My experience with 'triangular stars' on GSO 12" RC
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 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 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 and here.
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.
Last edited by Logieberra; 31-05-2015 at 09:18 PM.
Thanks for the write up. Im sure it will come in handy for people.
One question though. Is your version the type that has the focuser that screws onto the cell, or the type that has the focuser screwed onto the back plate?
Bert. A mate (Barry) has the version where the focuser screws direct to the primary cell. We see triangular stars.
I have the new and improved version where the focuser screws direct to the solid back plate. Triangular stars. The fault is in GSO's inadequate mirror cell...
My first light, post-Velcro mod, can be found here and attached. Round stars.
Last edited by Logieberra; 02-06-2015 at 01:37 PM.
Also, consider this. Look closely at the cell design in the bigger brother 16" RC (attached). That is a proper 18pt mirror cell.
Looking at my cell, attached, we only get a 3pt cork support system on the the 12".
It seems that GSO has modelled their new new and improved 16" RC support system on the stargazerslounge guy's fix (mentioned above) by providing a robust 18pt mirror cell. Pics of his expensive 12" RC after-market mod can be found here and attached.
The thing I like about my simple mod is that it's 100% reversable. No damage to components, no cutting of metal or drilling. You can go back to cork pads (Bunnings sells them), silicon in cell and mirror clips if needed. The idea of velcro might be new to some, but the big dob guys - experts in primary mirror cell design and support - use it with great effect I'm told.
Last edited by Logieberra; 02-06-2015 at 01:39 PM.
I used to own a GSO 12" newt and I can say the mirror cell had a proper triangular support system (12 point I think it was). Three point mirror supports can work very well but are not usually found in modern production 12" mirror cells. I have a three point silicon PLOP designed mirror support system under my 10" GSO mirror in the imaging newt that I built and it works just fine - nice round stars, so that in itself may not be an issue in an imaging scope. What have other RC owners found re this type of mirror support and has GSO addressed this in a redesign?
Yeah me too Glen. I owned a 12" GSO imaging newt. It had a proper 9pt support. Not sure why they did away with them on this... being a more demanding, longer focal length system (1200mm F/4 v.s. 2438mm F/8).
Last edited by Logieberra; 31-05-2015 at 02:47 PM.
I have found that the tension of the baffle is most likely the cause for pinched optics on the scope. My mirror clips have never touched the mirror at all and silicon while stiff is also an unlikely source for pinching due to its flexible nature. However, if your clips are touching and the tension of the baffle is too tight I am reasonable certain this will cause pinching. In fact I did this test to see if that was the outcome and it was in my case. I like the idea of the velcro though. In any event you got resolution.
I have found that the tension of the baffle is most likely the cause for pinched optics on the scope...
Thanks Paul. Your guidance and support on these RCs has been really helpful. GSO owe you.
I've now eliminated the clips - as they don't serve any purpose. My baffle was very loose for these tests. As you know, the baffle makes no contact and applies no pressure to the mirror itself. It threads directly into the removable locking ring. I've seen no difference in star shapes based on a tight vs. loose baffle. That's been my experience...
P.s. now, if the locking ring isn't locked in place by the two grub screws, and further turns of the baffle result in further turns of the locking ring onto the primary - that would pinch for sure. The best way I can explain the baffle on these, is that it just threads into and essentially becomes an extension of the locking ring. It's the locking ring itself that makes contact with the primary and presses down on it, by way of an o-ring.
Last edited by Logieberra; 02-06-2015 at 01:41 PM.
Good question. Some of us may wish to recoat the primary down the track. I can tell you that it's a cleaner removal in my configuration, compared to its stock configuration, permanently bonded in three places by black silicone!
Personally, I'm not after any "popping" action on my RC mirror... it's there to stay unless it's essential that it be removed. As you know, the beauty of Velcro - break the seal and reuse. I might try fewer Velcro dots on next install later in the year, following my mirror recoat. This should make for easier removal, but at the expense of primary mirror support... I'm not sure. The dob/newt guys can do the calculations. You can also buy different Velcro dot sizes to adjust tension (e.g. 16 or 22mm).
