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Can a Celestron C102-HD Refractor be collimated?

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#1 audioaficionado

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Posted 09 May 2013 - 03:58 AM

Even with a new star diagonal and eyepieces, the view isn't right. There's blurriness in the image that can't be focused.

Apparently the previous owner took it apart and didn't know what they were doing.

Would this Agena 1.25" Cheshire Collimating Eyepiece for Refractors do the trick?

Does my Celestron C102-HD Refractor have the possibility for lens collimation adjustments?

#2 Jon Isaacs

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Posted 09 May 2013 - 05:28 AM

Even with a new star diagonal and eyepieces, the view isn't right. There's blurriness in the image that can't be focused.

Apparently the previous owner took it apart and didn't know what they were doing.

Would this Agena 1.25" Cheshire Collimating Eyepiece for Refractors do the trick?

Does my Celestron C102-HD Refractor have the possibility for lens collimation adjustments?


Hi:

At what magnification does this "blurriness" occurs, when do start seeing it? At high powers (200x), what does a star look like? Can you see the diffraction rings?

Jon

#3 Eddgie

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Posted 09 May 2013 - 07:22 AM

I too would like a description of how stars appear.

If this is the Celestron achromat that I am thinking of, then the answer is that no, it is not collimatable

Most important to know is that the reason that these scopes were not designed to be collimatable is because they don't need to be.

Moderate speed doublet refractors simply are not all that sensitive to mis-collimation errors. At the center of the field, the image will not be affected by miscollimation, and even at the edge of the field, the aberation that is introduced is a very slight defocus, but this should not be very easy to see visually.

So what this means is that unless there is a very serious problem, collimation should not matter.

What problems could there be?

If someone took the scope apart in the past, they could have put the lenses in wrong (backwards or someting) or they could have failed to get the lenses seated against one another (not fully against all three spacers or not fully seated in the cell.

If you have reason to suspect this, one thing you can do is to loosen the ring retaining the elements, then with the focuser pointed down, tap around the outside of the lens cell. This may help "Settle" the lenses into the proper postion.

The other problem with these scopes is that the focuser tubes sometimes get knocked out of alignement (or come from the box originally with the focuser tube out of alignment).

This can usually be sight checked by racking the focuser tube out and standing back and sighting downt the tube.

Center the light from the inside end of the focuer tube in the rear opening, and then as best as you can, site down to the objective to see that the inside end of the focuser tube looks concentric to the lens opening.

A Chesire will show the lens out of collimation if the focuser is tilted, so you won't know if the focuser tube is your problem or the lens is tilted, so why not start with this check.

And again, if it is not one of these things, there may be nothing you can do.

#4 csrlice12

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Posted 09 May 2013 - 07:52 AM

Check out this video. While the guy is using an ST80, the concepts (and for the most part everything else) would apply to the 102HD. Unique method of collimating the scope (Yes, it can be collimated wsing a Cheshire or laser believe it or not).
http://www.youtube.c...ature=fvwp&NR=1

#5 dan_h

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Posted 09 May 2013 - 10:16 AM

Even with a new star diagonal and eyepieces, the view isn't right. There's blurriness in the image that can't be focused.

Apparently the previous owner took it apart and didn't know what they were doing.

Would this Agena 1.25" Cheshire Collimating Eyepiece for Refractors do the trick?

Does my Celestron C102-HD Refractor have the possibility for lens collimation adjustments?


Read what Eddgie wrote and then read it again. I very strongly agree that these scopes don't require collimation to provide a good image, as good as it gets for these scopes.

Most likely culprit is the crown lens element has been put in backwards. This lens has two convex curves but they are not equal. The steepest curve should be against the spacers on the flint glass, the flatter curve points towards the sky. You can determine which curve is the steepest by catching the reflection of a lamp on the lens surface. The steepest surface will show a more magnified reflection.

dan

#6 audioaficionado

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Posted 09 May 2013 - 10:46 AM

Thanx for that YT link. I saw some similarities to my scope.

Weather's been kinda dodgy this week, but it clears up late at night after the thunderheads disperse. You know I got new eyepieces LOL.

#7 Jon Isaacs

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Posted 09 May 2013 - 11:22 AM

Thanx for that YT link. I saw some similarities to my scope.

Weather's been kinda dodgy this week, but it clears up late at night after the thunderheads disperse. You know I got new eyepieces LOL.


What did you see? What objects did you see this "blurrines" on, what magnifications, how steady was the seeing?

If you are looking at Jupiter at 200x, I would not be surprised if the image were somewhat "blurry."

Jon

#8 audioaficionado

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Posted 09 May 2013 - 12:21 PM

I saw Saturn and the only thing that told me it was Saturn was the general oval shape. I also looked at a street light with tree leaves around it and it also didn't look sharp enough. I'll get out tonight again, weather permitting, and get more details on what I'm seeing. I thought it was a bad eyepiece or star diagonal. This is not the case as they've been replaced with new.

