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Collimation Help


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#21 MistrBadgr

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Posted 13 June 2020 - 09:19 AM

Hi John,

 

At this point, it can get to be a bit of a guessing game, since very small moves in the secondary can make a big difference.  Most likely, the secondary mirror is still out of position a little bit, either rotationally or axially.  It could, however, be the way the primary mirror is made.  It could be off a tiny bit. However, I think secondary mirror position or rotation is probably the culprit.  If we call the focuser up and the opposite side from the focuser down, if you have to move the primary mirror up or down a bunch, compared to its position when you are aligning it from the front of the scope, I am thinking it means there is something still wrong with the mirrors position along the axis of the scope.  If the movement is in a different direction, rotation of the secondary mirror is most likely involved,

 

I do think a star test is in order, because that is the final verdict on collimation.  You can make adjustments and try to get the primary mirror into its best collimation by judging how a bright star looks through your 9mm eyepiece with the star centered.  When you turn a screw, the whole image will move.  In order to make a judgment on the move you made, you have to move the scope to get the star back into the center of the field again. 

 

I have a mental block on remembering which way you want the star to move in the field in order to get the defocused image to go circular, when it is not round or you have coma.  Seems like you move the star in the direction of the elongated part or the direction the coma is going.  However, I always have a doubt that I need to go in the opposite direction.

 

Strengthening the focuser is a good idea and helps keep it truly perpendicular to the tube and pointing directly at the main tube axis.  To get things dead on, the axis of the draw tube and the axis of the main tube have to intersect at a 90 degree angle, which is hard to hit or even to measure. 

 

I have made attempts to verify the focuser position and direction using a laser collimator before, but that gets pretty exhaustive.  I am reluctant to suggest that you do that.  My method was to pull the secondary mirror out of the way, then use the laser to shine on the far side of the optical tube.  Using a ruler, I made a best guess at how far down into the main tube the red spot was located, and then measure the best I could to the center of the draw tube from the front of the scope.  The distances should be the same.  That is terribly crude.  I then eyeballed the position of the red dot circumferentially by peeking around either side the spider leg adjacent to the red dot.  Again very crude. 

 

I do not know if the final position was really good enough, but I did find the dot maybe a quarter of an inch off of where I thought it should be and tweaked the metal a bit to get the dot where I thought it should be.  After I fixed that finder attachment piece, I checked the position again and had to tweak it back to about the same spot it started at.  With that said, I am hoping you do not have to so something like that.  You have worked over that finder piece already.

 

Taking the secondary mirror out, making changes like that, then getting things all back together is a pain.  Then you have to go through the whole collimation process again.

 

Well, that is probably enough advice and rabbit trails followed.

 

Hope it all helps,

 

Bill


Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma

#22 John

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Posted 13 June 2020 - 03:48 PM

Hi Bill,

 

Sorry for the delayed response, I didn't know there was a second page to this topic. Lol.

 

Thank you for helping me through this, I definitely appreciate the help. 

 

If the spider looks centered through the focuser but the star test shows that the scope is not collimated, then what I need to do is adjust the secondary mirror? 

 

I have also noticed that when I go to focus on an object, the focusing tube that the eyepiece goes into always shifts down. This is what is messing up my collimation, right? I hope to fix this when I reinforce the focuser. Speaking of which, how did you remove the focuser tube from the focuser body? I see a metal ( or plastic, I'm not sure) block at the end of the focuser tube that prevents the focuser from sliding all the way out. 

 

Thank you for the advice, 

 

John



#23 MistrBadgr

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Posted 13 June 2020 - 05:30 PM

Hi John,

 

If the spider looks centered through the focuser but the star test shows that the scope is not collimated, then what I need to do is adjust the secondary mirror?  Most likely, you are correct.  Either that or the draw tube is not pointed at the secondary mirror exactly right (read close enough that you cannot tell the difference in the stars)

 

With that situation, I recommend that you make primary mirror adjustments with the stars and get things looking as good as you can.  (Here is where a laser came in handy for me one time.)  When I was back inside, I used the laser to make a spot on the primary mirror, to see how far the collimation had been off.  I then moved the secondary mirror in the opposite direction by that amount to move the red spot to the middle. (I think it was that amount, but I could have gone half way instead)  I then moved the primary mirror to put the red dot back through the hole in the middle of the laser screen.  (I cannot see a way to duplicate that with a Cheshire, but there may be one. I will think about it.)  The next time out, I repeated the process.  Seems like the second time I did that, I hit the mark as far as I could tell.

