Where's my donut

[freewheeler]

I am trying to collimate my Celestron AstroMaster 114 EQ.. I have a laser collimating tool. The tool is accurate and I understand the precautions necessary when working with lasers. All the tutorials and you tube videos make it look so easy when collimating the secondary mirror. Just bring the laser dot to the centre of the donut which marks the centre of the primary mirror. This donut supposedly being visible with or without the laser switched on. I just cannot see any donut. do all newtonians have one? Is mine an exception? Any pointers would be greatly appreciated.

[johnfosteruk]

Some scopes don't have em mate. You'll have to install one if you think you can stand the stress of removing the primary. A ring binder hole reinforcer will do the job. 

[Timebandit]

Hi  and welcome to SGL. If you cannot see a donut when the mirror is examined then it was not fitted at the  factory or has come off. You will need to remove the primary mirror and measure out dead centre of the mirror and stick your own donut on. There are a few sites on the web shows it in more detail. I hope this is of help to you ☺.

[John]

Is your 114EQ the one that has a shortish tube but a focal length of 1000mm ?

 

[freewheeler]

Thank you all for your useful replies. I was beginning to think that the donut was like one of those optical puzzles that you have to stare at for a long time before you see it. At least I now know that if I can't see it it's not there...obvious really but newbies often think they must be doing something wrong.  In answer to the last post, yes  45 mm tube length, 1000 mm focal length

[John]

I think it's what is known as a "bird jones newtonian". It has a lens at the bottom of the eyepiece drawtube to create the focal length which is why a 1000mm focal length can be squeezed out of a relatively short tube.

The relevence to collimation is that that particular design of scope might not enable a laser collimator to work as intended. I might be incorrect though so happy to be corrected by those who do use a laser collimator with these designs.

 

[Charic]

 

Freewheeler, some of the Celestrons, like my 127EQ are not fitted with any  alignment rings, so you'll need to fit one yourself, but if as John is suggesting, you have a short tube Newtonian, there should be a small corrector type Barlow lens installed inside the bottom of the focuser tube. This will prevent you from collimating your scope in the standard format, and this lens was not designed for the user to remove at will?

I centre spotted my 127EQ making no improvement to the level of collimation achieved by simply aligning the 3 primary mirror clips, detailed in the Celestron user guide.

But by adding the donut, will afford you the ability to strip and rebuild a clean scope, if your not detered by the task.

[Cornelius Varley]

The Celestron Astromaster 114 is known as a Jones Bird. Unfortunately this type of telescope cannot be collimated using a laser collimator so you will need to collimate it using either a cheshire eyepiece or collimating cap.

[freewheeler]

White dwarf. There is a lens at the bottom of the focuser tube. With the eye piece removed you look down through it on to the 45 degree secondary mirror which is adjustable in the usual manner by 3 allan key  headed screws.  With the laser in and on it works as intended as far as the primary mirror is concerned by returning the spot to the bullseye target built into the laser collimator

 

[freewheeler]

Peter. Thanks for your answer. Will I still need a donut added to my primary mirror if I collimate with either a cap or a cheshire

[Charic]

The corrector lens is there to facilitate  a shorter optical tube?

It's unlikely that the laser beam will remain un-affected passing through that lens twice, although  a method that is employed by adding a Barlow lens on my Skyliner ( Barowed Laser ) in order to receive a shadow of the donut which when used correctly, allows the use of an un-collimated laser tool.

This method works well, quickly checking the alignment of a parabolic mirror, your corrector lens does not work exactly the same way.

 

[freewheeler]

Thanks Charic. Out of interest, to attach a donut to my primary mirror I need first to remove the mirror cell. This is attached by Philip head machine screws with unsecured nuts and washers on the inside. So I need to be able to get my arm down the tube. To do this I have to remove the front crosspiece with the secondary mirror attached, same method of securing but I can reach the nuts OK. To do this I have to remove the eyepiece assembly because the plastic moulding that forms the crosspiece is extended to surround the eyepiece mount.   Don't make it easy do they

[johnfosteruk]

2 hours ago, freewheeler said:

Thanks Charic. Out of interest, to attach a donut to my primary mirror I need first to remove the mirror cell. This is attached by Philip head machine screws with unsecured nuts and washers on the inside. So I need to be able to get my arm down the tube. To do this I have to remove the front crosspiece with the secondary mirror attached, same method of securing but I can reach the nuts OK. To do this I have to remove the eyepiece assembly because the plastic moulding that forms the crosspiece is extended to surround the eyepiece mount.   Don't make it easy do they

You shouldn't have to do that. You should find 6 screws on the back of the primary cell. 3 are for collimation 3 are for fixing so you just unscrew the 3 fixing screws. Unless it's different with a bird Jones than an ordinary newt. 

