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ATTENTION DON PENSACK

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#1 alonzo

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Posted 26 April 2014 - 03:40 PM

....Don you have a collimation article in CloudyNights using a cayseye sight tube with crosshairs. My question is which should I believe the barlow laser combo or sight tube with crosshairs. I can get the sight tube with crosshars zeroed in with my sight tube on the primarys doughnut in both secondar and primary collimation. When I use the barlow laser it shows the primary as being of a little bit and primary needing a little more adjustment??????

Thanks
Alonzo

#2 sixela

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Posted 26 April 2014 - 05:21 PM

First of all, sorry for NOT SHOUTING and for not being Don Pensack ;-). [If you want to address a post to someone in particular, it's better to use the Private Message feature of the forum.]

Crosshairs are not very precise for collimating the tilt of the primary, but I assume that if you write "CatsEye" you mean you have a TeleCat or BlackCat and not just a TeleTube with crosshairs (which is a fine tool to place the diagonal under the focuser and to set the tilt of the secondary, but _not_ for the last tweaks in setting primary tilt. That's why the BlackCat exists, and why the TeleTube exists in its "TeleCat" incarnation that includes a BlackCat Cheshire).

The Cheshire ring concentricity with the centre spot reflection should essentially give you the exact same cues as the barlowed laser (the geometry is _the same_),...

...so if you see a difference then the reflected focuser axis isn't pointing to the primary precisely and one of the tools for setting the tilt of the primary isn't placed well for a balanced reading (i.e. one that is insensitive to the error in setting the tilt of the secondary).

To place a Cheshire well, first determine where the focal plane is (e.g. by pointing the scope to the moon and seeing where a focused image of the moon appears with a piece of paper in front of the empty focuser) and place the tool so that the focal plane is half way between the pupil of the tool and the Cheshire reflective ring.

To place a barlowed laser well, rack the focuser in/out until the minor axis of the silhouette of the centre spot matches the daimeter of the spot itself.

#3 alonzo

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Posted 27 April 2014 - 08:33 PM

....SIXELA if everything is correct,focuser alignment,secondary alignment and primary alignment which would I choose,laser barlow or sighttube. The laser barlow still shows it being a little off.

Alonzo

#4 pga7602

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Posted 27 April 2014 - 10:53 PM

....SIXELA if everything is correct,focuser alignment,secondary alignment and primary alignment which would I choose,laser barlow or sighttube. The laser barlow still shows it being a little off.

Alonzo


What brand of laser are you using? Not all of them are collimated out of the box.

#5 Starman1

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Posted 27 April 2014 - 11:44 PM

Some clarifications here:
1) to align the secondary under the focuser: Catseye TeleTube sight tube
or TeleCat combination sight tube and cheshire.
2) To align the focuser axis to the center mark on the primary by adjusting secondary tilt: Catseye TeleTube or TeleCat or simple laser beam.
3) To align the primary axis with the focuser: Catseye Black Cat cheshire, or TeleCat combo tool or barlowed laser.
4) To eliminate residual errors: Catseye Infinity autocollimator.

The barlowed laser is a primary adjustment tool. The sight tube is a secondary adjustment tool. You cannot compare them.

So, if the simple laser beam and sight tube don't agree?
--check the exact centering of the crosshairs in the tool. Sometimes a little bending will make them more accurate. Use a caliper.
--check to make sure the tool is registered accurately in the focuser**
--check the laser in a V-Block to make sure the laser is collimated. If it is not, collimate it first. The beam should rotate on itself without lateral movement at a distance of 15-20 feet.
--sight tube and laser beam will agree when the focuser axis is aligned with the primary mark.

If the barlowed laser and cheshire don't agree?
--look to the registration of the tools in the focuser**
--look to the visibility of the center hole in the shadow of the primary center mark on the bottom of the barlow or barlow window.

Note: the crosshairs don't enter into the picture in primary collimation. The barlowed laser is not used for secondary adjustment.
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**To insure alignment between the collimation tool and the focuser:
--make sure that only one thumbscrew in the focuser is tightened on the tool, and that it is the top-most screw.
--make sure the top screw does not tighten on the tool with a brass compression ring in between the screw and the tool. Move the ring around the focuser until the thumbscrew tightens directly on the tool.
--do not use 1.25" tools in a 2" focuser.

#6 sn1987a

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Posted 28 April 2014 - 12:15 AM

--make sure that only one thumbscrew in the focuser is tightened on the tool, and that it is the top-most screw.
--make sure the top screw does not tighten on the tool with a brass compression ring in between the screw and the tool. Move the ring around the focuser until the thumbscrew tightens directly on the tool.


Hmmm I learn something new every day :foreheadslap:


Doh'

#7 sixela

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Posted 28 April 2014 - 05:58 AM

....SIXELA if everything is correct,focuser alignment,secondary alignment and primary alignment which would I choose,laser barlow or sighttube.


As the sight tube is the tool for setting the tilt of the secondary and the barlowed laser (or BlackCat) is the tool to set the tilt of the primary, the barlowed laser.

