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Focuser alignment?

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#1 Kim2010

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Posted 25 January 2023 - 05:14 PM

So I know this maybe a bit on the OCD side, but since the event of this post:

 

https://www.cloudyni...xis/?p=12447258

 

where the diagonal bumped into the control box strapped to the tripod leg, I have been quite obsessing whether it affected the focuser. It surely hadn't affected the objective, right, as that end did not touch anything-- but the focuser and diagonal, yup, these were the "impactees" with the control box.

 

So how do I check if there was anything up with the focuser/diagonal end of the SW 100ED? I don't feel any play in the focuser tube, except when I insert a diagonal which I think is normal, right? I mean the play is between the diagonal and the focuser tube which tightens as soon as you tighten the retaining knobs.

 

 



#2 Couder

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Posted 25 January 2023 - 05:20 PM

I check mine by using a laser. I put it in a "VEE" block on a table and make sure the red dot on the wall does not move when I rotate it. Make sure it is a tight fit in the focuser. See if it hits the center of the lens. Then rotate it maybe 90 degrees and see if it still hits the middle. Move the focuser in and out too. I believe this will tell you if it is centered with the lens.



#3 Kim2010

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Posted 25 January 2023 - 05:26 PM

I don't have a laser pointer that can fit the focuser.

 

If there was a misalignment, what would be the "symptoms'?



#4 Oddyse

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Posted 25 January 2023 - 05:57 PM

.... Daytime pick something a couple of hundred yards away. If your scope focuses with low, medium, and high power eyepieces (assuming you also use it for visual) it is probably okay. Regards


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#5 Kim2010

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Posted 25 January 2023 - 06:06 PM

Yup focuses fine :)



#6 Eddgie

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Posted 25 January 2023 - 06:06 PM

I don't have a laser pointer that can fit the focuser.

 

If there was a misalignment, what would be the "symptoms'?

At f/9, it is unlikely that there would be any symptom at all. At this kind of focal length and focal ratio, a bit of focuser misalignment is meaningless unless you are imaging. If you are imaging, focus wont be the same in all four corners of the chip.

 

Visually, the only test you can do is kind of insensitive but if you don't have a laser, then it is all you have.  To do this test, you need a very bright star like Sirius and a wide field eyepiece.

 

Bring the star exactly to the center of the field and defocus about 10mm. 

 

Starting from the very center of the field, move the star all the way in any direction. As you get to the edge of the field, you should see the front end of the focuser baffle cutting into the light cone. You will know it when you see it.  Here is the subjective part. You have to estimate how many millimeters from the edge of the field stop that happened.

 

Next, move the defocused star to the opposite side and do the same observation. If the focuser is tilted one of those directions, you will see that the cone starts to cut off a bit sooner or later than the other side.

 

Repeat this at 90 degrees and 270 degrees from where you started. 

 

The thing is that you need more than just a small amount of tilt to actually be able to see this happen unevenly unless you have a very precise way to measure where the cutoff occurs and in an f/9 scope, the fully illuminated field is going to be really large, maybe like 30mm or so, so your field stop has to be at least this big and probably a big bigger, but the closer the field stop is to the fully illuminated circle, the more senstive the test will be.  If the circle is 30mm and you use a field stop that is 31mm, the cut should start 1mm inside the field stop in all directions.

 

Does that make sense?

 

You can buy a laser for $25. Much easier and more accurate.

 

Here is the bad news though.. In most new scopes, if the focuser is titled, you really can't do anything about it. 

 

The good news though is that because of the way they are made, it is highly unlikely that you have bumped it out of alignment and at f/9, it would take some pretty serious tilt to cause a problem. 


Edited by Eddgie, 25 January 2023 - 06:07 PM.

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#7 Kim2010

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Posted 25 January 2023 - 06:11 PM

At f/9, it is unlikely that there would be any symptom at all. At this kind of focal length and focal ratio, a bit of focuser misalignment is meaningless unless you are imaging. If you are imaging, focus wont be the same in all four corners of the chip.

 

Visually, the only test you can do is kind of insensitive but if you don't have a laser, then it is all you have.  To do this test, you need a very bright star like Sirius and a wide field eyepiece.

 

Bring the star exactly to the center of the field and defocus about 10mm. 

 

Starting from the very center of the field, move the star all the way in any direction. As you get to the edge of the field, you should see the front end of the focuser baffle cutting into the light cone. You will know it when you see it.  Here is the subjective part. You have to estimate how many millimeters from the edge of the field stop that happened.

 

Next, move the defocused star to the opposite side and do the same observation. If the focuser is tilted one of those directions, you will see that the cone starts to cut off a bit sooner or later than the other side.

 

Repeat this at 90 degrees and 270 degrees from where you started. 

 

The thing is that you need more than just a small amount of tilt to actually be able to see this happen unevenly unless you have a very precise way to measure where the cutoff occurs and in an f/9 scope, the fully illuminated field is going to be really large, maybe like 30mm or so, so your field stop has to be at least this big and probably a big bigger, but the closer the field stop is to the fully illuminated circle, the more senstive the test will be.  If the circle is 30mm and you use a field stop that is 31mm, the cut should start 1mm inside the field stop in all directions.

 

Does that make sense?

 

You can buy a laser for $25. Much easier and more accurate.

 

Here is the bad news though.. In most new scopes, if the focuser is titled, you really can't do anything about it. 

 

The good news though is that because of the way they are made, it is highly unlikely that you have bumped it out of alignment and at f/9, it would take some pretty serious tilt to cause a problem. 

 

The thing is I am still shocked how a seemingly gentle and short bump cause the Dec axis to malfunction/disengage? I still have not opened it up yet, so I don't really know what the issue is. I am just using the mount manually PLUS the RA tracking. So tracked, but no goto for now.



#8 Oddyse

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Posted 25 January 2023 - 06:56 PM

.... Hope it's fine. As Eddgie says an f9 focuser really has to be out of whack to see it, at least for visual use. Regards


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#9 Rutilus

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Posted 26 January 2023 - 07:30 AM

Here is the bad news though.. In most new scopes, if the focuser is titled, you really can't do anything about it. 

If things are really bad and the scope has not been sent back., then you can  make a rear

adjustable  plate to get collimation. I did this with my 6 inch f/15 refractor, so that I can look through

the eyepiece and  adjust collimation from the back of the scope. Once set it would take the weight

of heavy eyepieces and cameras.

p.s. I've just this week started work on such a back plate for a very fast f/2.8 refractor.


Edited by Rutilus, 26 January 2023 - 07:34 AM.

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