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Polaris Reflectors


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#1 Tumblebug

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Posted 13 November 2020 - 09:36 PM

I thought I would post a picture of my Polaris Reflectors, the 130, 127, and 114, in that order left to right.  The 130 and 127 required extensive modification to combat my light pollution, but the 114 actually has the same or slightly better contrast and light pollution control as it comes out of the box with no modification for that problem.
 
Another change I made to all three is the support strips that hold the focuser draw tube in place, centered in the focuser body.  As they come, which is the same way focusers in all brands of entry level reflectors are made these days, is to have a single strip running down the tube, opposite the rack and pinion used to move the tube in and out.  A flat metal spring pushes the pinion gear into the draw tube's rack which keeps the draw tube pushed against the support strip.  That is OK as long as the scope is pointed horizontally, but no stars there!  The situation gradually changes as the scope is moved upward.  By the time the scope is pointed straight up, both the support strip and the pinion gear are on the sides of the draw tube and not above and below as before.  The draw tube can sag with a heavy eyepiece in place, affecting the scopes collimation. 

 

What I do is replace the original single support strip with three of my own devising, one in place of the original strip, and two more strips identical to the first one, placed 120 degrees from the first on either side of the rack and pinion.  This holds the draw tube in place evenly and lets the rack and pinion simply do the work of pulling the draw tube in and out.
 
For the Polaris 114, with its long focal ratio, the effect may not be much at all.  For the Model 130, with its shorter focal ratio of 5, compared to 8 of the 114, the effect of miscollimation should go up inversely proportional to the square of their focal ratio, or 1/8^2 for the Model 114 and 1/5^2 for the Model 130.  Another way to put that is any  miscollimation affects the Model 130 2.56 times as much as the Model 114. 
 
For the Polaris 127, whose primary mirror has a focal ratio (as best as I can tell) of 3.5, the effect for the primary mirror alone is 5.22 times the Polaris 114.  Then, there is the booster lens in the way, with massive amount of spherical correction in it to counteract the spherical abberation of its primary mirror....I have no idea how to calculate that.  Let me just guess, that miscollimation from any source is pretty much intolerable in the Model 127.
 
Another way of putting it:  The Polaris 114 is a pretty forgiving scope when it comes to optical errors.  The Polaris 130 does not like errors much and will show you in the view.  The Polaris 127 cannot stand them at all and will do really weird things to stars, as well as mess up what you can see on planets.  The 127 will do reasonably well with the Moon with some miscollimation, but don't get wild with it.
 
I will be talking about these scopes for a while and thought I would start with this.
 
If anyone has any questions about any of them, let me know.

 

Thanks,

 

Bill Steen

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#2 Tumblebug

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Posted 16 November 2020 - 12:30 PM

I had the three scopes out last  night, looking at the Double Cluster in Persius.  In case someone does not know where it is, look in the sky at Caseopea.  The main figure is a wonky W shape to the North, sort of on the opposite side of Polaris from the Big Dipper.  Go to the last two stars in the flattened half of the W.  Put the cross-hairs or red dot of your finder on a spot on the South side of those two stars.  The spot would make an equilateral triangle with the two stars, but go about half again farther away.  Look around a little bit through your scope with the eyepiece that will give you a wide view.  You should be able to find it in that general area.

 

Through the Polaris 114 and 127, that eyepiece is a 40mm Plossl type.  It gives me a 5mm pupil size, the largest my eye can take with my light pollution.  The Polaris 114 almost has a 2 degree field of sky and the 127 about 1 3/4 degrees.  For the Polaris 130, with its shorter focal length, I have to use a 26mm eyepiece to get the pupil size down to my 5mm limit.  The field is slightly larger than 2 degrees.

 

All three scope had reasonable contrast, with the 127 and 130 pretty well the same, and the 114 having the best.  All three had a sharp field in the middle.  The field around the object was tighter with the 127, and I would have enjoyed the view a bit more with the additional space.  I spent a good 15 minutes over all with each scope, look at this and that in the clusters with each scope.

 

The Polaris 130 showed some of the eyepieces field curvature in the outer third of the field. (looks out of focus, but can be brought into focus with the center of the field going out of focus.)  That scope also had some coma on bright stars, pointing away from the center of the field.

 

The Model 127 did not have the same kind of coma, but did have some spikes going through the star (like toward the center of the field and directly away from the center in a radial direction).  The spikes were not very long nor were they very dominant.  Going from one side of focus to the other, the direction of the spikes would change from a radial direction to circumferential.  The field also had some distortion which made the view look a bit surrealistic in the outer 60%.  Though I am no expert with all the various kinds of aberration, I believe this is pin cushion.