[rwilkey]

Hi Freewheeler, I guess you are new to astronomy, and if your telescope is also new you should not need to collimate as yet, it is usually done at the factory.  If it gives good sharp views it does not need collimating.

[Charic]

Your scope has a spherical mirror which is factory set, and as long as you can see the three primary mirror clips through the finder,  without an eyepiece fitted, then to be honest, you won't receive any improvement, from further alignment The design of the scope allows for a tool less setup, no adjustments required? It's the design and nature of that type of telescope!

Like I said, go ahead and strip the scope, it encourages and allows you to better understand how scopes work, but don't expect too much with regards to the final image to what you have now.

My 127EQ was cleaned, collimated, I even used better eyepieces, and sorry to say, no improvements, the reason for having the scope I own now.

Sorry it's not what you want to hear, but those were my findings from my first scope, Celestron 127 EQ, very similar in design and layout to your own scope.

[freewheeler]

Thanks everybody. I've learned a lot from these answers to my post.Thanks for taking the time. I'm not worried that my scope is a pretty basic beginners one. There is no way I had the knowledge to invest in a more expensive one. If I continue on this learning curve maybe in a year or so I'll have a good idea of what I want in a scope and then will be the time to spend my hard earned pennies.

[Charic]

.........damm tablets, half my text is missing? on PC now?

I went on to say that the mirror should come out complete with the cell. This is normally achieved by removal of the perimeter screws that run around the outside of the  scope holding the mirror cell in place?

The screws on the base of the mirror cell  ( adjusters and locks control only  mirror alignment ) they should not be touched if just removing the whole mirror cell in one piece?

You said.." There is no way I had the knowledge ".......so for looking, learning or just  cleaning I encourage folk to be brave, who dares wins!  sometimes its the only way to understand how our system work, but if you fail or come unstuck, there lies a problem? But you also state your awareness that the scope is "basic"  but functional. My first views through my 127EQ on the Moon were  quite stunning, and the scope worked in daylight too, with its correct image adapter ?

 

 

 

[freewheeler]

I saw these screws but assumed that like all the other machine screws there were non captive nuts on the back side simply either waiting to fall off or once loosened just allow the screw to spin endlessly without undoing.  However now that I've looked closely I can see that the  nuts are bonded into the plastic end flange. So removal of the cell is straightforward. I accept the other advice which says that I am unlikely to actually achieve an improvement but I am very likely to go ahead and attach a donut to the primary mirror just for the experience.

 

 

[freewheeler]

I am now the proud owner of a donut...Thanks to 250 micron acetate sheet ( I had to buy a pack of 5 A4 sheets just to get a 120 mm square), a ring binder reinforcing ring, a cut down marker pen taped to a compass to draw rings on the acetate sheet), and a double whiskey to get my courage up. Actually it was simplicity itself. I attached the ring binder reinforcing ring to the back of the acetate by a dab of superglue on the removable centre disc which is designed to be pulled out and discarded.

[Timebandit]

Hello. Well done getting your donut. I see you got the order of priority correct with a whiskey being a key element?.

Sometimes a daunting task becomes so much easier especially after you have done it. I like the picture it's like something off blue Peter but a bit more important when it comes to getting your telescope set up. 

The next task is getting it all back together in the telescope and collimation of the scope and now with your donut in place hopefully a nice accurate collimation telescope. This way you can get the best from your scope.

Well done ☺

[Charic]

......I'm glad you had a go, simply for learning the mechanics of your telescope, and the blue makes it prettier than white, visually.  But without the donut, your laser tool  would have nothing to aim at, so you had to fit one in order to use your laser, but do  recall that a spherical mirror is less critical than a parabolic mirror, just requiring the  visual alignment of the primary mirror clips  as seen from the focuser. If the donut was essential to this scope, Celestron would have fitted one in the first place! And with the  Jones-Bird (that is correct! some say Bird-Jones? ) corrector lens that is installed in the base of the focuser,  it will disperse the laser beam from a pinpoint  to a spread of light? This spread of light illuminates the  mirror surface and the spot is seen as a  reflected shadow on the face plate of the laser tool, so this may cause some issues if not fully understood when you try to setup using the laser. Not sure if the blue donut, under a red laser light is affected in anyway to how visual the shadow is, compared to a white donut? You won't know until the time of testing arrives. I also don't think that the corrector lens should be removed in order to facilitate laser collimation.

However, once you have fully assembled the scope and collimated to the best of your ability, and it works, you will have become a master? or some way further to mastering  the art of collimation, and  how your telescope works, all beneficial when tuning your next telescope