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Meade etx 125 and contrast problem / soft focus

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#1 Okcman

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Posted 04 June 2019 - 05:41 PM

I have acquired a Meade etx 125 ,and recently viewed  Jupiter - with both my Celestron C6r refractor and my Meade ETX 125 - with the refactor I could see the bands clearly but with the etx125 - it seems I can focus ok - but I cannot see the bands as clear or as sharp - any Meade etx users that can throw a tip to help me out - to be fair this was essentially a 20 year old time capsule - stored in a garage - and it suffered some outgassing - I believe the grease  has transferred a little bit on the primary mirror - is this enough to cause me less than stellar images?



#2 Asbytec

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Posted 04 June 2019 - 05:54 PM

I dunno if the outgassing is severe enough. Maybe. I'd clean it, anyway. Is it well collimated and thermally stable? Jupiter should look better than it sounds like your are describing. ETX have a reputation for better optics, so I doubt that or the obstruction are causing soft images. Time to troubleshoot. Clean and collimate, if ETX can be collimated. At least check it.

#3 markb

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Posted 04 June 2019 - 07:08 PM

I would expect the etx125 to meet or come close to the refractor if the chromatic abberation of the refractor is considered.

 

ETX125s do not share the ETX90 reputation. Many are said to be over or undercorrected, and a star test may help, match to Aberrator for an idea of the possible issues. Extra-focal test comparisons can give an idea, but MAKs can give deceptive results per the experts; but my extra-focal quality results matched the in-focus Aberrator results.

 

The outgassing or contamination might cause softness but the optical experts would know better. I assume it would diffuse light and cause softness, while focus is okay. I also assume contamination may not seriously affect collimation and possibly Suiter star testing. Both assumptions.

 

In an SCT I would suspect a poorly aligned rear cell resulting in a misaligned optical center line (but I don't know if this Is possible on this design).

 

Of couse, the first stop is checking collimation. 90s are easily collimated despite warnings to the contrary, but final collimation must be done in focus only. 125s should be the same, although the collimation screws differ in layout. I have not had to collimate one, but the in focus test matched the table top result, see below.

 

The collimation screws are easily reached by loosening the focus knob setscrew, removing the knob, then removing the 3 screws in the back (2 are deeply recessed). The tube can be gently slid out out of the cheesy plastic back and placed on a towel arranged to prevent rolling, or, ideally, a wood or plastic miter box from lowes etc that safely cradles it. Collimation screws can then easily be accessed BUT limit changes to 1/2 turn per the advice of others; I agree that small changes should be enough. Start with the "tabletop kitchen" method below.

 

'Table top' 'kitchen' collimation by centering front reflections works remarkably well to get near perfect collimation without working in the cold and dark, see http://www.robincasa...ro/collimation/ for an SCT version. Finish on a nighttime target. This has worked extremely well for me, pre-and post- final collimation.

 

This method also lets you check collimation before disassembly, and to collimate the bare tube very close to final without popping it in and out of the plastic back. When I did this on an etx90 I found that just slipping it in without securing it was fine up until the final check.

 

1 Collimate, 2 star test extra- and in-focus, 3 clean would be my thought.



#4 Okcman

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Posted 04 June 2019 - 08:45 PM

Well so to fully understand my plight - the c6r refractor is my grab and go lol - the test was done in my front yard - where I am next to a street light - I live in the city where light abounds as well-
I used an explore scientific 11 mm 82 degree eyepiece in both the refractor and the cat - it is possible that the flip mirror is not that efficient or out of collimation - temperature may be getting me - although the temp difference between the garage and outside my front yard shouldn’t be that great of a difference

#5 jgraham

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Posted 04 June 2019 - 08:56 PM

If you have a film of grease on the mirrors that can't be good. The ETX-125 has a long focal length and the optical performance will be affected by the condition of the mirrors. You might want to take a look at a bright star like Vega and see if there is a halo of haze around the star when focused. I use a Bahtinov mask with mine to set the best possible focus. Thermal equilibrium is also very important.
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#6 markb

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Posted 05 June 2019 - 12:44 AM

All the etx90 and 125 flip mirrors I have testing during collimation (camera port and flip port), 5 so far, had no changes on tedting, so they must routinely get flat mirrors. Not my first choice for a problem source, and easily checked with a camera port adapter.

I do recall getting a scope with a cloudy secondary (not corroded, just a scummy buildup at the edges in, it cleaned off) and it did impair contrast.

I have found daytime tests very telling for contrast, snap to focus, sharpness impression and chromatic aberrations. I also use a Hubble (?) laser drilled artificial star and kithen table Casady collimation before moving to night testing.

It has worked extremely well for me, and bypasses cloudy or poor seeing nights.

I would stick to the collimation, star test and then cleaning if good optics hampered by filmy surfaces, order of attack.

Focusing with a Bhatinov mask is a great add-on.

Edited by markb, 05 June 2019 - 12:45 AM.


