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Hi gang, I have a small issue with my CPC & need some information from other CPC owners. 

Basically I was having a look at the Fastar connection for Hyperstar but noticed that the whole Secondary assembly was a little loose & could be rotated on its own leaving the corrector plate securely fixed. 

Here’s where I made my first mistake, I lost track of the original starting position of the assembly but it didn’t end there. 

The only way to tighten the assembly & stop it rotating freely the corrector plate needs to be removed as the Secondary unit is tightened on the underside of the corrector. 

Next mistake, I forgot to mark the corrector position to aid refitting, not considering the fact that it’s position would be important on reassembly. 

Turns out that my corrector has no significant markings on it except in 2 places on the edge, about 45° apart are an arrow pointing to the outer face & the number 4. 

Through much head scratching & the use of digital calipers I think I have found the correct orientation of the corrector plate in order to reassemble but I am still stuck with not knowing the correct position of the Secondary assembly. 

I have read various articles that contradict each other stating that one of the 3 collimating screws should be at 12o’clock or 6o’clock. 

What I have noticed though is that when the outer retaining ring is unscrewed from the Secondary assembly, there is a small locating grub screw that sits within a slot on the assembly housing, see pic:

 

79C8A4AE-9D7F-4C1F-B765-A002F5DBF185.thumb.jpeg.12cd76969413dc9d73b2d43cea8e009c.jpeg

a big ask I know but is it possible for other CPC owners, mine being a 9.25” if it matters, to unscrew the Secondary retaining ring & note the position, on a clock face (top of OTA as 12), of this locating slot. It would be much appreciated. 

Thanks

I know there’s plenty of discussion pertaining to Primary & Correctors as marching sets, although from what I gather this was for earlier SCT models, plus 365 Astronomy lists the corrector as a replacement part: https://www.365astronomy.com/Celestron-C9.25-Replacement-Corrector-Plate-50984-1S.html

so I’m hopeful that nowadays the matched pair issue is not relevant & ive got away with my stupid mistake.

Regards..,

Kirk

 

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Before I pulled off my corrector plate of my 8" SCT I did as much research as possible also, and like you found conflicting info, namely one that stated that the corrector orientation is not important and the other that the corrector has to be in a particular orientation. To be safe I marked the corrector before pulling off.

The way to check whether the optical patch is operating as normal, you need either a bright star (or an artificial star) and go through the collimation procedure... If your scope looks collimated on one end of the focus but not the other, and vice versa, you know that the optics are misaligned, but if collimation looks good on both sides of focus and you can reach focus than you're lucky and all is good in the CPC world.

Also my understanding of the secondary is that it's orientation is not crucial, hence the whole need to tilt it to the light path, i.e. Collimation... Otherwise there would be heaps of people with mis-orientated secondary mirrors who use fastar.

Another bit of info I found on another forum, CN I think,  is that the triangle on the corrector is meant to be lined up with a corresponding marker on the primary mirror...

Only other thing I can think of, other than trial and error adjust and check if the focus/collimation test fails, is see if you have any close up pics of the scope before disassembly and see if you can see any tell tale sign you can follow.

Worst case is that you will need to rotate the corrector and check collimation and focus until it is good... I know it's a long process but that might be all you have left... 

Keep us updated, I'm very curious about how this will develop.

 

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19 minutes ago, MarsG76 said:

Before I pulled off my corrector plate of my 8" SCT I did as much research as possible also, and like you found conflicting info, namely one that stated that the corrector orientation is not important and the other that the corrector has to be in a particular orientation. To be safe I marked the corrector before pulling off.

The way to check whether the optical patch is operating as normal, you need either a bright star (or an artificial star) and go through the collimation procedure... If your scope looks collimated on one end of the focus but not the other, and vice versa, you know that the optics are misaligned, but if collimation looks good on both sides of focus and you can reach focus than you're lucky and all is good in the CPC world.

Also my understanding of the secondary is that it's orientation is not crucial, hence the whole need to tilt it to the light path, i.e. Collimation... Otherwise there would be heaps of people with mis-orientated secondary mirrors who use fastar.

Another bit of info I found on another forum, CN I think,  is that the triangle on the corrector is meant to be lined up with a corresponding marker on the primary mirror...

Only other thing I can think of, other than trial and error adjust and check if the focus/collimation test fails, is see if you have any close up pics of the scope before disassembly and see if you can see any tell tale sign you can follow.

Worst case is that you will need to rotate the corrector and check collimation and focus until it is good... I know it's a long process but that might be all you have left... 

Keep us updated, I'm very curious about how this will develop.

 

Hi, thanks for replying. 

Yeah im waiting for a clear night now to check collimation. 

