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Airy disc distortion

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#1 Cirus

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Posted 20 February 2019 - 07:56 AM

Hello,

 

I have 80mm / 400 refractor, Meade Adventure scope

 

At x133 magnification, a focused star looks like this

 

star mis.png

 

 

Is this caused by miscollimation?

 

Another issue that might be related, when I unscrew the focuser dust cap, the focuser tube will turn with it.

 

Can this be fixed by tightening the focuser tube? Is it tightened by the 2 allen key screws near the focuser tightening knob?

 

Thanks in advance for any advice.



#2 AxelB

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Posted 20 February 2019 - 08:17 AM

This certainly looks like miss collimation. I would first look for a focusser miss alignment.

My AR127 suffers from a leser case of the same. The previous owner obviously dropped the ota and there’s sign of it on the focusser. I plan to buy a laser to help me fix this potential focusser miss alignment. If doing that is not enough, I’ll adjust the lens cell.
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#3 siriusandthepup

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Posted 20 February 2019 - 09:04 AM

Make a small cardboard circle to fit over the objective. This disk should have a small centered hole - say an 1/8" or so.

 

Align the focuser and tighten it down with the laser shooting out the front of the hole.

 

Now you can adjust the lens cell, if it needs that too, using the standard reflection centering method. I like to use a Cheshire for that.


Edited by siriusandthepup, 20 February 2019 - 09:11 AM.


#4 AxelB

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Posted 20 February 2019 - 09:46 PM

You can also use a star to fine tune collimation if the seeing is good. You can also get pretty close using an artificial star if you have enough space in your house to place it far enough to achieve focus. The longer the focal length, the more space needed. 400mm will fit in most house if you have a long corridor or lots of open space in the basement. I just checked today and my 825mm barely fit to focus my artificial star across the length of my whole house! I had to put the star in the furnace room so it would be far enough!

I corrected the small miss collimation but I still need some still and clear night sky to do the final adjustment or confirm everything is now perfect.

Edited by AxelB, 20 February 2019 - 09:53 PM.


#5 Cirus

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Posted 21 February 2019 - 01:06 AM

Thanks for your help guys!

I'm going to star test again tonight to ensure it wasn't an anomaly.

I will also rotate the diagonal to ensure that it isn't the issue.

How do I tighten the focuser to prevent the focuser itself from rotating?

To collimate, do I tighten and loosen the 3 screws that are on the OTA tube near the focuser?

Edited by Cirus, 21 February 2019 - 01:38 AM.


#6 Ernest_SPB

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Posted 21 February 2019 - 01:59 AM

> Is this caused by miscollimation?

 

Certainly...

Namely this coma is caused by displacement of one element relating another in objective lens. Quite usual defect in fast refractors.

 

> Another issue that might be related, when I unscrew the focuser dust cap, the focuser tube will turn with it

 

It can not be related to the coma in center FOV. It could cause astigmatic distortion, but not a coma. 


Edited by Ernest_SPB, 21 February 2019 - 02:00 AM.


#7 Redbetter

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Posted 21 February 2019 - 02:15 AM

Hello,

 

I have 80mm / 400 refractor, Meade Adventure scope

 

At x133 magnification, a focused star looks like this

 

attachicon.gif star mis.png

 

 

Is this caused by miscollimation?

 

Another issue that might be related, when I unscrew the focuser dust cap, the focuser tube will turn with it.

 

Can this be fixed by tightening the focuser tube? Is it tightened by the 2 allen key screws near the focuser tightening knob?

 

Thanks in advance for any advice.

That looks like a fairly harmful level of coma from miscollimation or misalignment somewhere.  It will kill detail.

 

I took a look at the stock focuser (since removed and replaced with a 2").  The focuser tube can't turn because of the rack.  But the visual back attached to it can rotate, if it is not screwed on tight.  Tighten that onto the draw tube and it should stay put.  The 2 tiny allen screws on the top should press down on the focus tube to maintain draw tube tension so that it doesn't move in and out.  I believe they are 1.5mm socket.  I doubt that the focuser draw tube is so loose that it is causing that level of misalignment, but it could be off to some extent or the focuser or objective cell could be misaligned with the tube or each other. 

 

Below about 40 degrees the Synta glue...errr...grease on the focuser gets stiff--and by the mid 20's problematically stiff from what I remember.  I eventually disassembled the bottom bracket covering the ST80's focuser spring and gears so that I could clean all of that tacky goo off of the rack and gears.  It has worked better without it, and I no longer end up with sticky gunk on my hands if I inadvertently touch the rack.   I had to adjust the tightness of the focuser rack & pinion cover plate screws a few times until it meshed properly in cold weather, until I finally removed that blasted grease.  It has behaved since then.

