12 replies to this topic

[akdwivedi]

over the weekend, a friend showed me his C6. the secondary mirror freely rotates in the secondary mirror holder. I did some search and it appears that there is indeed some issue with sometimes secondary mirror housing getting lose. 

 

The posts also say the etching/notch on the secondary mirror holder should point to 3 O'clock assuming the dovetail is pointing towards the ground. 

 

I checked some pics online and the attached shows the front view of C6 from celestron website. While our C6 isnt same (it came with evo - silver tube), when I point the dovetail to the ground, it does look the same.

 

I would like to confirm that the secondary mirror etching/notch is indeed at 3 o'clock (opposite to focuser which is at 9 O'clock) before I take out corrector plate and tighten the secondary holder.

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[akdwivedi]

I don't intend to change the corrector alignment but is the picture correct. do all C6 have their dovetail directly opposite to Celestron logo on the corrector retainer ring?. 

[James_5474]

The attached is what it looks like for my C6; however, do note that I moved the finder shoe to the 11 o'clock position (from the original 1 o'clock position; if viewing from the corrector plate face) to make it easier to use on my AZ5 alt/az mount.

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[Oldfracguy]

The C6 and larger SCTs have extra pairs of holes on the back so you can either 1) reposition, or 2) add another finder scope mounting shoe.  It is pretty common for people using Alt-Az mounts to reposition their finder scope so that it is in a usable location with the OTA mounted with its dovetail rail on one side or the other, at the 3 o'clock or the 9 o'clock position:

 

 

 

 

Here is a photo of my old C6:

 

 

 

 

When you re-install the corrector plate after securing the secondary mirror housing, make sure you only retighten those six screws around its edge just so they are snug and not overly tight.  Many times people complain about the views through SCTs being "soft" compared with other types of scopes.  Aside from having to acclimate the SCT temperature-wise so that the internal air currents die down enough to allow decent views, either by setting the scope outside for a time or wrapping the outside with that "reflectix"-type of reflective barrier insulation, many times the reason for the views not being as good as they can be is traced to the six screws that attach the front corrector plate being cranked down way too hard.

Edited by Oldfracguy, 02 August 2024 - 11:43 AM.

[akdwivedi]

Thanks.. So it's confirmed that notch on secondary mittor holder is at 3o clock.

[Echolight]

over the weekend, a friend showed me his C6. the secondary mirror freely rotates in the secondary mirror holder. I did some search and it appears that there is indeed some issue with sometimes secondary mirror housing getting lose. 

 

The posts also say the etching/notch on the secondary mirror holder should point to 3 O'clock assuming the dovetail is pointing towards the ground. 

 

I checked some pics online and the attached shows the front view of C6 from celestron website. While our C6 isnt same (it came with evo - silver tube), when I point the dovetail to the ground, it does look the same.

 

I would like to confirm that the secondary mirror etching/notch is indeed at 3 o'clock (opposite to focuser which is at 9 O'clock) before I take out corrector plate and tighten the secondary holder.

I think that's wrong.

 

For an OTA from an Evo or SE, the alignment pin should be on the same side as the dovetail. So six o'clock in your picture.

 

When mounted, the serial number should be read upright. Not sideways.

 

But I don't know if the secondary rotation is critical or not.

I have always read that the corrector plate should be kept at a certain rotation though.

 

Edit:

 

But maybe there is a huge misinterpretation.

 

You say the secondary freely rotates in the secondary mirror holder.

But that's impossible, because there is an alignment pin on the secondary and a groove in the secondary holder.

 

I can only guess that what you are calling the secondary mirror holder is not the secondary mirror holder, but actually the corrector plate. The glass lens.

And the corrector plate does need to be correctly positioned. As it is rotated for best correction from the factory.  

 

Is the "secondary mirror etching/notch" that you refer to a mark on the glass? Or the notch in the secondary housing?

Edited by Echolight, 04 August 2024 - 01:03 AM.

[Echolight]

I don't intend to change the corrector alignment but is the picture correct. do all C6 have their dovetail directly opposite to Celestron logo on the corrector retainer ring?. 

No.

 

On the Evolution 6 and 6SE, when the dovetail is at 3 o'clock, the Celestron logo is at 11 o'clock.

 

Edited by Echolight, 04 August 2024 - 01:22 AM.

[Echolight]

Thanks.. So it's confirmed that notch on secondary mittor holder is at 3o clock.

No. On your Evo optical tube. The notch would have originally been on the same position as the dovetail.

[akdwivedi]

You say the secondary freely rotates in the secondary mirror holder.

But that's impossible, because there is an alignment pin on the secondary and a groove in the secondary holder.

 

I can only guess that what you are calling the secondary mirror holder is not the secondary mirror holder, but actually the corrector plate. The glass lens.

And the corrector plate does need to be correctly positioned. As it is rotated for best correction from the factory.  

 

Is the "secondary mirror etching/notch" that you refer to a mark on the glass? Or the notch in the secondary housing?

The secondary mirror housing fits insite the secondary mirror holder and then tightened with the retainer ring on the top. When I say secondary mirror rotates freely I mean the entire secondary mirror assembly including the parts which are secured on the corrector. the whole thing rotates freely. 

 

I dont know the terms - but this is what I mean.

1. There is a two part assembly which is screwed on the corrector plate. one part is on inner side of corrector and other part is on the outer side of corrector.

2. The outer ring of the 2 part assembly has an etching/notch.. 

3. the secondary mirror with the collimation housing slides in this notch.

4. An outer ring is then screwed around secondary mirror to hold the secondary mirror in place.\

 

I am not sure if I am explaining correctly.

[JOEinCO]

That explains it perfectly.   

 

The inside of the secondary mirror holder is loose. If I remember correctly, the baffle itself is what threads onto the outer portion's nose that is sticking through the corrector plate. 

 

Unless someone knows a trick, you're going to have to remove the corrector plate.

[akdwivedi]

I have removed the corrector on my c6 a few times and am comfortable with process.. it’s the secondary mirror notch that I am not sure how it should be aligned wrt dovetail or focuser.

[davidc135]

Is the secondary assembly free to rotate because it grips the corrector plate too loosely?

 

Can the 2ndry mirror notch be seen with the mirror in situ or does the mirror need to be removed?

 

As far as 2ndry orientation goes, I'd check the collimation and if the now coma free, high mag image of a star is free of astigmatism, then the set up is as it should be.

The corrector plate can then be removed and the 2ndry baffle assembly tightened a little. Following reassembly another check on collimation.

 

It can help to use an artificial star if you have say 12-15 metres space available, especially if there multiple adjustments being made.

 

I could be wrong but I doubt if the 2ndry rotation will make any difference. If it's easily done, it would be interesting to see the result of turning the 2ndry by 90^o, and after recollimation, compare x200 star images.

 

If I'm wrong and there is a problem with the collimated star image then the 2ndry assembly can be rotated in 10^o steps until the best orientation is found, re-collimating after each adjustment of course.

 

David

[akdwivedi]

Thanks.. I tightened the rings on both sides of corrector plate. Currently secondary mirror notch is at 3o’clock. I will try and collimate when Thor allows..and ask for more advice.. as you have mentioned, it might require some trial and tests. The corrector plate had markings, probably previous owner did that for cleaning the corrector plate. So corrector and primary should be aligned.