Collimation Question/Issue

I have an 8" Celestron Star Hopper that I've been happy with for over 10 years. It's the second generation "white rocker box" type that I believe were made for Celestron by Discovery.

While I've enjoyed the scope, I have a nagging suspicion that I have never been able to accurately achieve proper collimation.

Some background:

The secondary is held in place by a single post. This post is in turn mounted to the inside of the OTA and held in place by one of the screws that are used to mount the focuser.

Looking down the focus tube without an eyepiece, I can swear that it appears that the secondary looks like it's mounted too far "down" the OTA, and should be moved "up" closer to the "front" of the OTA.

I say this because it "looks" like the secondary isn't centered under the focus tube. I can clearly see the leading edge of the secondary as well as it's "back".

When I try to collimate the scope I go through the following process:

I marked the center of the primary years ago.

I use the Celestron supplied Collimation Tool that came with scope for the initial "rough" collimation.

This is followed up by a "fine" collimation using an Orion Laser Mate Deluxe.

Everything appears to go smoothly up to this point, but when I go out and do a star test, the shadow of the secondary and post is always off center, and the location of this off centered shadow is consistently in the same location, no matter how many times I repeat the process.

The diffraction rings are concentric and consistent – both in and out of focus – around this off center shadow, and the images from the scope are on par with that found on my other scopes.

If I move the primary to try and center the shadow when performing a star test, the resulting images are no better - or in fact may be a hair worse.

While I'm basically happy with the scope, I always have this nagging feeling that if I moved the secondary to a more "centered" position underneath the focus tube I could improve the situation.

Since I'm afraid to start "hacking away" at the scope, I wonder if anyone else has had the same issue, and what did they do about it?

Any help would be appreciated.

Thanks.

Mike Garretson

Mickael:

I don't really have a fix for you, I have had a couple of these scopes over the years so I think I can picture your problem.

- First, after talking to Terry Ostahowski who was the optical half of Discovery, it is my understanding that while the larger Celestron Starhoppers were made by Discovery, the 6 inch and 8 inch were made by Celestron. Both these scopes use a mirror with a spherical back (like and SCT) that is mounted by a single point in the center.

- Make sure that your Laser Mate is properly collimated, it is quite normal for inexpensive collimators to be out of collimation, they need to be checked prior to each use by rotating them in the focuser and seeing is the dot remains in one spot or if it rotates in a circle. If it rotates, then you need to collimate the collimator...

- For anyone unfamiliar with these Starhoppers, they have a single vane secondary mount but unlike other single vane secondaries which use a rod and adjustment is made by bending the rod, these are cast aluminum (I believe) and have three screws, normally nylon thumb screws if memory serves me correctly, to adjust the tilt of the secondary.

There is no easy adjustment to position the secondary in the scope, ie center it or offset as one desires, it's where it is...

(The Celestron First Scope 114 Newtonians (Standard F/8's) use this same design._

- I am not sure if there is a way to easily move the secondary towards or away from the mirror, I just can't remember... If there is a center screw of some sort, (and there "should" be), then the three tilt screws push against it and it can be loosened or tightened to move the secondaries position.

- If you are not seeing a round ring when everything thing seems to be in collimation, it means you may not be seeing the entire mirror and that the secondary does need to be moved. Or it maybe be that when the tilts are adjusted, because of the position of the secondary, the edge of the tube is partially blocking the image. The basic problem here, as you well know, is that the scope lacks the necessary adjustments...

- What I would do is start out by carefully measuring everything, particularly the position of the secondary in the tube. If you are going to do anything about this, you need to know what needs to be adjusted. A tape measure can be used to see how well centered the secondary is (offset is only about 0.075") in the tube. I would suggest a Cheshire or an Orion combination tool to determine the fore-aft position of the secondary, as previously mentioned, you may be able to do something about this. You also should check the squaring of the focuser.

Once you know what the problems are, if any, then you can decide what to do about them...

- If it were me though, I would be tempted to replace the single stalk with a standard vane type mount for the secondary. The single vane is nice because of the diffraction spikes but the lack of adjustment would frustrate me.

As a matter of interest in something unique, here's a photo of the primary mirror and mount taken from the side. The cell is quite heavy and probably slows down the cooling... Somewhere i heard the mirror was actually made from a C-8 blank but I don't know...




Good luck...

jon

Attached Image:

Michael Garretson said:

I have an 8" Celestron Star Hopper that I've been happy with for over 10 years. It's the second generation "white rocker box" type that I believe were made for Celestron by Discovery.

While I've enjoyed the scope, I have a nagging suspicion that I have never been able to accurately achieve proper collimation.

Some background:

The secondary is held in place by a single post. This post is in turn mounted to the inside of the OTA and held in place by one of the screws that are used to mount the focuser.

Looking down the focus tube without an eyepiece, I can swear that it appears that the secondary looks like it's mounted too far "down" the OTA, and should be moved "up" closer to the "front" of the OTA.

I say this because it "looks" like the secondary isn't centered under the focus tube. I can clearly see the leading edge of the secondary as well as it's "back".

When I try to collimate the scope I go through the following process:

I marked the center of the primary years ago.

I use the Celestron supplied Collimation Tool that came with scope for the initial "rough" collimation.

This is followed up by a "fine" collimation using an Orion Laser Mate Deluxe.

Everything appears to go smoothly up to this point, but when I go out and do a star test, the shadow of the secondary and post is always off center, and the location of this off centered shadow is consistently in the same location, no matter how many times I repeat the process.

The diffraction rings are concentric and consistent – both in and out of focus – around this off center shadow, and the images from the scope are on par with that found on my other scopes.

If I move the primary to try and center the shadow when performing a star test, the resulting images are no better - or in fact may be a hair worse.

While I'm basically happy with the scope, I always have this nagging feeling that if I moved the secondary to a more "centered" position underneath the focus tube I could improve the situation.

Since I'm afraid to start "hacking away" at the scope, I wonder if anyone else has had the same issue, and what did they do about it?

Any help would be appreciated.

Thanks.

Mike Garretson

Mike,
I agree with Jon Isaac's advice.
Have you looked at the collimation threads on "that other site?"
Bill Meyers