 

I hope you are not doing this test with the stock diagonal.  That RACI is pretty cheaply made so I wouldn't trust it for evaluating the optics.  I never even tried this one, because several times I have had terribly misaligned cheap diagonals as well as two that have demonstrated considerable astigmatism.  Use a known good diagonal that produces good star tests on a known scope.  When possible I sometimes troubleshoot without a diagonal, but I don't recall if the scope can reach focus without a diagonal in it. 


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#8 Cirus

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Posted 21 February 2019 - 08:10 AM

That looks like a fairly harmful level of coma from miscollimation or misalignment somewhere. It will kill detail.

I took a look at the stock focuser (since removed and replaced with a 2"). The focuser tube can't turn because of the rack. But the visual back attached to it can rotate, if it is not screwed on tight. Tighten that onto the draw tube and it should stay put. The 2 tiny allen screws on the top should press down on the focus tube to maintain draw tube tension so that it doesn't move in and out. I believe they are 1.5mm socket. I doubt that the focuser draw tube is so loose that it is causing that level of misalignment, but it could be off to some extent or the focuser or objective cell could be misaligned with the tube or each other.

Below about 40 degrees the Synta glue...errr...grease on the focuser gets stiff--and by the mid 20's problematically stiff from what I remember. I eventually disassembled the bottom bracket covering the ST80's focuser spring and gears so that I could clean all of that tacky goo off of the rack and gears. It has worked better without it, and I no longer end up with sticky gunk on my hands if I inadvertently touch the rack. I had to adjust the tightness of the focuser rack & pinion cover plate screws a few times until it meshed properly in cold weather, until I finally removed that blasted grease. It has behaved since then.

I hope you are not doing this test with the stock diagonal. That RACI is pretty cheaply made so I wouldn't trust it for evaluating the optics. I never even tried this one, because several times I have had terribly misaligned cheap diagonals as well as two that have demonstrated considerable astigmatism. Use a known good diagonal that produces good star tests on a known scope. When possible I sometimes troubleshoot without a diagonal, but I don't recall if the scope can reach focus without a diagonal in it.


Thanks for your help!

Yep you're right, I think it's the visual back that's rotating, how can I tighten it onto the draw tube, is it via those 2 tiny allen key screws?

I'm using a $30 USD skywatcher 90 degree mirror star diagonal, it should perform decent enough, unfortunately I wasn't able to star test tonight because of clouds.

Tomorrow night is also forecast to be clear, hopefully I'll have better luck and will be able to prove whether it is or isnt the diagonal.

I don't have any collimation tools, I'll need to get a cheshire and do the paper/cardboard alignment which may take a week to arrive.

#9 Redbetter

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Posted 21 February 2019 - 11:05 AM

The visual back just threads onto the draw tube, so hand tighten it back into place.


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#10 daquad

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Posted 22 February 2019 - 08:02 PM

Hello,

 

I have 80mm / 400 refractor, Meade Adventure scope

 

At x133 magnification, a focused star looks like this

 

attachicon.gif star mis.png

 

 

Is this caused by miscollimation?

 

Another issue that might be related, when I unscrew the focuser dust cap, the focuser tube will turn with it.

 

Can this be fixed by tightening the focuser tube? Is it tightened by the 2 allen key screws near the focuser tightening knob?

 

Thanks in advance for any advice.

At f/5 it could be miscollimation.   However, I had a 6" jaegers f/5 RFT that always put up perfect Airy discs.  I Built the OTA myself and never checked the collimation.  If not collimation then your problem is decentering of the lens elements.  That is a more difficult problem to overcome.  Check the collimation with a Cheshire to make sure you are collimated (optical axis and mechanical axis coincident.)  If that is OK and you still have the asymmetrical Airy pattern, the problem is centering of the elements.



#11 Eddgie

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Posted 22 February 2019 - 11:36 PM

Well, it looks like coma, but this would mean that the scope has a cemented doublet.  If it were air spaced, it would not have coma.

 

Now, if it is air spaced, even if the lens were tilted, it should not show as coma.   If one sees coma, in an air spaced lens, it would usually indicate that the lenses are tilted with respect to one another (not seated against the spacers).

 

Here are things to try.

 

#1 And most likely.  The focuser is tilted with respect to the tube.  Very common on cheap refractors.    Just rack the focuser all the way out and look down the tube from a couple of feet away.  Position your eye so that the end of the focuser nearest you and the end of the focuser furthers from you are centered, and then see move closer or further while watching the outside edge of the lens cell and ensure that the tube is not tilted.   Better to use a laser.

 

#2 The lens cell is tilted, or the lenses inside the cell are tilted.   Easiest thing to do for this (assuming there is a retaining ring on the lens cell) is to back off the retaining ring and shake the cell from side to side to settle the lenses in the cell.