 

Speaking of which, how did you remove the focuser tube from the focuser body?  Movement in the draw tube inside the focuser can definitly cause problems like what you are seeing.  I think it is definitely a big part of the problem at this point.  This business of supporting the draw tube firmly inside the focuser is one thing I wish the manufacturers could agree to do and not leave out the extra two strips of slick material.  What could it cost to put those in when assembling a focuser, 25 cents?

 

Speaking of which, how did you remove the focuser tube from the focuser body?

 

Look on the underside of the focuser, where the knob shaft goes through.  There is a plate holding the knobs, shaft, and the pinion gear in place.  There is a piece of spring steel in there, bent to hold the pinion gear pushed up against the rack on the draw tube.  Take those screws out and those things can come off.  They put some sticky gooey grease on the gear and rack, so watch out and keep it off of your fingers.  Or be sure to clean it off your hands if you do get some on.  That stuff can end up on an optical surface.  The draw tube can then be pulled straight out.

 

Looks like you are making progress with your scope and also with your vision on how to make it work.  You are getting there!

 

Hope this helps,

 

Bill


Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma

#24 John

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Posted 13 June 2020 - 06:20 PM

Hello Bill, 

 

At this point I feel like I am going in circles. Every time I think I have it, something else is wrong that needs to be fixed, which then messes up what I have fixed already. I will admit, I am starting to get a little frustrated. I will try adjusting the focuser to see if I can get that to help, but if that doesn't work, I am clueless on what else to do. 

 

Thank you so much for sticking with me through all of my confusion and for helping me try to get my telescope in better shape. 

 

John



#25 John

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Posted 14 June 2020 - 06:50 AM

Hi Bill, 

 

I have reinforced the focuser using your method, and it is a lot more stable. It doesn't move side ways at all when I move it in and out. That is a genius idea. 

 

Now all I have to do is try and recenter the primary mirror and get that aligned, but I think that I will be able to do it. I will update you on my results in a bout a week or so, as I will not have wifi or cell service for the next week. 

 

Thank you for all of your help, I really appreciate it. 

 

John



#26 MistrBadgr

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Posted 14 June 2020 - 07:06 AM

Hi John!

 

Stabilizing the focuser will definitely give you more consistency, which will diminish the problem, if not allow you to completely solve it.  I will be very interested in how this turns out for you! :)

 

You are definitely welcome for the help! :)   That is the reason I am here on this forum.  when I can help someone, it brightens my day.

 

Best Regards,

 

Bill


Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma

#27 John

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Posted 22 June 2020 - 04:37 PM

Hello Bill, 

 

I have fixed the mirror alignment issue. I got the secondary mirror centered in the focuser, and then I used wood chips to move the entire focuser assembly to get the focuser centered in the secondary mirror as best as I could do. It is not perfectly centered, but it is as close as I can get it.  I then star collimated, and everything is working properly. 

 

Thank you for helping me though this and for helping me achieve good results. 

 

I just have a question on upgrading eyepieces; would the Meade Wide angle aspheric eyepieces that cost $35 be a good upgrade from the included Modified Achromat eyepieces? Or would it be better for me to by the more expensive ones?

 

Hope you are well, 

 

John



#28 MistrBadgr

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Posted 22 June 2020 - 08:00 PM

Hi John,

 

Glad you made it through all the jungle of "things" and ended up with a good collimation! :)  The major victory from all of this is you now know how to do it and are very familiar with the functional process of your scope.

 

You are most welcome for the help!  Having the opportunity to help someone like you every now and then is what keeps me going!