Regarding the use of silicone to secure optics, I just want to repeat some A+ advice I came across from Australia's Mark Suchting, one of the best:
" ... from my own direct experience - Silicone is absolutely capable of warping optics -it is only warping on a sub -micron level we are talking here . This is my own practical experience of testing optics on and off silicone mounts in a highly sensitive bench star test . Diagonals are also susceptible to warping if not mounted just right if silicon [is] used. I would always recommend optics to be free floating both from the edge and back for the best possible level of support without possibility of induced strain at different angles of altitude. It is certainly possible to pull off a silicon mounting but it willl rarely be as good as a free standing optic and can certainly end up much worse if it is not done right."
I believe that I have now arrived at a 'free floating', or 'free standing' system at the back - and edges, by removing the black silicone, mirror clips and instead using Velcro
Last edited by Logieberra; 02-06-2015 at 01:44 PM.
Thanks Paul. Your guidance and support on these RCs has been really helpful. GSO owe you. I've now eliminated the clips - as they don't serve any purpose. My baffle was very loose for these tests. As you know, the baffle makes no contact and applies no pressure to the mirror itself. It threads directly into the removable locking ring. I've seen no difference in star shapes based on a tight vs. loose baffle. That's been my experience...
P.s. now, if the locking ring isn't locked in place by the two grub screws, and further turns of the baffle result in further turns of the locking ring onto the primary - that would pinch for sure. The best way I can explain the baffle on these, is that it just threads into and essentially becomes an extension of the locking ring. It's the locking ring itself that makes contact with the primary and presses down on it, by way of an o-ring.
My correction, I meant the retaining ring too. Not the actual baffle. i have had a long day.
Shortly after posting the above comments, I emailed GSO direct and provided them with the link for this thread. I was contacted earlier today by a GSO rep, who advised:
'thanks for your mail. We have been change the design of the RC12A last year, all CNC made machine parts now'.
So yeah, it looks like they've known about the 12" RC cell issues for some time.
The thing I like? In the first pic, see the salmon/pink coloured cell - it's thick, nearly as thick as the 12" RC back plate! Yay. There goes the bending in the current 3-4mm thick cell. The additional support points (from 3 to 18) is welcomed too, but as I said previously, there's a real science to calculating mirror weight distribution in cells - and I have no expertise there.
I called the good guys at BINTEL and have placed an order for this new mirror cell and bits. I'm sure the cost will be reasonable. I look forward to it
Last edited by Logieberra; 05-06-2015 at 11:01 PM.
I will investigate further. I got a back plate last year in February, but I have not heard there was this development, which is kind of odd since I regularly talk to Bintel. Both Bintel and I have had a lot of contact with Jim (GSO) about development.
I think it is a good thing for sure and it will sort some stuff that people are having trouble with. I think it is kind of odd about the mirror clips though. Looks like there are six of them now. The mirror support is a good idea.
Might consider getting this too. Did Bintel tell you they could get the parts and which Bintel did you contact? Sydney?
This might be news to Bintel, Sydney as well... I sent the pics and email to them today. I recently discussed the 12" RC in detail with Michael and Don, but this 'new cell' did not come up. The guys are very forthcoming, as you know. Please do investigate further. You've got the contacts. Keep us posted. Michael in Sydney is looking at sourcing the bits for me. Fingers crossed
Clips. Yes, bad idea! The central retaining/locking ring is enough to secure the mirror in RCOS and CDK 12" scopes...
Quote:
Originally Posted by Paul Haese
I will investigate further. I got a back plate last year in February, but I have not heard there was this development, which is kind of odd since I regularly talk to Bintel. Both Bintel and I have had a lot of contact with Jim (GSO) about development.
I think it is a good thing for sure and it will sort some stuff that people are having trouble with. I think it is kind of odd about the mirror clips though. Looks like there are six of them now. The mirror support is a good idea.
Might consider getting this too. Did Bintel tell you they could get the parts and which Bintel did you contact? Sydney?