The plastic lens retaining ring looks like it's not quite square either. I can see the spacers are still intact and in the correct 120 degree spread. I'll check out the crown element and make sure it's not backwards. The focuser needs a closer look too. I'm going to order that Cheshire EP.

While I have the lenses out, I might as well clean them and blacken the edges with a felt permanent marker.

#9 Jon Isaacs

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Posted 09 May 2013 - 01:20 PM

A few comments:

At what magnification were you working?

In your situation I would be cautious about basing any conclusions on Saturn. From San Diego, at 9pm Saturn is at 30 degrees and culminates at about 45 degrees. You are 9 degrees north so those numbers would be about 9 degrees, with Saturn so low on the horizon and the seeing were so-so, I would not be surprised to see Saturn as a blob.

A star test of a star near the zenith is best.

Also, taking apart an objective is not as easy as it seems and getting it back clean and without chipping the edge doesn't always happen.. The orientation can matter so mark it before you take it apart.

Myself, I would wait until I really knew there was a problem. An off-axis view of a star should tell you if the something is reversed in the objective.

JOn

#10 audioaficionado

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Posted 09 May 2013 - 01:41 PM

50x & 111x. 20mm & 9mm w/~55° AFoV.

The next local star party is June before I can have another person look at it.

Maybe I'm going blind and just don't know it yet. This scope has always sucked since I got it. I threw some money at it, but now I'll have to use my brains.

I'm going to order that Cheshire tool and verify the collimation.

There are smears on the inside lens surface which tells me either the factory has idiots assembling the scopes or the last owner was the idiot.

I'll get back after I've reevaluated the scope according to input in this thread.

Thanx everyone for the help and advice given so far.

#11 csrlice12

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Posted 09 May 2013 - 02:04 PM

With the smears, probably someone took it apart. Probably is the front lens in backwards or maybe a thin greasey coating from somebody's fingers who tried modding the scope before you got it. That grease could really affect the view. I don't believe the 102HD had a collimateable front cell (outside of how it was done in the video).

#12 audioaficionado

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Posted 09 May 2013 - 05:19 PM

Nope the front isn't adjustable, but I'll still make sure the elements are in the correct position and order. I'll loosen the retaining ring and gently shake it like in the video to make sure they're seated. It looks like the rear section/focuser interface can be tweaked a little.

#13 csrlice12

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Posted 09 May 2013 - 06:01 PM

If that doesn't work, you (or someone you trust) could check out the front crown lens and make sure it didn't get turned around. I'd say mark the lenses to put them back together properly, but, I'm willing to bet that horse has long left the barn. Be sure to clean the lenses while you're at it, and maybe flock/paint the OTA. Might as well do it while the scope is apart.

#14 TG

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Posted 09 May 2013 - 07:07 PM

If that doesn't work, you (or someone you trust) could check out the front crown lens and make sure it didn't get turned around. I'd say mark the lenses to put them back together properly, but, I'm willing to bet that horse has long left the barn. Be sure to clean the lenses while you're at it, and maybe flock/paint the OTA. Might as well do it while the scope is apart.


I suspect this as well. It could be either that the whole lens assembly is reversed (flint toward object) or an individual element is reversed. I'd first try reversing the whole assembly and see if that helps.

The crown lens is obviously bi-convex and goes first. It may even have equal curves in which case it wouldn't matter which side is toward the front. The flint lens is strongly concave on one side (toward the crown element) and weakly convex or even plane toward the eyepiece end. In between there are spacers which are placed 120* apart. Be sure to save them and put them back if they fall out. If they're stuck to the lens, it could be that the orientation of that element is correct but you never know if the previous owner did this wrong as well.

Tanveer.

#15 buddyjesus

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Posted 10 May 2013 - 01:42 AM

the crown is bi-convex but didn't focus when turned the wrong way entirely. all I saw was diffraction rings.

I collimated mine with a little work. I took off the three screws that hold the objective to give a little play that might center your optics(I had to enlarge the holes that hold the focuser on to get it dead on.)

procedure: remove objective screws and then put clear cello over the end of the dew shield. Mark the center with a dot. Put a collimated laser into focuser and rotate it to make sure the laser "circle" goes around the center.

#16 audioaficionado

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Posted 10 May 2013 - 03:11 AM

OK I went out at midnight and looked at Saturn again. Could see Titan too. Looked at Spica as it was bright enough to see under my orange-red skies. The biggest issue I could see was coma. Real big coma that didn't change orientation as I rotated the eyepiece or moved the star around in the field. When I defocused, the point was on one side of the fuzz balls, not in the center. Even made a ring of light around the ball initially during the process. There were also diffraction rings around the star that were in the middle of the coma touching the star point. The image looks slightly better next to the field stop, but not by much. I used my 20mm, 9mm & 5mm. The 5mm wasn't too good as could be expected. None of the views was acceptable. I've seen Saturn last summer through CATs and DOBs and one refractor and my scope looks like there is a sheet of Saran Wrap over the objective while their views were great.