 

The Model 114 was reasonably sharp all over, did not show any noticeable aberrations, and was very nice in just about every way.  The stars were not as pinpoint as a good refractor might have, but were definitely better than the other two scopes.

 

As time went by, more tiny pinpoints of light showed up as my brain figured out what I was viewing until maybe double the number of stars had resolved themselves by the end of the session as I had at the start.  I did not notice any significant difference in the number of resolved stars in the view from telescope to telescope.  Over all, I would say the 114 gave the best view, the 130 second, and the 127 third.

 

I hope to view M33 next and will write about it soon.



#3 Tumblebug

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Posted 23 November 2020 - 10:35 AM

Viewing M33 was a partial success.  It is located in the Triangulum Constellation, which lies south of Andromeda and north of Aries.  M33 goes by a couple other names, the Triangulum Galaxy and the Pinwheel Galaxy.  It is the third largest galaxy in our local group, after M31, Great Andromeda Galaxy, and our Milky Way.

 

Triangulum is one of the constellation where the designations of the stars is a bit out of order.  Beta is the brightest at magnitude 3.0, Alpha is about 3.5, and Gamma is 4.0.  Alpha is the sharp point star on the triangle, located southeast of the other two.  The triangle is very long and narrow, reminding me of a spear point.  From my back yard, with my light pollution, I can see Beta all the time, but Alpha and Gamma are not immediately visible.  I have to look directly for one of them and wait a second or two for it to register in my mind.  The same goes for Gamma and I can only see one of them at a time, along with Beta.

 

Alpha Triangulum is the reference star that I use for finding the galaxy.  I put my red dot finder on the star, then look through my eyepiece and center it.  With the Polaris 130 and a 26mm Plossle or the Polaris 114 with a 40mm eyepiece, I have to go about 2 and a half fields to the west and a half field to the north to put me roughly in the area.  Then with a little searching, I can find the right spot.  This time, the object was east of me, but about 80% of the way to zenith.  Since I keep my focuser on the opposite side of the scope from the counterweight, the focuser was on the west side of the optical tube.  From that position, my reference was a rainbow shaped group of stars.  The galaxy looks like a thin fog at the top of the rainbow.

 

There is another star under the peak of the rainbow that forms a lopsided diamond with the three top stars of the rainbow itself, with the bulk of the fog I could see being contained inside.  From that point, the first night I observed it, I waited with my finder eyepiece to see what would show up with time.  Slowly, a few dimmer stars showed up and the fog became more apparent.  Eventually, the relatively uniform fog began to take on more definition with some differences in density in patches.  I went through this process with all three of the Polaris reflectors with the same basic result.

 

With my light pollution, my pupil does not open up wider than 5mm, even though it can open up to 6mm with the drops my eye doctor uses.  Because of that, I limit the eyepiece on the Polaris 130 to the 26mm, which gives just over a 2 degree true field of view.  With the 127, I can use the 40mm eyepiece, but with the longer focal length of the scope and the 44 degree apparent field limitation of the eyepiece, the field is 1 and 3/4 degrees.  For the Polaris 114 and its 900 mm focal length, the 40mm eyepiece allows for a field just under 2 degrees.

 

The Polaris 114 had the best contrast, though not greatly better than the other two scopes, but had the over all sharpest stars and the best definition of the stars.  The Polaris 127 had the least contrast and had some small spikes on the stars, but not any big coma as it has when more out of collimation.  I could see the patches in the galaxy but not quite as clearly as the other two scopes.  The Polaris 130 had contrast between the other two scopes, the stars were not quite a sharp as with the other two and had bits of thin coma away from the field center in the outer quarter of the field radius.  I think the grey tones showed a bit more subtlety in this view, but I am not totally positive.  However, the patches in the fog were just as apparent to me with this scope as with the 114, just not quite as obvious.

 

On a second night, with the 114 and the 130, I did have a momentary sense that there was a spiral in the patches, but it was so fleeting that I really cannot say I saw it or that it was anything other than my imagination.  I took the magnification down on all three scopes to a pupil size of about 1.25mm.  I had read that with larger scopes and, I think, more light pollution that this was a level to go to to get the light level at a point where the differences in grey were most apparent to the eye.  For me and my smaller scopes, I think this is too far.  I wanted a third night to investigate this more, while my eyes and optical memory are tuned in for that galaxy, but the weather intervened.  My best guess, at this point is that a pupil size of around 2mm will be about right.  As far as seeing the swirl goes, I have read that it takes maybe an 8 inch scope for that to show up.  However, I question that and intend to try again and see if I can get anything to show beyond random patches.

 

Bill Steen






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