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Baader Morpheus range - General chat


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3 minutes ago, Louis D said:

I believe Ernest uses a telescope with micrometer eyepiece to look into the exit pupil of the eyepiece to measure the aberration spot sizes.

I also believe he always uses the center of the lens and moves the eyepiece across its central axis to take measurements so lens edge aberrations do not figure into the equation.

Interesting - so very much like the approach I suggested above - except he is using telescope + EP + eyeball instead of camera lens + camera (in principle one can also use telescope + camera as well).

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9 minutes ago, vlaiv said:

Interesting - so very much like the approach I suggested above - except he is using telescope + EP + eyeball instead of camera lens + camera (in principle one can also use telescope + camera as well).

How would you go about moving the EP off axis while the telescope/lens stays on axis to measure edge aberrations? 
This is really needed to stop telescope off axis aberrations being captured. (Which is much harder to minimise with careful telescope selection than is the case on axis.)
I wonder how Ernest did it?
I think we need a 3D printer to .....  are we drifting off topic??? 🤔

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54 minutes ago, globular said:

How would you go about moving the EP off axis while the telescope/lens stays on axis to measure edge aberrations? 
This is really needed to stop telescope off axis aberrations being captured. (Which is much harder to minimise with careful telescope selection than is the case on axis.)
I wonder how Ernest did it?
I think we need a 3D printer to .....  are we drifting off topic??? 🤔

Great idea!

Yes, we are probably just a tad off topic here :D - but yes, 3D printer and some clever mechanism to be able to slide EP left and right would help with keeping star image on axis for telescope but off axis for eyepiece.

Think of EP revolver except not moving in circle but rather left / right. A bit like this filter slider:

image.png.50474fd2f76fd82dc7bf64553c29e0a4.png

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2 hours ago, globular said:

How would you go about moving the EP off axis while the telescope/lens stays on axis to measure edge aberrations? 
This is really needed to stop telescope off axis aberrations being captured. (Which is much harder to minimise with careful telescope selection than is the case on axis.)
I wonder how Ernest did it?
I think we need a 3D printer to .....  are we drifting off topic??? 🤔

If you have an optics test bench as Ernest does, you just move a vernier to slide the eyepiece side to side independently of the image forming lens.  Something along the lines of putting this:

spacer.png

on this:

spacer.png

If I came across some well priced, surplused equipment in this realm, I might go this route someday.  However, I'm not really that serious about getting exact results.

Actually, I just remembered I have one of these macro focusing rails that could be used for this with a few clamps:

spacer.png

Edited by Louis D
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5 hours ago, Don Pensack said:

The only figure where I disagree with Louis is the eye relief of the 30mm APM.  I couldn't see the entire field at 16mm effective eye relief, yet it's easy to do so.

I agree it feels like 18mm in actual use, but the point where the image circle is smallest is 16mm from the flipped down eye cup using the projection method.  I remeasured it a few times because it didn't align with my ER experience with it.  It's not the only eyepiece with this divergence between measured and "feels like" distances.  For instance, the 22mm AT AF70 also has a repeatably measured 16mm or usable ER, but feels like 18mm in use (eyeglasses just above rim).  The 22mm NT4 has 14mm of measured ER, but feels like 16mm in use (I have to push in a bit on my glasses).  The 17mm ES-92 has a measured 16mm ER, but feels like 17mm in use (eyeglasses just touching).  The 35mm Baader Scopos Extreme has 16mm of measured ER, but feels like 18mm.  That last one actually has about 35mm of design ER thanks to its 47mm diameter eye lens, but the eye lens is recessed about 17mm, IIRC.  As an experiment I screwed off the top and put a flat lens retainer over it to get to about 30mm of ER.  Even for an eyeglass wearer, that was too much ER!  I had trouble holding the exit pupil because I had no frame of reference touching either my glasses or nose.

I guess I need a measured ER and a "feels like" ER.  The problem with the latter is that it is very subjective.  I'm going to try to measure it someday with a cardboard tube wrapped around the eyepiece and pushing it down with the phone's camera until the field stop pops into view.  I would then measure and record the distance from the top of the eyepiece to the top of the tube.  After measuring a bunch of eyepiece ER distances this way, I should be able to come up with a constant offset that accounts for the location of the camera's entry pupil inside of it by comparing these numbers against the ER distances measured using the narrowest projected image circle.