I was under the impression that you were using a BlackCat Cheshire. If you're using 'the sight tube' and trying to make the cross hairs overlap the cross hair reflections, then that will be much less precise for setting the tilt of the primary than a barlowed laser.

But if the sight tube _then_ shows residual errors, then that means that you haven't done what you were doing with the sight tube precisely enough.

It's hard for us to guess what it all is, since you keep mentioning what tools you use but not exactly _what_ you do with them.

As Don pointed out, in your posts you're claiming two tools "disagree" while on the face of it they usually are used for different purposes, which means they can't actually 'disagree' on their _main_ visual cues.



#8 alonzo

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Posted 28 April 2014 - 08:41 AM

....Well here goes nothing. I will use my Hotech laser because it shoots a straight line to my primarys center dot in my doughnut.The Hotech provides accurate registration for this procedure. I will then use the hotech with my Seibert 2X barlow and use the registration device on it. The Seibert barlow is a very fine machined unit and appears to have good registration in my focuser. I will see how this works out.

Alonzo

#9 Starman1

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Posted 28 April 2014 - 08:53 AM

....Well here goes nothing. I will use my Hotech laser because it shoots a straight line to my primarys center dot in my doughnut.The Hotech provides accurate registration for this procedure. I will then use the hotech with my Seibert 2X barlow and use the registration device on it. The Seibert barlow is a very fine machined unit and appears to have good registration in my focuser. I will see how this works out.

Alonzo

Be sure to use a paper cover on the bottom of the barlow as in this tutorial:
http://www.cameracon...collimation.pdf
Hint: the paper can be on the barlow itself, and not on a filter housing.

If you let the beam pass through the barlow twice and look at it on the rear screen of the laser, the accuracy goes down, both because of a more defocused/slightly distorted primary marker shadow and because of the registration inaccuracies that are inevitable.

#10 alonzo

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Posted 29 April 2014 - 12:08 PM

....Don and to all others. I used the hotech laser to align my secondary and finished up with the barlow laser combination and all looks well. I checked it with the sight tube and all is concentric enwhich surprised me. The only concern I have is you have to be able to center the dot in the doughnut showing on the bottom of the barlow. I centered it the best I could but will always wonder if I am still a little off.

Alonzo

#11 alonzo

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Posted 29 April 2014 - 12:08 PM

Duplicate post deleted.

#12 Starman1

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Posted 29 April 2014 - 01:50 PM

....Don and to all others. I used the hotech laser to align my secondary and finished up with the barlow laser combination and all looks well. I checked it with the sight tube and all is concentric which surprised me. The only concern I have is you have to be able to center the dot in the doughnut showing on the bottom of the barlow.

Alonzo

Your last sentence confuses me.
One uses the laser beam without a barlow to center in the primary's center marker (secondary adjustment), then uses a barlow with the laser to center the shadow of the centermarker around the hole in the paper on the bottom of the barlow (primary adjustment).

If you meant "center the hole in the center marker's shadow on the bottom of the barlow", then that is correct.
Hint: If the hole in the paper is just a *trace* smaller than the hole in the center of the shadow, then a small thin ring of bright surface will show between the hole in the paper on the barlow and the hole in the shadow. If the hole in the paper is larger than the hole in the shadow, it could be harder to make sure the shadow is centered on the barlow. So making a hole of the right size in the paper on the bottom of the barlow might be a little bit of 'trial and error'.

#13 alonzo

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Posted 29 April 2014 - 03:03 PM

....Don I have a very distinct doughnut showing on my white disk at the bottom of my laser. The doughnut is white in its middle because of the white disk attached to my laser. There is a black dot with red in its center that can be moved with the primary adjustment knobs. I make adjustments to center the black dot as center as I possibly can inside the white area of the center of my doughnut..My barlow has a white disk on the bottom with a small hole in its center and was purchased from AstroSystems.The disk is from their laser-barlow combination that they sell.It fits my barlow perfect
and what I just described is all I know to do.

Alonzo

#14 Starman1

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Posted 29 April 2014 - 03:35 PM

....Don I have a very distinct doughnut showing on my white disk at the bottom of my laser.


OK, so far.

The doughnut is white in its middle because of the white disk attached to my laser.


OK, that's a sign the hole in the white surface is smaller than the hole in the shadow of the center marker. No issues so far.

There is a black dot with red in its center that can be moved with the primary adjustment knobs.


This is probably the small black area surrounding the hole in the laser, and the black area has nothing to do with the shadow of the center marker, though, I suppose, it might be helpful to use it to center the shadow of the center marker. The red in the center is the laser itself.

I make adjustments to center the black dot as center as I possibly can inside the white area of the center of my doughnut..


Sounds OK.

My barlow has a white disk on the bottom with a small hole in its center and was purchased from AstroSystems.The disk is from their laser-barlow combination that they sell.It fits my barlow perfect
and what I just described is all I know to do.
Alonzo


It sounds like you're doing it right. This should result in good collimation of the primary.






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