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O Sigma 410 and my Polaris 130

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#1 MistrBadgr


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Posted 26 September 2015 - 04:07 PM

I decided to help on Sissy Haas's study on various separations and varying magnitude differences. As part of that process, I decided to use my three f/5 reflectors as well as my longer refractors up to 100 mm. The three reflectors are a Polaris 130, LX 70 8 inch, and Lightbridge 12. In R.W. Argyle's Book, Observing and Measuring Visual Double Stars, he talks very convincingly about reaching the Dawes Limit with reflectors with equal magnitude stars and a very good collimation. He did say that he doubted that a scope with as short of a focal ratio as f/5 probably could not make it due to its increased intolerance to collimation error compared to longer focal ratio scopes. As part of all this, I thought it would be a nice intellectual quest to find out how far an f/5 reflector can be pushed...hence, the f/5 reflectors in this process.

Last night, with a very bright moon along with my dark red zone light pollution, I did not think it would be a good time to start any tests on the subject stars, since that much interference would, I thought, certainly affect the results. Instead, I decided to test out the Polaris 130 with O Sigma 410, since it is at the Dawes' Limit for this telescope and is very near zenith during the times I can view it.

O Sigma 410 is located in Cygnus. Find Gamma Cygni, which is the middle star in the "cross." Then look for Nu Cygma to the east and a little bit North of Gamma. The symbol for Nu looks like a "V" with the right leg normally curved. Going from Gamma to Nu, stop about 40% along the way and turn South a tiny bit, and you will find it. I looked this up in several atlases and took my Sky and Telescope's Pocket Star Atlas with me. There are three stars in an angling row that are reasonably the same brightness in a large eyepiece field. The target star is the southern-most one.

I spent quite a bit of time getting the collimation just right, switching back and forth from lazer to cheshire collimators. After much fiddling, I seemed to have it dead on with both methods agreeing.

Taking the scope outside and finding the star was a bit more of a challenge than with the LX 70 8 inch when I viewed it before, for some reason, but I eventually did find it. I think that if I had a 6X30 RACI finder on the scope as well as a red dot finder, like on the LX, this would not have been a problem. Eventually, I did find it with no doubt.

At first, I used some of the eyepieces that I normally use for planetary observing. This specialized set has eight eyepieces with six elements. Try as I might, I did not have enough contrast and brightness on the star under the bright moon and light pollution conditions to determine if they were actually split by a hair or were just kissing. I finally gave up, deciding that I needed a better night with that bright moon gone.

I recently recieved one of the new sets of Plossles. I decided to get those out and take a first look with them. I worked my way up from the 32 mm, through 18, 13, 9, then 6 mm, on O Sigma 410. The image seemed brighter somehow. I went to the 2X Barlow with the 9 mm and finally the 6. With the 6mm and 2X Barlow, I was at a magnification of about 217X in the 130mm f/5 reflector, looking at a roughly mag 6.7 double star with a 0.9 arc second separation. The two stars were definitely split by a hair. I kept watching the object for quite some time, moving it from one side of the field to the other and letting it drift across many times. I could see the split over about 60% of the field.

I have proved to myself that a good f/5 Newtonian reflector with a top notch collimation can reach the Dawes' Limit on an equal pair near f/6.

I did see some coma along the way in the outer region of the field, this is normal for Plossles in an f/5 scope. I also saw some straight rayed starbursts at times. I normally see these from car headlights and believe that was coming from my eye lens and not the scope so much. I did not, however, see anything in the field that I would not consider normal under the circumstances. I will be trying out other things with these eyepieces and will let you know about what I find.

Now to find out what else this combination can do!!

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

#2 jiangshi



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Posted 29 September 2015 - 03:17 AM



This is an excellent post, and a good way to do some performance testing on the Polaris 130. As for collimating the 130 - the day before the Lunar eclipse, I decided to check the collimation on my 130, as I had purchased a laser collimator when I bought my scope. The factory collimation was off a bit. The laser point was about half way between the center of the mirror and the mirror's edge, so I decided to take a whack at doing my very first ever collimation. As expected, it is a learning process, and one that is going to continue for awhile. I recommend the process to anyone serious about maintaining their own equipment. The net results of my effort was that I did not ruin the scope, and I am no longer afraid to fiddle with the secondary and primary mirrors (I actually had them out of the scope). I have yet to do a star collimation to see just how good a job I did, but I did manage to get some pictures (digiscoping) of the eclipse, and the Moon was still round.


You mention a Cheshire collimator. I did not know about those, so while writing this, I did some googling. I see more learning experiences in my near future. And this of course, is one of my main enjoyments in amateur astronomy. By the way, which is better, the longer or shorter Cheshire?



#3 MistrBadgr


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Posted 29 September 2015 - 12:30 PM


Which Cheshire is best, probably depends on who you talk to. I have had both. I have not been able to find my long one, so I have been using the short one that was designed for refractors. With the short one, I can see all the way around the secondary mirror and see if it looks round, is a bit out of round or whatever. When I started this recent process, I found that I had my mirror cocked. It looks a bit oval, but the long axis was at about a 45 degree angle. The laser was lightning up right, but things really were not right. I then looked down the main tube and could see that the mirror was canted to the left and turned a bit. Sighting down the main tube, I loosened the center screw and turned the secondary mirror until it was straight and then just backed off a screw on the right side until the mirror looked centered. I was then able to get back on to the cheshire and make little changes until the mirror looked round and most things were centered....just not the cross-hairs in the Cheshire. I then did a collimation with the laser and tweaked in the secondary first, then the primary. I then went back to the Cheshire. Everything was centered up except for the cross-hairs. I had a screw in the right place on the focuser base, where it mounts to the main tube. I undid it and made a shim out of thin cardboard. With that in place, the cross-hairs was centered with the rest, but a laser check was now off. I tweaked that in, then went back to the Cheshire. Finally, everything was centered up.

When I did a star test, everything looked like it should. That is when I was able to make the split on O Sigma 410. Incidentally, in some catalogs, like the Washington Double Star or WDS Catalog, this object goes as STT 410.

If I find my long Cheshire, I will use it as well, just to see what the difference is, but so far, the short one has given me a nice double check by allowing me to see around the outside of the secondary mirror. I don't think I could do that with the longer one. I expect the cross-hairs on the longer one would be less fuzzy and easier to see, being farther away from my eye.

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

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