How to collimate a Celestron C150-HD

[Harry Wise]

I have a Celestron C150-HD with a lens between the primary and secondary mirrors.  I was hoping that someone could help me in collimation as the instruction manual is rather sketchy in this area.

When I look down the eyepiece hole without a lens I can’t see a reflection of my eye as I am sure I used to.  Does this mean the primary or secondary is out of alignment?  From various posts I have read it seems that the secondary is difficult to adjust so I will leave that for now, but will tackle the primary.

I removed the lightweight plate at the end of the tube to expose three Philips head screws; this is when I had a problem.  The manual states that collimation is achieved by  “loosening the three screws  and adjusting the three knurled knobs until the secondary mirror is centered in the reflection of the primary mirror.”   I can’t see knurled knobs and didn’t want to proceed without advice.  Can anyone help?

Also I would like to remove the mirror with a view to its cleaning, I undid the six Philips screws on the outside of the tube end and assumed that this would free the mirror.  Do I need to use some force to free it?

[Alan64]

It sounds to me that you have a Jones-Bird type of reflector.  Have you researched the type online, and the troubles folks have with them in so far as collimating?

Does your kit appear as this one...

If so, then that's a Jones-Bird type, and practically uncollimatable for the vast majority of folks.

I've never had one myself, but wouldn't mind tinkering with one.  It's not a true Newtonian, but a catadioptric in that it has an integrated lens, and overall quite a poor design.

Best of luck, and clear skies to you always.

[Harry Wise]

Thank you Alan,

Yes, that is my scope.  The instructions manual gives instructions on collimation of the primary mirror so at least this can be done.  I was hoping that someone could give some advice on the subject before I jumped-in and maybe mess things up.  I don't know if I should remove of loosen the three phillips screw at the back of the mirror or just loosen them.  There are supposed to be Knurled adjusting knobs but they are not obvious.

I have read negitive reports of this model of scope but it has given me good service over about 12 years.

Harry

[Ricochet]

How many screws can you see in the end of the tube? Is it only three? There should be two sets of three screws/knobs. One set are locking screws and one set are adjustment screws. Perhaps if you take a photo of what you can see then we can advise you on which screws are which. You only need to loosen the locking screws if they are what you can see but without identifying the adjustment screws there is no point in doing that. 

[popeye85]

1 hour ago, Harry Wise said:

I have read negitive reports of this model of scope but it has given me good service over about 12 years.

Harry

12 years without collimation? That's impressive!!

[John]

I think the best way to do this type of scope is to use a star test to determine the state of the collimation and then, if needed, to confirm the effect of adjustments made.

A laser collimator does not really work with the bird-jones type design because the correcting lens at the bottom of the drawtube has the effect of dispersing the laser beam which renders the device useless.

A simple collimation cap might help to see that the priciple elements are at least generally aligned. The star test should show mis-collimation, if it is present.

 

 

[Alan64]

10 hours ago, Harry Wise said:

Thank you Alan,

Yes, that is my scope.  The instructions manual gives instructions on collimation of the primary mirror so at least this can be done.  I was hoping that someone could give some advice on the subject before I jumped-in and maybe mess things up.  I don't know if I should remove of loosen the three phillips screw at the back of the mirror or just loosen them.  There are supposed to be Knurled adjusting knobs but they are not obvious.

I have read negitive reports of this model of scope but it has given me good service over about 12 years.

Harry

In that event, I would get a collimation-cap, and perhaps even a Cheshire, and among the first three items within this listing...

https://www.firstlightoptics.com/other-collimation-tools.html

Once you remove the corrector-lens, you can insert the cap where an eyepiece goes, and view the entire scene of the optical system inside, and consisting of the primary and secondary mirrors only...

...and then to collimate it in the manner of a traditional Newtonian.  A Cheshire is useful in centering the secondary mirror directly under the focusser, and in ensuring that the eye and eyepiece are aimed directly at the center of the primary mirror, once the mirror is center-spotted...

This iillustrates a common location of the lens, and attached to the bottom of the focusser's drawtube...

I'd clean the lens, too, whilst at it, in addition to the secondary mirror as well. 

This is a Celestron PowerSeeker 127mm, the largest of Celestron's Jones-Bird reflectors currently, and with the focusser disassembled and the corrector-lens appearing as a separate component at far right within the fourth image down...

http://www.cloudynights.com/topic/502410-celestron-powerseeker-127eq-rebuild/?p=6624249

The primary mirror and its cell is easily removed from the optical-tube.  Simply mark its position in relation to the tube, remove the screws, and as you've already done, then carefully work it out.  Simply click on the images to enlarge...