#17 csrlice12

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Posted 10 May 2013 - 08:15 AM

Sounds like a mess. You didn't use buddyjesus's idea and forget to take off the platic wrap?

Sounds like it's tear into it time, you obviously have something in there backwards or that's some really nasty stuff on inside of the lens (I wonder if someone modded this scope's focuser first, had Lithium grease on his hands, and decided to tackle blackening the lenses or flocking/painting the OTA. Regardless, I'm willing to bet they took out the lenses and separated them without noting how they went together. I'm not an optics expert, but the fron lens more curved side should go towards the OTA, and rest on the spacers with the other lens. It's also possible this lens is backwards or something too. There's really only about 4 combinations you could make, give it a try, bet you'll have it up and running in a very short time.

#18 Eddgie

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Posted 10 May 2013 - 08:47 AM

Well, coma in a medium focus achromat does not indicate that the entire lens stack is out of collimation.

The reason for this is that achromats are corrected for coma, and tilting the stack won't introduce it.

If you are really seeing coma, then this would suggest that an individual element was tilted with respect to the other.

Are you sure it was coma and not astigmatism?

severe coma would not be a possiblity in a properly designed acromat even with mis-collimation unless there was some tilt in one of the elements.

If an air spaced doublet, it is the gap between the air gap and different curves between the lenses that corrects for coma.

If this gap is set correctly, the design would be coma free. This means that even if you tilted the objective lens pair toghether, there would not be any coma induced.

The effect of mis-collimation (by tilt) of a doublet is that the outside of the field becomes defocused (inside of focus on one side, and outside of focus on the opposite side).

This is why it usually isn't necessary to collimate a slow doublet. Unless the tilt is bad enough, our eyes don't see that the field is not evenly focused.

We don't collimate doublets to eliminate coma because coma is not present in doublet (though it can be in oil spaced tirplets).

When we collimate reflectors, it is for coma, but not doublets. Doublets are coma free.

Double-check to see if you are not seeing astigmatism.

If it is really coma, then this would indicate that some other condition besides tilt is present.

The first is that the lenses are tilted with respect to one another.

The second is that one lens was rotated with respect to the other.

Suppose that one lens has some astigmatiems in it.

If this condition exists, you can mate it with another lens that has the opposite condition.

When this is done, the leneses are usually marked along the outside to show where they should line up.

If you rotate one lens with respect to the other, the correction for astigmatism is lost.

If you see astigmatism, the best thing to do would be to drop the lens stack out and check for registration markes on the edges of the glass.

If you see astimatism, I would also look for tilt of the focuser tube.

Anyway, coma is not a normal condition in a doublet, and if you really see it, it indicates that the lens stack is not spaced properly.





#19 audioaficionado

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Posted 10 May 2013 - 09:23 AM

Well it looks triangular like a comet with a short fan like tail. It doesn't look oval like the fuzz balls I see at night without my glasses. I have astigmatism.

I loosened the plastic retaining ring and gently rapped the ota as it was vertical to set the elements and then retightened it until they just stopped making any noise. I didn't want to over tighten and stress the glass. The silver colored spacers appear to be intact between the objective elements.

The lens stack is coming out for inspection/cleaning and the focuser is going to get cleaned and tightened also.

#20 Zamboni

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Posted 10 May 2013 - 10:33 AM

Triangular with a fan tail? Sounds like a combination of one lense being tilted in respect to the other combined with pinched optics.

#21 audioaficionado

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Posted 10 May 2013 - 10:55 AM

Glass is a super cooled fluid. I hope the dozen years it was stashed in the previous owner's garage wasn't enough time to permanently warp the lens elements while they were under stress.

#22 csrlice12

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Posted 10 May 2013 - 11:22 AM

I imagine after you clean it all up, and get the front lens configuration set right, and tune up the focuser, it'll work fine. Those C102HDs are actually a pretty decent scope. Be sure to look to see someone didn't put the spacers back on the wrong side of the lens too..seems the front lens assembly is where your problem lies.

#23 Eddgie

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Posted 10 May 2013 - 10:55 PM

Attached is a picture that shows three conditions.

The top is coma. When you say "triangular" some could see that the in focus star has a somewhat triangular shape.


The second image is astigmatism. notice that in focus, the star looks like a cross, and out of focus, the pattern is stretched in different directions on either side of focus.

The last image is triangular and is "Pinch" caused by having the retaining ring to tight.

Which of these most resembles what you see?

Attached Files



#24 audioaficionado

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Posted 11 May 2013 - 12:01 AM

Coma looks much closer than the pinched one. Astigmatism doesn't look at all like what I'm seeing.

I suppose it's possible to have a combination too.

Too busy today working on my front brakes, but I'll get into the guts of this scope soon and see what I can find out.

#25 buddyjesus

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Posted 11 May 2013 - 01:15 AM

might still be a bit too tight. I hear a tiny click when moving between horizontal and vertical just to give you an idea.

what eyepieces are you using to test? Plossl eyepieces give me about a 20% area around the margin that has aberrations.






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