Have downloaded Al’s Collimation Aid to make things easier plus I’m going to replace standard Phillips collimating screws with hex head ones for ease. 

What exactly do you mean by ‘both sides of focus’?

i realise that part of collimation is deficusing on a bright star to get concentric circles aligned, is that not the end of the matter?

regards..,

 

Kirk

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11 hours ago, 1CM69 said:

What exactly do you mean by ‘both sides of focus’?

I mean that you defocus to get the concentric rings and when you move towards focus, move past focus to get concentric ring on the other side of focus... if both are symmetrical circles that everything is ok, but if one side of focus looks like good collimation, but the other side looks off, that means that the corrector is mis aligned.

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On 3/10/2018 at 02:48, MarsG76 said:

I mean that you defocus to get the concentric rings and when you move towards focus, move past focus to get concentric ring on the other side of focus... if both are symmetrical circles that everything is ok, but if one side of focus looks like good collimation, but the other side looks off, that means that the corrector is mis aligned.

 

OK, thanks for clarifying 

Edited by 1CM69
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OK, with probably more luck than judgement I got it sorted.

I had just recently setup my CMOS with my focuser so I new exactly where perfect focus should be.

So I setup my scope & left it for more than 3 hours top cool minus dew shield & dew heater. I did consider wedge mounting my scope but as I had limited clear sky forecast & limited time, I decided not to bother. Obviously wedge mounted & Polar aligned would have made it whole lot easier keeping a star centered than just AltAz, plus my view is only to the south - tricky at the best of times but most people who have SCTs do not necessarily have a wedge & they have to manage collimating without.

I started up SharpCap & Al's Collimation Aid located Procyon, which was at 44deg, so a good mid altitude, noticed that the star had a slight egg shape with pointy end pointing to about 1:30ish, double checked my focus with Bahtinov & found it to be pretty damn good.

I set my focuser to 4000 steps IN, fiddled a bit setting up the circle sizes in Al's Collimation Aid & exp/gain in SharpCap just enough to make the centre spot visible to aid alignment of the circle mask overlay of Al's Collimation Aid.

It was evident that my scope was out of collimation but not by much at all, just the tiniest of tweaks to 2 of the collimation screws was enough to bring it all back inline.

Took an age though because everytime I went to my scope to adjust it, cloud came rolling by hiding the star so I had nothing visible to see on my laptop, plus each time I made an adjustment the star moved on the screen, then I had to recentre & check again. Anyway, eventually I got it.

Then I set my focuser to go 4000 steps OUT from focus and checked collimation, thankfully it was spot on.

I was so relieved.

I then went through the whole procedure of checking my actual focus point & found that my initial point, which a week ago was spot on focus, now was not quite. Turns out in the end that my new focus point was 210 steps away from my initial point, not much I know & had me scratching my head for a while wondering why the point had changed.

I assume that part of the reason was that when I found this focus point last week, it was a lot colder & I had my dew heater on but the main thing is that I had focus & perfect collimation.

I dodged a bullet there but now I have put markings on my corrector plate & my secondary housing to know exactly where they should be located if I take it apart again.

Phew!! :icon_biggrin:

 

 

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Hi,

I recently bought a Nexstar 11 GPS and i to was able to rotate the centre , the scope needed some collimation but all turned out fine afterwards.

Havent paid any attention on the collimation nobs orientation though.

Other question, i see you have the pro wedge, how does that work out for you ?

I often see it isn't all that an that one should better safe up and buy a EQ-mount instead...

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21 minutes ago, Miguel1983 said:

Hi,

I recently bought a Nexstar 11 GPS and i to was able to rotate the centre , the scope needed some collimation but all turned out fine afterwards.

Havent paid any attention on the collimation nobs orientation though.

Other question, i see you have the pro wedge, how does that work out for you ?

I often see it isn't all that an that one should better safe up and buy a EQ-mount instead...

Glad to hear that you had no problems collimating your scope. 

As for the wedge, it took lots of trial & error to figure out its use but recently I have managed 20min subs with perfect stars & could probably go longer if needed. 

Also, mine is a portable setup, I have no permanent pier so I need to redo all alignments every time I use it. I suspect if fitted to a permanent pier there’d be no reason that it would not compete with a far more expensive EQ mount, plus the wedge does not require meridian flips. 

There always seems to be plenty for sale second hand, so at the right price it’s probsbly worth a try. 

For me, I am considering an EQ mount but not for this scope but an RC. I personally don’t think I would get any more from my CPC after spending 1000’s on a pro mount. 

Regards..,

Kirk

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18 hours ago, 1CM69 said:

As for the wedge, it took lots of trial & error to figure out its use but recently I have managed 20min subs with perfect stars & could probably go longer if needed. 