The primary mirror assembly exploded...

When reinstalling the mirror, you don't want to snug let alone tighten down the clips.  Use a slip of paper or a business card for a proper clearance between the surface of the mirror and its clips...

Else, the mirror will be pinched by the clips, and you would see that effect in the images themselves.  The clips are only for keeping the mirror from falling out of the cell, not in battening it down to where it's immovable.

Center-spotting the primary is absolutely necessary for collimating with the tools, and isn't that difficult...

https://stargazerslounge.com/topic/286250-collimation-secondary-doesnt-seem-centered/?do=findComment&comment=3138680

Collimation instructions, which can be helpful with the Jones-Bird as well...

http://www.forumskylive.it/Public/data/serastrof/201281510358_Astro Babys Guide to Collimation.pdf

http://www.schlatter.org/Dad/Astronomy/collimate.htm

I would take photographs and make some notes during the cleaning, center-spotting and collimating, throughout.

The plate that covers and seals the back of the primary cell should be left off permanently.  The primary mirror needs air-circulation, in order to acclimate to the outdoor conditions, and for the ability to conduct high-powered observations that much more quickly...

The plate only serves to smother the mirror. 

The secondary mirror assembly is the most finicky portion of the optical system.  It moves in most every direction imaginable, the motions akin to that of a gyroscope, in tilting and rotating, but not as fast...

https://www.youtube.com/watch?v=DsOU0WEc6OU

But the secondary mirror must come to rest in only one position, directly under the focusser, and circular in shape...

To make adjusting the secondary and primary mirrors more easily, take one of the small set-screws from each assembly to a local hardware that sells screws and what-not loose in bins, size them up, metric most likely, and replace them with longer socket-cap bolts, then to adjust with the hand rather than with tiny tools...

You can also replace the larger screws of the primary cell, and replace the rubber grommets with sturdier springs instead which will help hold the collimation better.

The C150-HD f/6.6 catadioptric is larger than what's currently marketed, and should be a bit easier to work on as a result. 

I know that I wouldn't mind having one to explore. 

[Alan64]

I got to looking around. It could be that the corrector-lens is as this within yours, and this from the larger G8-N...

If so, you may need to remove the lens in order to collimate the secondary mirror, focusser and primary mirror as one, then reinstall the lens.

[MoonNut]

Hi Harry,

 I have owned a C150 since 2000, it has given me great service over the years and I have collimated it many times.

Firstly, you should not need to remove the corrector lens, it is positioned in the secondary mount between the primary and secondary as in the photo in Alan64 second post above. 

There is a metal plate covering the Primary mirror cell held on with  3 small phillips screws, remove them and the plate, you shoulld then be able to see the back of the primary mirror and 3 large phillips screws.

The 3 phillips screws are the adjustment screws for the mirror cell (these are knurled knobs on later models of the C150) next to each is a small allen drive grub screw,  these are used to lock the mirror in place after adjustment.

So first wind them out until they are well clear on the cell.

Then adjust the screws to align the mirror.

Between the inner face of the mirror cell and end of the OTA tube, around each of the 3 adjustment screws are 3 (I think) orange rubber washers, these provide the tension to hold the mirror in place and allow adjustment, you should be able to see them when you remove the cover plate.

The secondary is not so easy to mess with, the adjustment screws are glued into position during manufacture, unless there has been some major "trauma" with the OTA they should not need adjusting.

I have always used a collimation cap and followed up with a star test, this works well enough. I have also looked down the focus tube (with the OTA pointed at  bright wall or blue sky) and aligned the ring that you can see with the  secondary mirror arms that are visible and the secondary mirror tube, this works well for me.

I can send some photos later if you need them to clarify anything.

MoonNut

[MoonNut]

Forgot to mention, yes, the mirror cell "carrier" at the end of the OTA needs some force to remove it, I seem to remember it was stuck on with glue or double sided tape.

[MoonNut]

But to remove the mirror you only need to remove the collimation screws and the mirror cell will fall out, fitting the rubber washers on re-assembly can be tricky.

[Harry Wise]

Hi popeye85. thanks for your help I have as suggested.

I will follow the methos advised by MoonNut and see how I get on.  Any photes to aid the procedure would be appreciated.  Thank you.

[Alan64]

With its metal focusser, and a boon towards considerable ease in collimating, along with its 150mm aperture, that particular model is one of the best of its type.

[Ricochet]

If you are still unsure about which screws are which I have marked the primary adjustment screws (phillips) in red and blue and the primary locking screws (Allen key type). It looks like the screw I have marked in blue might be slightly damaged so I would try to collimate using only the other two screws if possible.