 

As far as eyepieces go, I cannot comment on the aspherics that Meade is selling since I have not used them.  My instincts lead me to the conservative approach of recommending Plossl eyepieces to you.  They are more expensive than the ones you mentioned, but that design has been worked over by different people and the result has been a lot of bang for the bucks. 

 

What Meade is currently selling as their 4000 series Super Plossl eyepieces are good, honest workhorse eyepieces.  They are sharper in the middle of the field than the wider field eyepieces in their price range.  They are the best ones I know about for lunar, planetary, and double star work. 

 

All Plossls being sold in that price range have a design issue related to field curvature, which means the exact focal point for objects on the edge of the field is slightly different than objects in the middle of the field, so the edge objects will seem a little fuzzy when you focus on a middle field object.  However, if you focus on an outside object, then it can be sharp while the middle field object gets a little less sharp.  This is normally not a big deal, except for people who have been used to using high dollar eyepieces.

 

I think what I need to do is to get out my Polaris 127 and use my Plossls with it, and maybe compare those to some other eyepieces, then get back to you.  With the scope having the need for the booster lens to not vary much from a particular location, the choice of a particular type of eyepiece can be important.  We have some rain forecast, but there should also be a reasonable night, not too far away. 

 

Again, congratulations on getting your scope going with a good collimation!

 

Best Regards,

 

Bill


Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma

#29 John

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Posted 23 June 2020 - 10:03 AM

Hi Bill, 

 

Would the Plossl eyepieces be good for deep sky viewing? That is what I use my telescope for 90% of the time. 

 

Thanks, 

 

John



#30 MistrBadgr

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Posted 23 June 2020 - 04:21 PM

Hi John,

 

They can be.  They are sort of a "jack of all trades" type eyepiece by today's standards.

 

Investigating deep sky viewing with f/8 scopes is a project I have been working myself up to for a while and the Polaris 127 is one of the scopes I will be using.  I have a 60mm refractor that I put together with a focal length of 487mm, a 100mm refractor with an 800mm (old Meade DS-2102 optical tube that has been upgraded in several ways), a Polaris 114, and the Polaris 127 are the test scopes.

 

My intent is to be using some eyepieces that are affordable and justifiable for a new person in astronomy.  The designs are known as either Erfle, or I think, Konig with possibly some modern alterations.  Most are listed on line as house brand eyepieces at several on-line stores.  They will normally be listed as WA and SWA.  The SWA normally claim to have a 70 degree field of view, and will normally have five elements in the lens, even though there is one 6mm that has six elements.  Those are, I think, Erfles or derivatives.  Mr Konig was trying to make a less expensive eyepiece than Erfles by designing eyepieces with four elements instead of five.  These Konigs will normally have a 65 degree field of view, have the designation of WA for  eyepieces in the 15 to 20mm range and 60 degrees for shorter focal lengths.  There are, however some exceptions at 15mm where the eyepiece goes all the way to 70 degrees.

 

All of these eyepieces are not quite as sharp in the middle as a Plossl, but have a wider field of view and a better image toward the outside.  They work very well with f/8 scopes, but I am not sure yet for the Polaris 127 type.

 

With an f/8 scope, a 10mm eyepiece will give a pupil size (diameter of the image going into your eye), which seems to be about the right situation for decreasing the effects of light pollution and allowing your eye at light levels that it is good at detecting small differences in light intensity.  You might want a few of those for deep sky viewing.  Unfortunately, Meade does not sell any of those eyepieces at the lower priced end of things.

 

With a 60 degree field of view, the Meade HD 60 eyepieces work very well in my Polaris 127 are are really my go to eyepieces for that scope.  I have not priced them recently, but I would expect them to run around $90 and are good for all types of viewing.

 

I will talk more later.

 

Best Regards,

 

Bill


Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma

#31 John

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Posted 24 June 2020 - 04:36 PM

Hi Bill, 

 

I think what I will do is wait a little bit on upgrading for now. 

 

Thanks again for the help on collimation and eyepieces!

 

John






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