20 min subs with a cpc..wow.. with any long focal length scope that's really good..what size cpc are you using and what's the setup on guiding?

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4 hours ago, newbie alert said:

20 min subs with a cpc..wow.. with any long focal length scope that's really good..what size cpc are you using and what's the setup on guiding?

9.25” & using Lodestar x2 via OAG with PHD

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2 minutes ago, newbie alert said:

Nice..which OAG are you using..and do you thing a lodestar is a necessity  (I'm trying to make my Skyris see something but failure so far)

This one: https://www.celestron.com/products/off-axis-guider

i got a Lodestar after been told I would need it to be able to find guide stars in such a small FOV. 

Not used any other guide cam so I cannot comment on whether it is necessary. 

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24 minutes ago, 1CM69 said:

This one: https://www.celestron.com/products/off-axis-guider

i got a Lodestar after been told I would need it to be able to find guide stars in such a small FOV. 

Not used any other guide cam so I cannot comment on whether it is necessary. 

I'm using the same oag..looks like I need to get saving my pennies

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3 minutes ago, newbie alert said:

I'm using the same oag..looks like I need to get saving my pennies

I’m currently in the process of changing over to SX Filter Wheel & OAG but am waiting for some C Mount extenders to arrive, should be fun trying to get stars in a smaller prism ?

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  • 2 months later...

OK, back to this collimation lark - seems all was not as perfect as it seemed back-a-long.

I had been trying to image Jupiter and getting nothing but blurry washed out image, I know the planet is fairly low but this niggle in the back of my mind kept on telling me that maybe my original collimation attempts weren't that great. There was nothing for it but to dispel this niggle, I had to check my collimation at the next available opportunity.

It was while setting up some different equipment the other night & watching my screen while the focuser was running down to the focus point, I noticed that the Poisson Point which should be central & a single point was in fact 3 points of light forming a triangle.

Last night there were clear skies forecast all night so I decided to set aside the time to get this job done.

Problem 1 - my backyard has limited sky view

Problem 2 - because of Problem 1 I was unable to Polar Align & hence no guiding either.

So I was relying on a 2 star EQ North Align

Anyway, went ahead as best as I could. I slewed my scope to Denebola, which was at 45degrees & proceeded to attempt to collimate and bring these 3 points together.

No matter what I did however, nothing seemed to work. I was altering the collimation alright as the centre circle was moving away from it's central position but these 3 points of light were not coalescing to form a single point.

I then set the collimation back as best I could given that my tracking was not perfect & it was a complete pain keeping the star within the circles of Al's Collimation Aid.

I played around with my focuser for a bit, I started at 5000 steps above the known focus point, (previously achieved via Bahtinov), and set the focuser to run down to the focus point.

I noticed that the 3 points of light didn't really form a single point as such until almost at focus.

OK, I thought, what about now going 5000 steps below focus, so again, I set my focuser going & this way the Poisson Point was indeed a single point as it should be.

So I recorded a short video of this process to better explain. This video starts at 5000 steps above focus & ends at 5000 steps below focus.

What I found was that could run my focuser below the focus point by say 1000 steps & then it back up to the actual focus point and the Poisson Point would stay as a single point.

Excuse the quality of the video above, I captured the data in a .ser file but YouTube doesn't like these and so it was converted to an .avi in PIPP before uploading to YouTube.

So my original capture showed the star dancing around a bit because of tracking so I processed the video through PIPP to centralise the image. I also ran off individual image files for each frame.

With these images I was able to find a good quality one and open it in Photoshop, centralise the image with the Ruler tool and then use Al's Collimation Aid overlay, much more precise than mucking around with a moving target.

The image below shows the defocused star exactly centralised with the overlay in place. It is obvious that the central dark ring is slightly off in the NE direction, pretty much the same amount as the Poisson Point which is just about visible.

Untitled.png.caccf891df5bc49c258b22c943339d10.png

In this next image I have removed the Rulers Guides in Photoshop, added a grid and highlighted the Poisson Point to make it easier to see.

Untitled2.png.4503b73d42e1887f29dca15c86610136.png

Last night I didn't think of this but the only difference to my setup from my previous seemingly good collimation, was that this time I had my Reducer fitted.

Could the reducer cause the odd Poisson Point behaviour above focus?

I am going to have to thoroughly go through all of this again when I am properly aligned and can keep a star centered for long periods.

Just strikes me as odd that when adjusting the collimating screws, the central circle was very obviously moving although it seemed the multi Poisson Point was not.

Anyone have any other ideas what should be causing this & why it's only apparent above focus and not below?

Regards..,

Kirk

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