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Good Mirror?

Reflector
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#1 Ghost332

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Posted 05 August 2021 - 04:39 AM

Having had 3 Newtonians in the past ( 6" f5, 10" f5 and lately but already sold 8"f5) I always got a little disappointed at the image quality when the magnification was pushed above 70x. I could never reach pin sharp focus on bright stars. The planets looked a bit fuzzy, and in good seeing, even though I could see the Cassini Division easily for example, I could always just tell I am looking through a Reflector. I used cheshire and lasers to get a spot on collimation and star test after, but still, it was never that good, even after letting the scope cool down.

So After deciding to get the 120mm f8.3 Refractor to be my only scope, I liked the image quality much better, and the CA didn't bother me too much, expect on bright stars and Jupiter.

Still, a 120mm is a 120mm Aperture and I missed deep sky performance of the larger apertures, especially on Globular Clusters. Having used the Refractor for a couple of month, I decided to finally give a try to the mighty 8" f6 Dob (Skywatcher). To say I am surprised is an understatement.

Last weekend, the 3 of us picked a dark sky site at the my friends little garden with fences. Bortle 3/4ish. Both the seeing and transparency was average. We have waited the planets to come higher up in the Sky. We Picked Altair in both scopes, and I did a quick Star test on the dob to check collimation, since it was an hour and half drive to get there. Star test looked good.

First Target was Jupiter, and all 3 of us could see a major difference. Jupiter was pin sharp at mag 70x, 150x and 225x. Even though the planet was floating a bit due to seeing conditions, we could easily got that snap of sharp focus. After we checked Jupiter in the Refractor, all 3 of us agreed that it looks much better in the dob. Next Target Saturn. Same story. In fact, it felt like you are looking directly at Saturn without anything in between. It was super beautiful and clean. Cassini easily spotted and so the cloud belts. After admiring the beauty of the planets, we went to check the bright star magnification test. At 70x, where the other reflectors started to show a bulky star image, I could still snap the focus to get pin sharp image. At 150x, It wasn't that sharp anymore, but still better than the other 3 was at 70x. After that, we went to check large open clusters with a 2" 40mm eyepiece and all 3 of us agreed again, its all better in the dob. We did check on galaxies too, but personally, I couldn't see any difference. Galaxies are needy targets and screaming for excellent transparency and darker sky. All other DSO, planetary and Globular were noticeably better of course in the dob, in fact, the globulars felt better than they felt in the 8" f5 before, noticeably sharper pinpoint stars.

How is this possible? Could it be just a well figured mirror, and one can have this dramatic difference in these Chinese mirrors? Or did I have bad luck with the other 3? Just to note, the 6" f5 back then was bought brand new with a perfect collimation, but it was nowhere near this good in terms of image quality even in good seeing.

Knowing that mirrors can be this good, I would really want the 10" dob again, but I am afraid, I would get a so-so optics again.

Thanks,
Zoltan



#2 Jon Isaacs

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Posted 05 August 2021 - 04:51 AM

Zoltan:

 

There are many factors that go into getting a good view of the planets in a Newtonian.  Thermal equilibrium is important and with a fan to actively cool the mirror, it may never actually cool down.  Seeing is critical and the best test of seeing is the quality of the view at higher magnifications.  And collimation is critical.  Very often inexpensive lasers are out of collimation themselves and need to be collimated before use.

 

If I had to guess, I would say you probably just experienced a night of good seeing. 

 

Jon


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#3 Ghost332

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Posted 05 August 2021 - 05:07 AM

Zoltan:

 

There are many factors that go into getting a good view of the planets in a Newtonian.  Thermal equilibrium is important and with a fan to actively cool the mirror, it may never actually cool down.  Seeing is critical and the best test of seeing is the quality of the view at higher magnifications.  And collimation is critical.  Very often inexpensive lasers are out of collimation themselves and need to be collimated before use.

 

If I had to guess, I would say you probably just experienced a night of good seeing. 