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Frankly, I don't know where Ernest's figures come from.

 

Take his "essentially perfect" 10'.

In a 100° eyepiece, that would be 1/600 the width of the field if talking linear field size in an angular sense.

Would 1/600 the width of the field appear perfect?  No way.  A 10' star would be a blob.

 

So I view his figures as "relative", meaning an eyepiece yielding a 20' spot size would be a spot 2x as wide as one yielding a 10' spot size.

One eyepiece I know has a design 1µ spot size on axis and 1.5µ spot size just inside the field stop, and shows on his list as <5' on axis and 11' at field edge.

1.5µ is a lot smaller than the Airy disc in an f/5 scope, so, correcting for coma (many times as large), the eyepiece should yield stars at the edge

indistinguishable from perfect.  And his f/10 numbers are perfect, so f/4 just falls below perfect only at the very edge.

That is consistent with the design specs.

 

So, however the numbers are derived, they do have some basis in comparative reality.  

I do think choosing f/4 and f/10 is somewhat arbitrary, though.  Few scopes are as long as f/10 these days, and those that are have other significant aberrations affecting the star images.

And it is largely the very big scopes (say, 20" and larger) that are commonly as short as f/3-4.  And really big scopes have a "spot of bother" yielding tiny star images for a variety of reasons.

I would have picked f/4.5 and f/9 to more closely correspond to more of today's scopes.

 

One thing the numbers do tell, and that is if f/4 is too fast for the eyepieces to yield excellent star images across the field.

It's unfortunate his list doesn't contain more of the contemporary eyepieces.

 

 

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On 29/08/2021 at 12:44, vlaiv said:

I borrowed the idea from @Louis D who did wonderful set of comparison photos thru the eyepiece. Only difference is that I would advocate using large lens instead of phone camera because phone camera can act as aperture stop and reduce aberrations created by eyepiece.

We went round and round on this in another thread, and I tried to use large lenses and presented the mediocre results there, but the results were inconclusive.

I will restate that the image from the camera phone exactly mirrors my visual impression of the field of view, right down to where some eyepieces have zones around 70% out that go soft and then get sharp again toward the edge.  Visually, those areas look "jittery" depending on how you move your eye.  Clearly, there's some sort of aberration going on at the exit pupil, possibly SAEP related.

The one exception as I've previously stated is the portrayal of field curvature.  Eyepieces with a curved field look much flatter in the small camera lens than to my presbyopic eyes.  It probably mirrors how younger eyes perceive the field of view.  I suspect the enormous depth of field/focus of these tiny, wide angle lenses are able to bring the curved field into focus from center to edge.  Despite these lenses operating at f/1.7 to f/2.5, they still have lots of depth of field because of their incredibly short native focal lengths of around 4mm or less.

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2 hours ago, Dantooine said:

Can someone explain why there is a 12.5mm being so close to the 14mm?

sorry if the question seems a little dim 😊

One traditional approach to eyepiece focal length progression is a 1.4x - 1.5x step for each focal length. 1.4x nearly fits with the Morpheus focal lengths apart from the 14mm which seems to be an "in between" focal length.

Others favour a 1.6x step - I guess it depends on the scope focal ratio as to what works best ?

 

 

 

Edited by John
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The range without the 14mm does indeed look like the usual 2x steps in fov area... i.e. root(2) = 1.4 steps in fl.

When the range first came out, however, they couldn't get the 17.5 quite right.... so brought out a 14mm instead.... as that was the longest fl they could get right.

When they finally cracked the 17.5 they brought that out too... but (rightly) didn't withdraw the 14mm.

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3 hours ago, globular said:

The range without the 14mm does indeed look like the usual 2x steps in fov area... i.e. root(2) = 1.4 steps in fl.

When the range first came out, however, they couldn't get the 17.5 quite right.... so brought out a 14mm instead.... as that was the longest fl they could get right.

When they finally cracked the 17.5 they brought that out too... but (rightly) didn't withdraw the 14mm.

