

  • -----
  • topic starter
  • Posts: 20
  • Joined: 27 Jun 2020

Posted 02 July 2020 - 11:42 AM

I've gone through a lot of glass in my time and I offer a contrary opinion.  first, that crack won't be likely to affect imagery, nor will it be likely to propagate.  There is a mighty big difference between the mirror "crack" and the broken windshield

 

I find it a little strange that the telescope wasn't tried out before the OP experienced all this angst.

 

.

Thanks, this gives me a little console, I hope for the best in this, will see how it goes but also prepare mentally for the worst by replacing the primary mirror if it affect imagery 

 

I did do a lot of checking, there was no opportunity to use this when buying, it was in the shopping mall and bright daylight. I didn't realized that I have to look at the bottom part of the tube. 

 

 

Windshield repair glue may stop the crack. The glue works into the crack by capillary action but may compromise the optical quality by flowing onto the mirror surface.

 

Bruce

Thank you, I will try not to touch it then. 



#13 a__l

a__l

    Vanguard

  • -----
  • Posts: 2,021
  • Joined: 24 Nov 2007

Posted 02 July 2020 - 02:36 PM

I've gone through a lot of glass in my time and I offer a contrary opinion.  first, that crack won't be likely to affect imagery, nor will it be likely to propagate.  There is a mighty big difference between the mirror "crack" and the broken windshield

 

I find it a little strange that the telescope wasn't tried out before the OP experienced all this angst.

 

.

I’m wondering if you installed these mirrors on a stand with an interferometer, in order to find out really, does it influence or not? For example, astigmatism?

What is the huge difference between these glasses? Car windows are different. Hardened crumble from the blow. Triplet is no crumble. OP has Chinese glass. Which is not known. It can be ordinary window glass.

 

 

Windshield repair glue may stop the crack. The glue works into the crack by capillary action but may compromise the optical quality by flowing onto the mirror surface.

 

Bruce

 

This will not help. It is necessary to drill the tops of the formed cracks. Then glue it. But this may not help. In my case, it didn’t help.

 

Chips are more benign, because they have already separated and stopped further incursion.    

I wonder what prohibits such a chip (photo)?

Attached Thumbnails

  • crack_.jpg


#14 TOMDEY

TOMDEY

    Fly Me to the Moon

  • *****
  • Posts: 7,057
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 02 July 2020 - 03:24 PM

I’m wondering if you installed these mirrors on a stand with an interferometer, in order to find out really, does it influence or not? For example, astigmatism?

What is the huge difference between these glasses? Car windows are different. Hardened crumble from the blow. Triplet is no crumble. OP has Chinese glass. Which is not known. It can be ordinary window glass.

 

This will not help. It is necessary to drill the tops of the formed cracks. Then glue it. But this may not help. In my case, it didn’t help.

 

I wonder what prohibits such a chip (photo)?

It's the instability of the condition that is of concern.

Buying a new telescope would be cheaper than trying to repair this one.

At work, we routinely drilled stop-holes to defuse crack propagation... but that's mirrors that were worth the time, effort, and unavoidable cost.

So, your cheapest remedy is to replace yours.

The only incidental is who pays for it... the supplier, the dealer... or you!

You indicate that the other two are intransigent... that you have no recourse.

 

Solution: Buy another one, and get on with enjoying the hobby!    Tom

 

in·tran·si·gent
/inˈtransəjənt,inˈtranzəjənt/
adjective
unwilling or refusing to change one's views or to agree about something



#15 a__l

a__l

    Vanguard

  • -----
  • Posts: 2,021
  • Joined: 24 Nov 2007

Posted 02 July 2020 - 11:59 PM

Yes, I replaced the glass of my car with a new one.
There are no chips on my optics. I carefully treat it and do my best to prevent them from appearing.
For example, I do not glue secondary, I use felt under the primary, do not allow glass to metal contact etc.


Edited by a__l, 03 July 2020 - 12:12 AM.


#16 mandala8

mandala8

    Lift Off

  • -----
  • topic starter
  • Posts: 20
  • Joined: 27 Jun 2020

Posted 03 July 2020 - 11:38 PM

Unfortunately that's a no and a no. Glue will not help you.

 

It's an unfortunate issue to deal with. To prevent the break from propagating about the only option you have is to store the telescope in a climate controlled environment. Storing in a location that has a wide temperature variation on a daily cycle is a risk for crack propagation (ask me how I know - lol).

 

Use the scope and enjoy it as long as you can. Keep your eyes peeled for a replacement primary mirror. Yours is a common size and the mirrors come up for sale fairly often.

 

good luck!

Hi, can I know why it's a no no ? It does not help or it will harm it even more ? If it will harm even more, how does it harm ?

 

I was planning to use epoxy resin adhesive that is specifically for glass. The crack is just on the surface so far.

 

It's a Skywatcher 12 inch dobsonian. 

 

Planet views are still really sharp and clear. So there's no reason for a new telescope for now. I am still figuring out if there's any astigmatism, waiting for some collimator tool to come. 



#17 gregj888

gregj888

    Vanguard

  • -----
  • Posts: 2,476
  • Joined: 26 Mar 2006
  • Loc: Oregon

Posted 04 July 2020 - 12:45 AM

The surface of the mirror is very critical.  The crack can relieve or create stresses causing the surface shape to change enough to hurt the image.  Epoxy will have a different "coefficient of thermal expansion" so as the temperature changes it will create more stress, making things worse.