 

Jon

Hi Jon

I completely agree, but the 6" f5 should have been cooled more than once since I was using that scope for almost a year. With the 10", that might never happened, since I didnt have a fan, thats true. After the weekend I took the dob out again, the stars were twinkling more since we had a storm, and the transparency was excellent but according to the forecast the seeing was slightly below average. I still got the same level of image quality with the new dob on the planets, I just couldn't push the magnification that high. Its not necessarily the magnification only, its more the image quality that is noticeably different. I was thinking its because of f6, but that just comes with a slightly smaller secondary CO and less coma at the edge of the field. No idea what plays here, but if I had to compare to the 8" f5, I would say this feels premium compared to that mirror. Very pleasant experience nonetheless.



#4 Asbytec

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Posted 05 August 2021 - 05:56 AM

Well, it's my understanding and experience with Chinese optics are hit and miss. Anecdotal and my own, they seem to cluster around 1/6 PV of spherical aberration. Mine had astigmatism that I worked out using Lockwood's article, "Why Aren't my Stars Round". That alone improved my own scope, it might have been helpful to you, too.

Other differences you noticed could be observing conditions on different nights. One night observing is really not enough. Chinese mirrors are not premium, but you're likely to get a useable mirror or an occasional outlier that is rarely better or sometimes worse. You don't say much about an actual star test on any of the samples, other than a bright star magnification test (not sure what that is), so it's hard to really say much about the quality of the mirrors.

Edit: I agree with Jon. Dobs require a little prep prior to observing. A little TLC goes a long way.

Edited by Asbytec, 05 August 2021 - 06:04 AM.


#5 Jon Isaacs

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Posted 05 August 2021 - 05:58 AM

Hi Jon

I completely agree, but the 6" f5 should have been cooled more than once since I was using that scope for almost a year. With the 10", that might never happened, since I didnt have a fan, thats true. After the weekend I took the dob out again, the stars were twinkling more since we had a storm, and the transparency was excellent but according to the forecast the seeing was slightly below average. I still got the same level of image quality with the new dob on the planets, I just couldn't push the magnification that high. Its not necessarily the magnification only, its more the image quality that is noticeably different. I was thinking its because of f6, but that just comes with a slightly smaller secondary CO and less coma at the edge of the field. No idea what plays here, but if I had to compare to the 8" f5, I would say this feels premium compared to that mirror. Very pleasant experience nonetheless.

 

I have no idea either.  Collimation is certainly a possibility, at F/5, the primary should be within 0.63mm, at F/6, the tolerance is 1.1mm,  Have you checked the collimation of your laser?

 

My experience has been that the Asian mirrors are generally pretty good, certainly at 200x the views should be solid.  

 

Jon



#6 Asbytec

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Posted 05 August 2021 - 06:03 AM

My experience has been that the Asian mirrors are generally pretty good, certainly at 200x the views should be solid.

Jon


Yea, if the image is bad at 70x, something bad is wrong. Probably not the mirror.
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#7 CHASLX200

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Posted 05 August 2021 - 06:08 AM

Yea, if the image is bad at 70x, something bad is wrong. Probably not the mirror.

Collimation. Maybe Pinched optics or flat out bad optics. A good Newt in the 6" size will do 350x easy and a 8" 450x in my steady seeing. 10" 600x and on and on.


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#8 Jon Isaacs

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Posted 05 August 2021 - 06:10 AM

Yea, if the image is bad at 70x, something bad is wrong. Probably not the mirror.

 

I would say it slightly differently.  Something is probably badly out of adjustment.. 

 

It's unlikely that it's the mirror, which is a good thing.

 

Jon


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#9 Asbytec

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Posted 05 August 2021 - 06:18 AM

It's unlikely that it's the mirror, which is a good thing.

