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Old 31-08-2018, 02:51 PM
Hemi
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Information that may prevent a panic attack

Iím into my 4th year of Astronomy, but firmly a newbie.

My C925, is now 4yrs old as well. Itís been used a lot, but never needed much TLC. Whilst getting to grips with my new eq mount and the fun of polar alignments, I noticed that my collimation was off. I went to the front of the scope and noticed that my corrector was very dirty.

I looked for the collimation screws! They were hiding under a rotatable plastic cover, that did not rotate very well. What did rotate however, was the entire secondary housing! Gulp and panic! The secondary housing was loose! Not just the retaining ring but the assembly! Turning it would turn the orientation of the collimation screws!

Stopped, and closed my eyes and guesstimated the original position! And googled. My patients google (and itís generally not that helpful) but we astronomers google for answers all the time!

Googling got me answers that ranged from, the world is going to end, to relax dude and get a beer the suns still coming up tomorrow, no problem.

Any how, after a lot of cogitation, I went back to the scope....with courage and fortitude, I tightened that secondary retaining ring! Luckily it caught and did not spin! (Otherwise the corrector needs to come off Iím told). I took off the visual back, and looking in from the front and back, all seemed grossly lined up. Put back the diagonal and 12mm EP and focused on Antares......the collimation was totally screwed now. But that was it! Only collimation.

So I learnt how to collimate my SCT, it was a bit random but projecting your finger at the narrowest point of the concentric rings definately helps identifying which screw to turn. I donít think itís perfect, but I think I will uses a camera for that.

Sorry for the ramble, but to summarise my experience and my googling.

A rotating secondary on a sct is not much to worry about, it is a rotating corrector that is! It will mess up your collimation sure, but collimating an sct is easy! Just takes a little time and care, the Internet posts are correct in that you only want 1/8 turns, no more, even if you feel itís not doing much.


Clear skies

Hemi
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Old 31-08-2018, 03:31 PM
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multiweb (Marc)
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All elements in an SCT, corrector, secondary and primary have a mark to align them. Usually corresponds to the 3'oclock position facing the scope so opposite the focusing knob.
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Old 31-08-2018, 05:07 PM
Wavytone (Nick)
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You have much to learn, grasshopper !
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Old 31-08-2018, 05:37 PM
Hemi
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Quote:
Originally Posted by multiweb View Post
All elements in an SCT, corrector, secondary and primary have a mark to align them. Usually corresponds to the 3'oclock position facing the scope so opposite the focusing knob.
Thanks MW,

But to see those marks, I presume you have to take off the corrector?

I had a good look inside the OTA (through the Corrector) and couldn't see any alignment marks....I did look as several websites said as much.


CS

Hemi
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Old 31-08-2018, 07:03 PM
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multiweb (Marc)
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Quote:
Originally Posted by Hemi View Post
Thanks MW,

But to see those marks, I presume you have to take off the corrector?

I had a good look inside the OTA (through the Corrector) and couldn't see any alignment marks....I did look as several websites said as much.


CS

Hemi
For the primary and secondary it's usually a black line drawn from the mirror center to the edge, for the corrector it's an etching on the edge of the glass under the retaining ring and a > on the side of the corrector indicating which side faces out.
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Old 31-08-2018, 07:48 PM
Zuts
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Hi,

I have an edge HD 9.25 and had the same issue. I was thinking of getting a hyperstar so attempted to take out the secondary in the normal manner by twisting the large black locking nut.

Horror, the entire secondary assembly just rotated with the locking nut.

I took it to bintel and and they said this shouldn't happen and that basically the secondary assembly was loose. To fix it they removed the corrector so they could tighten the secondary assembly.

Edge HD's are made in the US and all have a mark indicating the correct orientation of the corrector, also they don't use shims anymore. Taking off the corrector tightening the secondary assembly and re-assembling took around 3 minutes courtesy of Don at Bintel, at no charge I should add, and I didn't buy the scope at Bintel.

I had to re-collimate the secondary and now everything is fine.

I can easily remove the secondary mirror now as the secondary assembly is now tight. Putting back the secondary mirror is simple as the Edge HD secondary has a pin and the mirror can only go in in one orientation.

However I am not sure if all the above applies to a regular C9.25.

Cheers
Paul

Last edited by Zuts; 31-08-2018 at 10:25 PM.
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Old 01-09-2018, 12:39 AM
Hemi
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Quote:
Originally Posted by Zuts View Post
Hi,

I have an edge HD 9.25 and had the same issue. I was thinking of getting a hyperstar so attempted to take out the secondary in the normal manner by twisting the large black locking nut.

Horror, the entire secondary assembly just rotated with the locking nut.

I took it to bintel and and they said this shouldn't happen and that basically the secondary assembly was loose. To fix it they removed the corrector so they could tighten the secondary assembly.

Edge HD's are made in the US and all have a mark indicating the correct orientation of the corrector, also they don't use shims anymore. Taking off the corrector tightening the secondary assembly and re-assembling took around 3 minutes courtesy of Don at Bintel, at no charge I should add, and I didn't buy the scope at Bintel.