 

My money is on #1 though.   I have seen at least half a dozen inexpensive refractors (and one very expensive one) that had tilted focusers. 



#12 Cirus

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Posted 23 February 2019 - 02:44 PM

Thanks everyone so much for the help!

I just did another star test today with 80mm Meade f/5 adventurer, it definitely is not the diagonal or one off conditions, same distorted airy disc pattern while in focus at x133. I tried my 100ed f/9 at x300 for comparison and the airy disc appeared flawless.

How do I know whether it's air spaced or cemented? It's a Meade 80mm adventure scope which should be the same as ST80

I don't have any collimation tools at the moment, is it better to get a standard Synta cheshire or a 1.25 inch farpoint 650nm laser?

Eddgie, for your test, should I be looking down the tube from the objective lens? Or from the focuser end?

#13 Redbetter

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Posted 23 February 2019 - 04:47 PM

Unless your Meade 80 f/5 is completely different than mine, it is air spaced.  When looking at the objective, you should see the three tabs that serve as spacers.

 

The only modern achros that I have seen which are cemented are very short ratio finder doublets.  My old generic 80 ~f/4 or perhaps f/3.75 is cemented, as is my old 50mm Tuthill ~f/4.  I assume cementing is still common for binoculars and finders, but I am not sure what achros are still cemented being sold as stand alone scopes.  The half dozen achros I have purchased in the past 3 years have all been air spaced:  two 80 f/5's, one 80 f/11.3, one 70 f/5.7. a 60 f/15, and a 50 f/12.   


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#14 Cirus

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Posted 23 February 2019 - 07:42 PM

Yep, it definitely has those 3 rectangle spacers.

#15 Eddgie

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Posted 23 February 2019 - 08:27 PM

Thanks everyone so much for the help!

I just did another star test today with 80mm Meade f/5 adventurer, it definitely is not the diagonal or one off conditions, same distorted airy disc pattern while in focus at x133. I tried my 100ed f/9 at x300 for comparison and the airy disc appeared flawless.

How do I know whether it's air spaced or cemented? It's a Meade 80mm adventure scope which should be the same as ST80

I don't have any collimation tools at the moment, is it better to get a standard Synta cheshire or a 1.25 inch farpoint 650nm laser?

Eddgie, for your test, should I be looking down the tube from the objective lens? Or from the focuser end?

Look down from the focuser end.   Stand maybe 3-4 feet away.  Try to center the opening in the far end of the focuser tube in the end closest to you.  Now, move further or closer to see if the primary looks concentric.     If the objective does not perfectly center in the concentric circles formed by the front and rear of the focuser, then the focuser is tilted.

 

Focuser collimatoin.jpg

 

If there are foil spacers visible at the edges of the lenses when you look into the front, it is air spaced.  Not seeing them does not mean it is not air spaced because now they tent to use plastic rings (better).


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#16 Cirus

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Posted 06 March 2019 - 09:35 AM

Just an update on my progress.

I bought a Cheshire and had a look at the alignment while the cap was on. The same technique advised by youtuber astronomyshed on the ST80.

I used a desk lamp as a light source to shine into the Cheshire.

The alignment was significantly off. I adjusted the screws that connect the OTA to the focuser and got it close to centre.

Stars at high power now show rounded airy discs, with a subtle circular pattern. It's not perfect, but it's much better now.

Thanks to everyone for their kind support!
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#17 Jeff B

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Posted 06 March 2019 - 03:39 PM

waytogo.gif waytogo.gif



#18 GDG

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Posted 09 March 2019 - 12:52 PM

I had a similar experience to Cirus with my recently purchased 80mm Meade Adventure Scope. Even with a good star diagonal and a good eyepiece I was unable to resolve the four brightest stars in the trapezium until I recollimated this scope. I purchased an inexpensive Chesire on ebay ($15) in order to do this. I first loosened the retainer ring for the objectives and tapped around the cell to reset the objectives, That was not enough. I too loosened the the focuser screws and adjusted them while looking through the Chesire. It almost but not quite centered, but the views are dramatically better now. I imagine I could widen the focuser holes on the tube a bit to collimate even better, but if the views are good I am not sure I should even try.

 

The trapezium is now easily resolved at 50x, and with my zoom it stayed resolved on a night of average seeing down to 30x or so. Views at 100x of the trapezium and of the crescent moon last night were still crisp. A star test at 100x showed perfect circles on inside and outside focus. I will try out higher mags next time.

 

This will be a fun and very light weight grab and go! My thanks to the CN contributors who pointed out the usefullness of this scope and how to tune it up so good images could be obtained!


Edited by GDG, 09 March 2019 - 01:01 PM.

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