Jon


I had to work with my scope a little to get it working well. First was learning collimation and adding a fan. Then chasing down some astigmatism. Once that was done, I was doing magnifications on planets more in line with Chas and the Airy disc was cleaner. No more fuzz balls.
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#10 Ghost332

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Posted 05 August 2021 - 07:29 AM

I have no idea either.  Collimation is certainly a possibility, at F/5, the primary should be within 0.63mm, at F/6, the tolerance is 1.1mm,  Have you checked the collimation of your laser?

 

My experience has been that the Asian mirrors are generally pretty good, certainly at 200x the views should be solid.  

 

Jon

Looking at google images and examples of possible causes, it may looks like a mild astigmatism with a maybe a hint of a slight miss collimation but unsure. Somewhere in between, Its the closest example I found with these previous f5 telescopes. I have checked the collimation of the laser by putting it into a corner and rotating the laser. That looked good, however, if I rotated the laser in the eyepiece adapter, the laser itself showed the centre in a slightly different location. So, testing with laser and cheshire back and forth, I think I managed to get the collimation of these f5 scopes spot on. Interestingly, the f6 dob travelled nearly 150 miles from the shop, then I travelled 1.5 hours with it on slightly rough road, but only did a quick star test for collimation, yet it still bested (when it comes to image quality) all previous mirror scopes without touching the collimation screws, even on the next day after another drive. It is honestly indistinguishable from a refractor, apart from a slight sign of coma at the edge, even at low power. Maybe its what the normal should be, I don't know, but even my Fiancé, who occasionally takes a sneak peek, came with me after the weekend and said the same thing, that this dob somehow looks noticeably better than all the previous mirror scopes. There is a possible club meeting this month here in Hungary, hoping to take it there, so the more experienced fellas can take a quick look.



#11 Tom Stock

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Posted 05 August 2021 - 03:41 PM

First thing I would suspect is mirror cell and secondary mounting.  My 16" showed stars as you describe.  They were not pinpoints but tiny blobs.  The views were not terrible, but I could tell something was wrong.  Even after letting the scope cool, and re-collimating, while things did get better my 10" XT10i was still showing sharper images.

 

The original mounting was the primary siliconed to the cell triangles with cork around the edge.

 

I rebuilt the aluminum cell.  I used a 90 degree edge support system (two nylon screws under the edge) and I removed all silicone from the cell, replacing it with felt furniture pads.  I also removed the cork, and installed mirror clips to allow the mirror to move freely.

 

Something slick for pads would have been better to allow the the mirror to slide a bit easier, but it's what I had at the time and expect to replace them eventually.

 

I also re-mounted my secondary because it was mounted with a large piece of double sided tape.  I mounted it with three small silicone blobs, mirror spaced from the holder with pennies until the silicone set up.

 

Now when I look thru the scope (once it cools down) the stars are nice pinpoints as they should be.

 

Next question, WHERE did you set up? Did you have the 10" set up on a driveway in evening, or looking OVER your house or other building?

 

I found that anything which emits heat (warm driveway) causes terrible seeing.  When my home A/C system kicks on, I stop observing towards the house because the hot air from the compressor fan destroys the seeing.  The driveway is a more of a problem for a dobsonian if the bottom of the tube is open.  The hot air runs right up the tube.

 

For comparison, try setting up at 3am, after everything has cooled down, and don't observe the sky directly over a building.


Edited by Tom Stock, 05 August 2021 - 03:48 PM.


#12 GeneT

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Posted 05 August 2021 - 05:09 PM

You raised a very important point. Reflectors can provide excellent images assuming quality figure, spot on collimation, and mirror/optics at ambient temperature just to name a few parameters. Refractors are loved and used by a lot of people. The issue is not which can provide the best image, rather are the other parameters ratchet up to high standards. There is a different flavor in the views, but both refractors and reflectors can provide excellent results. I always wished I had built an observatory at a dark sky site. I would have loaded it up a larger Dob, a refractor or two with a few SCTs or MAKs thrown in. I did not build an observatory. I ended up with a 12.5 inch, F5 Portaball, and being limited to a single telescope, it was an excellent choice and served me well.    




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