Agreed.  Also, there's a lot of wiggle room in the 11mm to 16mm space as to what is the best workhorse focal length.  It depends on the focal length of the scope, the apparent field of view of the eyepiece, and observer preferences.  For instance, I started out with a 14mm Pentax XL at 65°, but later found that the 12mm ES-92 covers more sky at a higher magnification without shrinking the exit pupil too much in the process.

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10 hours ago, globular said:

 

...When the range first came out, however, they couldn't get the 17.5 quite right.... so brought out a 14mm instead.... as that was the longest fl they could get right.

When they finally cracked the 17.5 they brought that out too... but (rightly) didn't withdraw the 14mm.

That sounds a plausible explanation :thumbright:

 

 

 

Edited by John
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On 31/08/2021 at 23:09, Dantooine said:

Can someone explain why there is a 12.5mm being so close to the 14mm?

sorry if the question seems a little dim 😊

 

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A 40% jump between magnifications, starting with the 17.5mm runs:

17.5...12.5...9...6.5...4.5  Now you know where the focal lengths came from.

 

So why a 14mm?  Because it was the longest focal length possible in the original formulation of the design.  It really wasn't in sequence with the other focal lengths.

They tried 3 different designs for the 17.5mm until they landed on a good design, but the 17.5mm is different internally than the others, and has different eye relief (longer), a different apparent field (slightly smaller),

and one note: the Baader data is incorrect--the field stop in the 17.5mm is 21.7mm, not the 23.5mm Baader quotes.  I think this was changed somewhere in between the 3 prototypes they tried.

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10 minutes ago, Don Pensack said:

the field stop in the 17.5mm is 21.7mm, not the 23.5mm Baader quotes

Does that mean the FOV is 71°?  Or do distortions push it back out to 77° or so?

Edited by globular
typo
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2 hours ago, Don Pensack said:

I recall it being measured at 74° +/- with the flashlight test.

With the other focal lengths at 78°, and just the 6.5mm at 79°

For my two Morpheus:

14mm: 77° by projection, 78° photographically

9mm: 78° by projection, 79° photographically

And before anyone suggests a systemic error between the two, the two measures match for many of my eyepieces, and the first is sometimes larger than the second.  They are generally within 1° of each other.  Eyepieces without a physical field stop are problematic for both methods because it's a judgment call to define where the usable field ends as it fades to black either in the projected or photographed image.

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So in this design, Baader cant go longer than a 17.5 - Which is a real shame, If they could produce longer and it performed as well as the 17.5 they would be onto a winner.

Can they go  the other way though? Would there be any incentive to produce an EP around the 3mm FL?

Baz

Edited by Barry-W-Fenner
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6 hours ago, Barry-W-Fenner said:

So in this design, Baader cant go shorter longer than a 17.5 - Which is a real shame, If they could produce shorter longer and it performed as well as the 17.5 they would be onto a winner.

At least not as long as they stay in a 1.25" barrel.  Were they to switch to a 2" barrel like the 17mm and 22mm NT4s, 14mm and 20mm Orion LHDs, and 22mm AT AF70/Omegon Redline SW, etc., then they probably could go with longer focal lengths.

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10 minutes ago, Louis D said:

At least not as long as they stay in a 1.25" barrel.  Were they to switch to a 2" barrel like the 17mm and 22mm NT4s, 14mm and 20mm Orion LHDs, and 22mm AT AF70/Omegon Redline SW, etc., then they probably could go with longer focal lengths.

Thanks for pointing out my mistake Louis, now ammended!

Baz

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I’ve been out once (20th Aug) with my 12.5mm Morpheus since I got it. Scope is a 300p dob. 

I have 32mm omni and 16,7,4mm OVL nirvanas for comparison. 

Eye relief is better than the nirvanas, field seems flat with pinpoint stars across the field of view

The moon was close to full which limited my choice of objects. Contrast seems very good, Jupiter was crisp and i was able to see the transit of Ganymede’s shadow at 120x.

I don’t feel like replacing all my nirvanas with morphi but the 9mm would fit in between 12.5mm and 7mm… ☺️

 

 

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