#7 Magnetic Field

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Posted 05 June 2019 - 12:58 AM

I have acquired a Meade etx 125 ,and recently viewed  Jupiter - with both my Celestron C6r refractor and my Meade ETX 125 - with the refactor I could see the bands clearly but with the etx125 - it seems I can focus ok - but I cannot see the bands as clear or as sharp - any Meade etx users that can throw a tip to help me out - to be fair this was essentially a 20 year old time capsule - stored in a garage - and it suffered some outgassing - I believe the grease  has transferred a little bit on the primary mirror - is this enough to cause me less than stellar images?

1. The ETX-125 is already fairly bulky with thick meniscus lens. Did you let it cool down?

 

2. What I would do to test the collimation: a star at high magnifications (>200x) check if you can see the airy disc with centred and round diffraction rings.

 

3. Also I would compare the ETX-125 against the Celestron C6r in daylight. If there are optical problems with the ETX-125 it will already show (yes the main mirror moves from factory optimised infinity focus against the secondary mirror and this  is different to the refractor where the drawtube moves; but I  think severe optical problems on the ETX-125 will show at either focus near and far).


Edited by Magnetic Field, 05 June 2019 - 01:01 AM.


#8 rkelley8493

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Posted 05 June 2019 - 04:07 PM

It may not be in issue with collimation or thermal conditions but an issue with too much magnification. In the past few months, I haven't been able to view Jupiter above 100x without getting atmospheric shimmer and blurriness. Try using an eyepiece that would give the same magnification yield in the ETX as the refractor to rule out the telescope being the problem. The ETX125 has a focal length of 1900mm which is very long. Try using an 18mm or 20mm eyepiece and see if that gives you a better view. I always had the best views in my ETX with a 15mm eyepiece.


Edited by rkelley8493, 05 June 2019 - 04:10 PM.

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#9 whizbang

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Posted 05 June 2019 - 11:40 PM

rkelley is correct.  The ETX125 has a longer focal length than the refractor.  You need to use a larger eyepiece.  At 11mm, you over magnified for the seeing conditions and wiped out the view.  Pilot error.


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#10 jgraham

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Posted 06 June 2019 - 06:50 AM

+1

 

Modern UWA eyepieces work great with these scopes offering a nice balance of magnification and field of view.


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#11 Okcman

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Posted 06 June 2019 - 07:24 AM

I  think that rkelley nailed it - simple over magnification - we have wave after wave of turbulent storms passing through Oklahoma right now - I did get to observe last night and tested a 18 mm  eyepiece and the views were much better - I did a quick star test on acturis and did notice that the out of focus view was not dead centered in the middle which may not be indicative of a collimation problem as well


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#12 Asbytec

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Posted 06 June 2019 - 07:58 AM

A 125 MCT should do 200x (0.6mm) on Jupiter without much trouble. Provided it's collimated, cooled, and in good seeing. Those are the conditions I observe in and the magnification (exit pupil) I use all the time. An 11mm eyepiece gives what, about 150x or a tad more? I use an 8mm eyepiece at 1950mm focal length for 240x in a 150 MCT. 


Edited by Asbytec, 06 June 2019 - 08:01 AM.


#13 Magnetic Field

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Posted 06 June 2019 - 01:27 PM

I  think that rkelley nailed it - simple over magnification - we have wave after wave of turbulent storms passing through Oklahoma right now - I did get to observe last night and tested a 18 mm  eyepiece and the views were much better - I did a quick star test on acturis and did notice that the out of focus view was not dead centered in the middle which may not be indicative of a collimation problem as well

Forget the out of focus star image either side of focus.

 

You need to check the in focus star pattern and airy rings!


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#14 BaysidePhotonics

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Posted 11 July 2019 - 03:07 PM

I know this is an old thread, but I have an Explore 127 Mak-Cass and a Celestron C6R and have compared them side-by-side on the moon and Jupiter, so I thought I'd add my 2 cents. I believe that the Explore is the same JOC F/15 telescope with different branding.

 

The C6R, especially with a UV-block filter, simply blew away the Mak. This is probably to be expected, because

 

1. the aperture is smaller on the Mak, and

2. the central obstruction on the Mak is about 40%, and there is none on the refractor.

 

Unless there is something wrong, there should be no contest in the transfer of contrast for intermediate spatial frequency targets like the bands on Jupiter.

 

My C6R has some 3rd order spherical aberration I would estimate on the star test at about 1/6 wave, and of course it has some chromatic. I did an interferometric measurement of the objective a few years ago, which gave a similar result (1/6 lambda spherical) at 633nm.

 

The Mak, after collimation, showed obvious 3rd and 5th order spherical on the star test. The in-focus image settled down fairly well, though. 3 or 4 rings were visible on a bright star (Arcturus), sort of what you'd expect from a 40% obstructed system. It just goes to show how higher order spherical can make the star test look terrible while not having very much of an impact on performance in focus.

 

I also had an Orion 102 Mak-Cass out that night. It had a much better appearing star test than the Explore, which we should expect from the smaller aperture, but it could not compete for detail on the moon.

 

The C6R and the ETX are different telescopes. The Mak is great when you don't have the time to deal with the big refractor but want to see some detail, especially on high-contrast targets.

 

Warren


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