I had to re-collimate the secondary and now everything is fine.

I can easily remove the secondary mirror now as the secondary assembly is now tight. Putting back the secondary mirror is simple as the Edge HD secondary has a pin and the mirror can only go in in one orientation.

However I am not sure if all the above applies to a regular C9.25.

Cheers
Paul
Hey Zuts,

The vanilla 925 (mine) has shims around the corrector.
Not sure if there is a pin ensuring only 1 conformation/orientation for the secondary as I didn't go that far. But like you am considering a hyperstar.
Have held off as I'm considering upgrading to a c11 edge hd. It's a marginal gain.

I completely agree that Don and the Bintel crew are excellent. He was my first port of call when this happened, but alas he was away. I do feel though that some of this stuff is lore! And stuff of myth and mystery. There's a great you tube video showing an experienced South African astronomer, stripping a c14 to clean it! Dispels a few things. I think taking apart an ota or mount for cleaning, repairs and upgrades should be reasonably achievable. I live in Darwin so popping into Bintel is not an option. And we have no Astro societies/community either. You should look at some corrector plate cleaning videos! Wow! I get the principles, but really only clean the corrector on a full moon in a leap year? And you must hold your breath for the entire process as saliva moisture might get onto the glass and eventually corrode the entire scope in 25yrs time

Best

Hemi
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Old 01-09-2018, 01:06 AM
Zuts
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Hi Hemi,

I used to have a C11 (circa 2000), which i sold. I had to remove the corrector on that and it was a pain. There was a little black mark to re-align and when i removed it all the paper and cork shims fell on the floor!

The edge has a ring holding the corrector on, you just remove 3 screws and pull it out. There is an etched registration mark for alignment when you put it back in.

The secondary mirror has a pin which fits into a slot in the housing so you literally can't go wrong putting it back in.

I was really amazed at the mechanical design improvement. I'm nowhere near a Don, but I reckon i could remove the corrector and re-install it in 5 minutes. The secondary removal and replacement is a 30 second job.

So things do improve, at the expense of mystery and lore!

Cheers
Paul
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Old 01-09-2018, 07:59 PM
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blink138 (Pat)
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yup this happened to me on my c11 ............. still not game to take the corrector off
.............. so it is the ring on the inside of the corrector that has come loose?
pat
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Old 01-09-2018, 09:02 PM
Zuts
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Quote:
Originally Posted by blink138 View Post
yup this happened to me on my c11 ............. still not game to take the corrector off
.............. so it is the ring on the inside of the corrector that has come loose?
pat
I believe so. The secondary housing is in two parts and they screw together over the corrector plate. The secondary fits in the holder and is secured by the external locking ring.
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Old 02-09-2018, 02:19 AM
Hemi
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The corrector is sort of sandwiched between the internal and external fittings of the secondary holder. If it continuously spins, the only way to tighten it is to take off the corrector and hold both sides of the assembly/holder.

Luckily on mine, gentle clockwise rotation of the external ring, managed to grip and tighten somewhat. Non gentle counterclockwise turning, to install a hyperstar for example may do it again though. We will see.

Zuts, Iím interested in your downsizing aperture. Are you predominantly an imager?
Iím thinking of going the other way, but oddly, I think you gain much more by a c11xlt to c925 edge hd, than a c925xlt to a c11 edge.


Hemi
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  #12  
Old 02-09-2018, 03:35 AM
Zuts
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Quote:
Originally Posted by Hemi View Post
The corrector is sort of sandwiched between the internal and external fittings of the secondary holder. If it continuously spins, the only way to tighten it is to take off the corrector and hold both sides of the assembly/holder.

Luckily on mine, gentle clockwise rotation of the external ring, managed to grip and tighten somewhat. Non gentle counterclockwise turning, to install a hyperstar for example may do it again though. We will see.

Zuts, Iím interested in your downsizing aperture. Are you predominantly an imager?
Iím thinking of going the other way, but oddly, I think you gain much more by a c11xlt to c925 edge hd, than a c925xlt to a c11 edge.


Hemi
I get a bit bored looking in Sydney as it's too bright. I have a TV 85 which satisfies my widefield astro ambitions. The 9.25 I have only imaged planets, but when it goes to a dark site I am happy with the views. I just got a reducer for the edge so i will see.

Anyway I want to use the TV85 for a bit and get back into widefield.

My C11 which i sold was really good for looking, but mechanically the Edge is far superior. I would buy an 11 inch edge but feel i should be satisfied with my 9.25 and since I have a reducer and feathertouch focus boss think I should persevere with it.

I really don't think you could go wrong with a 11 inch edge, they are manageable, mechanically sound and would give good views if they are like my 9.25 which gives pinpoint stars.

Anyway, both visual outside of a city and astro, it's all good.

By the way I lived in Darwin in Gardens Hill and Malak for around 5 years, a great place

Cheers
Paul
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