 

If you consider the mirror already ruined, there's no harm in trying it as it is or using some epoxy.  I would try the adhesive used to repair windshields if it were me, assuming you can get it as it will get into the crack and may stop it from spreading. 

 

If you find the image is really bad, mask the area over the of the crack.  Not ideal, but might get you started.


  • Oregon-raybender likes this

#18 mandala8

mandala8

    Lift Off

  • -----
  • topic starter
  • Posts: 20
  • Joined: 27 Jun 2020

Posted 04 July 2020 - 01:47 AM

The surface of the mirror is very critical.  The crack can relieve or create stresses causing the surface shape to change enough to hurt the image.  Epoxy will have a different "coefficient of thermal expansion" so as the temperature changes it will create more stress, making things worse.

 

If you consider the mirror already ruined, there's no harm in trying it as it is or using some epoxy.  I would try the adhesive used to repair windshields if it were me, assuming you can get it as it will get into the crack and may stop it from spreading. 

 

If you find the image is really bad, mask the area over the of the crack.  Not ideal, but might get you started.

Thank you, now I understand, that make sense, I won't touch it then. Unless if it grows further, I have put a mark on both ends of the line to track it, so far no changes.

 

Can I know if the lens has astigmatism so it will affect all kinds of views ? That will include the planetary & moon ? What are the symptoms to look for if the performance of the optic is affected ?



#19 davidc135

davidc135

    Apollo

  • -----
  • Posts: 1,121
  • Joined: 28 May 2014
  • Loc: Wales, UK

Posted 04 July 2020 - 02:50 AM

If you view a star at high power in good 'seeing' and rack the eyepiece from inside focus through focus to outside focus astigmatism will show as an elongation or ellipse whose long axis is mutually perpendicular inside and outside of focus. At best focus a cross replaces the Airy disc. Slight astigmatism shows as a thickening of the first diffraction disc at the points of a cross and would do little harm. If there is a problem I think you would have noticed it in use but check out a star. Even if the atmospheric 'seeing' or steadiness is only so so astigmatism should still be evident.

 

Would it not be worthwhile getting in touch with the retailer and ask him to do the right thing and replace the faulty scope? You wouldn't lose anything. It's an odd thing that it's the customer's responsibility to ensure that a new item is in good order and not the seller's. Especially if the customer is new to astronomy.

 

It's a shame that the law is so weak it allows customers to be cheated in this way.

 

But hopefully there are no ill effects and the crack remains stable in the future.

 

David


Edited by davidc135, 04 July 2020 - 02:53 AM.

  • mandala8 likes this

#20 mandala8

mandala8

    Lift Off

  • -----
  • topic starter
  • Posts: 20
  • Joined: 27 Jun 2020

Posted 04 July 2020 - 05:03 AM

If you view a star at high power in good 'seeing' and rack the eyepiece from inside focus through focus to outside focus astigmatism will show as an elongation or ellipse whose long axis is mutually perpendicular inside and outside of focus. At best focus a cross replaces the Airy disc. Slight astigmatism shows as a thickening of the first diffraction disc at the points of a cross and would do little harm. If there is a problem I think you would have noticed it in use but check out a star. Even if the atmospheric 'seeing' or steadiness is only so so astigmatism should still be evident.

 

Would it not be worthwhile getting in touch with the retailer and ask him to do the right thing and replace the faulty scope? You wouldn't lose anything. It's an odd thing that it's the customer's responsibility to ensure that a new item is in good order and not the seller's. Especially if the customer is new to astronomy.

 

It's a shame that the law is so weak it allows customers to be cheated in this way.

 

But hopefully there are no ill effects and the crack remains stable in the future.

 

David

Hey, thanks, I notice astigmatism, BUT i believe it is my eyes, i don't even realized that i had and what astigmatism is until this event. When I in focus a star it appear elongated or straight like | then when i out focus it turn 90 degrees _ , when i turn my head 180 degrees though that line follow my turning direction. I have been using right eye most of the time, i was very surprised that my right eye show a straight line star and then when i change to left eye with the same focus, the star appear much less straight and much more round. This is all using 25 mm for my F5 telescope.

 

When i change it to 10mm, the stars become much more round and a much milder astigmatism for both eyes. I am waiting for 5mm lens to come, will try this out on this. 

 

 

"At best focus a cross replaces the Airy disc" , I have not tried to observed more carefully on this, tonight if the weather permit, I am going to look for this. 

 

But I am so new in this, this is my first time looking at stars via telescope, so I don't know what exactly the perfect view of a stars look like in telescope, there are some illustration in the websites, but illustration are not realistic, I been looking at some youtube video.

 

If my primary mirror has astigmatism, should it affect the view on planet too? Because the 3 planets Jupiter, Saturn, Mars looks very clear to me. At 150x magnification Jupiter I could see the belt and great red spot and Mars i could see the white polar ice.

 

Thank you, you have renewed my motivation that I shouldn't let them go that easily, I have just wrote to them again to help on this matter and still try to maintain good relation with them, there's  consumer protection law here too, I prefer not to flex that muscle. They will get back to me on Monday again. 


Edited by mandala8, 04 July 2020 - 05:09 AM.

  • davidc135 likes this

#21 davidc135

davidc135

    Apollo

  • -----
  • Posts: 1,121
  • Joined: 28 May 2014
  • Loc: Wales, UK

Posted 04 July 2020 - 01:42 PM

The retailer could reasonably claim from the manufacturer if he received it in that condition. Good luck.  David


Edited by davidc135, 04 July 2020 - 01:46 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics