Logieberra
31-05-2015, 10:33 AM
I would like to provide a few comments in relation to 'star shapes’ produced by these scopes. It is quite possible that some of you will see triangular stars caused by pinched optics in your system. There are plenty of educated guesses out there on what the causes might be, and a range of potential fixes. For completeness, here are some of the posts out there, including my experience.
1. This iceinspace (http://www.iceinspace.com.au/forum/showthread.php?t=132919) member found that the secondary mirror should be mounted with great care. I found this not to be the case on my scope, but still an important contribution to the conversation.
2. This stargazerslounge (http://stargazerslounge.com/topic/187671-gso-12-rc-dissambly/page-6) member went all out and purchased a completely new, custom made cell at great expense – fixing the pinching issues all together. Their findings pointed to pinched optics arising from the mounting of the primary mirror in its cell, which has been my experience. This thread is an excellent resource, but long…
3. Lastly, this NZ customer. I'm pretty sure that he returned the scope for a refund. The pinching in his system was quite extreme, as you can see here (http://www.iceinspace.com.au/forum/showthread.php?t=120674) and here (http://www.iceinspace.com.au/forum/showthread.php?t=120976).
Don't let all this concern you. Your scope might be bang on. Even with the above issues, these scopes still represent wonderful value, and not all of us are after prefect star shapes. But, if you do see triangular stars, and you’re game to take the system apart – you might like to consider the following.
My experience
See the attached Horse Head pic. My star shapes are clearly triangular. I can confirm that my scope was very well collimated using a Takahashi collimating scope. I am using a quality focuser built to exacting standards (Optec TCF-Si) and not the cheapy GSO 4” focuser supplied with these scopes.
I have used the Takahashi collimating scope for over 10 years on a range of scopes, ranging from Mewlons, triplet refractors, doublets etc. I know how to use it.
I proceeded to take the scope apart and inspected the primary mirror housing. I am not an optical technician, but what I did see concerned me. I found a series of mechanical decisions made by GSO – typically involving three pinching points, which might explain the triangular stars. Here goes:
i. Black goop in three places pinching the side of the mirror. I understand that the goop is designed to stop lateral movement, but this is unnecessary. The one-piece primary mirror cell includes a central tube that the 12” mirror simply drops/slides into. The fit on my scope between mirror hole and cell tube is quite snug. The mirror must be positioned correctly before lowering/sliding the mirror into the cell. For this reason, lateral movement of primary in cell is minimal, at least in my setup.
ii. Three cork pads supporting a heavy 12” primary mirror. Newtonian guys out there would cringe at this, with their typical 16-point mirror cells etc.
iii. Three primary mirror clips. Why? It is impossible for the primary mirror in a GSO RCs to fall out (at least in the current truss line). As you’ll see from the pics, the primary mirror is kept in place by a central locking ring. The ring compresses a rubber o-ring with each turn of the locking ring. I no longer use the clips, as they are redundant and can easily cause pinching if set incorrectly. They also obstruct a percentage of light to the primary, be it small.
iv. Wafer thin aluminum mirror cell which is pushed/pulled in three places during collimation. If you’ve ever seen pics of an RCOS or CDK disassembled, you’ll see that mirror cells are chunky by design with zero flex. GSO has simply reused their budget cell design from their Newtonian line, being only a few mm thick. With a primary mirror ‘black gooped’ into the cell, it is possible that the three push/pull collimation points deform the back of the thin cell, which might transfer to the mirror itself when black gooped in place.
v. Central locking ring too tight. CDK 12.5 primary mirrors are kept place by a similar locking ring. Advice from a local CDK user is that the locking ring must not be overtightened. It is this locking ring which keeps the primary in cell. For this reason, it’s impossible for the mirror to simply fall out. Added to that, the locking ring includes two grub screws to lock the ring I place. Trust me, you’re mirror is never going to fall out of its cell!
My fix
· Cut the sides of the primary mirror free from the black goop using a Stanley knife.
· Scrape off the three cork pads from the mirror cell, clean the surface of the cell and mirror back and apply a number of quality Velcro pads in a nice, concentric supporting pattern (I used 12, but defer to the Newtonian guys on what is best). Each pad can hold 0.5kg x 12 = the weight of mirror at approx. 3kg. Further, these pads stop all lateral movement (not that there is any, see point i. above). They also stop the mirror from ever falling out (although unnecessary, see point v. above). Lastly, if the mirror was bending along with the wafer thin cell during push/pull collimation, the Velcro ‘gives’ a bit which would eliminate the transfer of that bending to the mirror, especially now that the mirror is free in cell and no longer ‘black gooped’ in place.
· Throw the three primary mirror clips over the fence into the neighbor’s yard for the dogs to eat.
· Screw the central retaining ring back in place and apply minimal pressure. Test under the stars to get it ‘just right’.
After this mod, I can confirm that it holds collimation perfectly after slewing, on both sides of the meridian and from night to night, and produces round stars.
I hope this information helps someone out there :)
Logan.
1. This iceinspace (http://www.iceinspace.com.au/forum/showthread.php?t=132919) member found that the secondary mirror should be mounted with great care. I found this not to be the case on my scope, but still an important contribution to the conversation.
2. This stargazerslounge (http://stargazerslounge.com/topic/187671-gso-12-rc-dissambly/page-6) member went all out and purchased a completely new, custom made cell at great expense – fixing the pinching issues all together. Their findings pointed to pinched optics arising from the mounting of the primary mirror in its cell, which has been my experience. This thread is an excellent resource, but long…
3. Lastly, this NZ customer. I'm pretty sure that he returned the scope for a refund. The pinching in his system was quite extreme, as you can see here (http://www.iceinspace.com.au/forum/showthread.php?t=120674) and here (http://www.iceinspace.com.au/forum/showthread.php?t=120976).
Don't let all this concern you. Your scope might be bang on. Even with the above issues, these scopes still represent wonderful value, and not all of us are after prefect star shapes. But, if you do see triangular stars, and you’re game to take the system apart – you might like to consider the following.
My experience
See the attached Horse Head pic. My star shapes are clearly triangular. I can confirm that my scope was very well collimated using a Takahashi collimating scope. I am using a quality focuser built to exacting standards (Optec TCF-Si) and not the cheapy GSO 4” focuser supplied with these scopes.
I have used the Takahashi collimating scope for over 10 years on a range of scopes, ranging from Mewlons, triplet refractors, doublets etc. I know how to use it.
I proceeded to take the scope apart and inspected the primary mirror housing. I am not an optical technician, but what I did see concerned me. I found a series of mechanical decisions made by GSO – typically involving three pinching points, which might explain the triangular stars. Here goes:
i. Black goop in three places pinching the side of the mirror. I understand that the goop is designed to stop lateral movement, but this is unnecessary. The one-piece primary mirror cell includes a central tube that the 12” mirror simply drops/slides into. The fit on my scope between mirror hole and cell tube is quite snug. The mirror must be positioned correctly before lowering/sliding the mirror into the cell. For this reason, lateral movement of primary in cell is minimal, at least in my setup.
ii. Three cork pads supporting a heavy 12” primary mirror. Newtonian guys out there would cringe at this, with their typical 16-point mirror cells etc.
iii. Three primary mirror clips. Why? It is impossible for the primary mirror in a GSO RCs to fall out (at least in the current truss line). As you’ll see from the pics, the primary mirror is kept in place by a central locking ring. The ring compresses a rubber o-ring with each turn of the locking ring. I no longer use the clips, as they are redundant and can easily cause pinching if set incorrectly. They also obstruct a percentage of light to the primary, be it small.
iv. Wafer thin aluminum mirror cell which is pushed/pulled in three places during collimation. If you’ve ever seen pics of an RCOS or CDK disassembled, you’ll see that mirror cells are chunky by design with zero flex. GSO has simply reused their budget cell design from their Newtonian line, being only a few mm thick. With a primary mirror ‘black gooped’ into the cell, it is possible that the three push/pull collimation points deform the back of the thin cell, which might transfer to the mirror itself when black gooped in place.
v. Central locking ring too tight. CDK 12.5 primary mirrors are kept place by a similar locking ring. Advice from a local CDK user is that the locking ring must not be overtightened. It is this locking ring which keeps the primary in cell. For this reason, it’s impossible for the mirror to simply fall out. Added to that, the locking ring includes two grub screws to lock the ring I place. Trust me, you’re mirror is never going to fall out of its cell!
My fix
· Cut the sides of the primary mirror free from the black goop using a Stanley knife.
· Scrape off the three cork pads from the mirror cell, clean the surface of the cell and mirror back and apply a number of quality Velcro pads in a nice, concentric supporting pattern (I used 12, but defer to the Newtonian guys on what is best). Each pad can hold 0.5kg x 12 = the weight of mirror at approx. 3kg. Further, these pads stop all lateral movement (not that there is any, see point i. above). They also stop the mirror from ever falling out (although unnecessary, see point v. above). Lastly, if the mirror was bending along with the wafer thin cell during push/pull collimation, the Velcro ‘gives’ a bit which would eliminate the transfer of that bending to the mirror, especially now that the mirror is free in cell and no longer ‘black gooped’ in place.
· Throw the three primary mirror clips over the fence into the neighbor’s yard for the dogs to eat.
· Screw the central retaining ring back in place and apply minimal pressure. Test under the stars to get it ‘just right’.
After this mod, I can confirm that it holds collimation perfectly after slewing, on both sides of the meridian and from night to night, and produces round stars.
I hope this information helps someone out there :)
Logan.