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Some general questions about field flatteners

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#1 Lumix.guy

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Posted 24 March 2020 - 11:53 AM

As a short refractor owner (AT60ED f/6, Evoguide 50ED) and also a C11 owner, I've been trying to learn more about field flatteners/correctors.  I haven't yet found basic info on how field flatteners are designed so I thought I'd see if knowledgeable people here on CN could provide some basics.  (I will note there is some info in the book Telescope Optics, but I need more help.)

 

For example, I have an AstroTech AT2FF field flattener that seems to work well on an AT72ED (now sold), but works significantly less well on an AT60ED (love the wider field).  I can't find any (increased) spacing info various (shorter) focal length scopes and the AT2FF doesn't even seem to be spec'd for the precise focal length (or field curvature) it is designed for.  I've done some experimentation with increased spacing to my M43-size sensor, but I haven't found the sweet spot yet.

 

There are some anecdotal reports that a Celestron 6.3 reducer/corrector works great on an AT60ED.  I haven't tried that yet, but I'm wondering if that would indicate the AT60ED suffers more from coma than field curvature.

 

There are adjustable flattener designs out there, notably from WO and Borg.  Do all those work by changing the back focus (only), or do some adjust the inner spacing of the elements themselves?

 

As a home machinist, I'm wondering if there is a simple way to construct your own field flattener?  I've wondered about using the individual elements from an air-spaced (short focal length) achromat or even the rear group of an SLR lens.

 

I'd appreciate any comments or tips.

 

Thanks!



#2 sg6

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Posted 24 March 2020 - 12:58 PM

If you look around you will find that a flattener is more defined by the f number.

The 60 is f/6 as is the AT72ED so it seems a little unexpected the 2 scopes end in what sounds like significantly different results. It may be the separation between front objective and the flattener, may not be possible to get the flattener in the right place.

 

The purpose is s seems obvious to flatten the final image. We draw optical images as if flat, they aren't, but the sensor is.

 

On a refractor the final image tends to be ) in shape, the sensor wants |.

I assume that most flatteners are a negative lens, to add some counter curvature.

Rather simply the intention is that:   ) + ( = |

 

If a Celestron 0.63 works on an AT60 then that is chance.

 

The adjustable WO does not alter the back focus. Back focus is a poor description. All it is is a required separation between the rear face of the flattener and the camera sensor. It is a length in mm. you cannot change it.

What the WO adjustable does is very simply allow you to adjust the separation you have to match the one you want - you wind it in and out. Then lock it when it is at 55mm. It is all spacing, in spite of the fancy name of "back focus".

 

Why the flattener may not work well on both scopes:

The new focal length of the scope with the flattener is given in simple optics by:

1/F = 1/Fo +1/Fr - (d/(Fo-Fr))

F = New focal length

Fo = focal length of objective

Fr = focal length of reducer

d = separation between them.

 

That is for a simple "thin" lens, the front one is a thick doublet and likely separated, and the reducer also I guess a thick doublet.

 

So I guess the AT60 flattener just cannot be placed at the right separation distance on the 72 in order to flatten adaquately.

 

From what I read a refractor has field curvature as ), whereas a reflector has field curvature as (.

Makes you realise that eyepieces can have a bad time and could be why some are good on one scope and others on the other.

 

The intention is that if the reducer is placed at the right position, then the reducer will create a flat and focused image at a distance that is Xmm from some datum face.

So you put the camera at the separation from the datum face,

Stick reducer in the focuser and move the reducer to the required position.

And if you can managed all that the image on the sensor should be both in focus and flat across the whole image plane, which is where the sensor should be.

 

The term back focus is poor. On a DSLR the distance is termed The Flange Distance, which is the distance from the front flange to the sensor. Seems to be more sense and accurate. The "back focus" is Reducer to Sensor Distance. And that is fixed.



#3 BGRE

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Posted 24 March 2020 - 04:24 PM

Its actually a combination of the Petzval curvature which is independent of element spacing and astigmatism (if present).
If astigmatism is zero then for a flat field the Petzval curvature must be zero.
If astigmatism is non zero then asuitable mount of astigmatism can be balanced against Petzval curvature to flatten the field.
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#4 Lumix.guy

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Posted 25 March 2020 - 04:12 PM

The adjustable WO does not alter the back focus. Back focus is a poor description. All it is is a required separation between the rear face of the flattener and the camera sensor. It is a length in mm. you cannot change it.

What the WO adjustable does is very simply allow you to adjust the separation you have to match the one you want - you wind it in and out. Then lock it when it is at 55mm. It is all spacing, in spite of the fancy name of "back focus".

Well, I think it's a matter of agreeing on semantics. I've seen various flattener with tables that specify a different "back focus" for different telescopes like this TS 2" Field Flattener - Universal Field Flattening Lens:

 

In principle this rule applies: the shorter the refractor´s focal length, the longer the working distance to the sensor has to be.

♦ focal length < 450 mm: 128 mm
♦ focal length 450-490 mm: 123 mm
♦ focal length 500-550 mm: 118 mm
♦ focal length 560-590 mm: 116 mm
♦ focal length 600-690 mm: 113 mm
♦ focal length 700-800 mm: 111 mm
♦ focal length ab 800 mm: 108 mm

 

I suggest the table above indicates the field curvature relates more to the focal length of the telescope than strictly the f-ratio of the scope.  The AT60ED and AT72ED are both f/6, but the petzval curvature is different since their focal lengths are different.

 

True flatteners appear to be a strong positive element followed by a strong negative element with a few mm of airgap between them. I'd sure like to better understand how varying the spacing between the flattener and the sensor changes the Petzval correction.

 

So BGRE's comment is very relevant:

 

Its actually a combination of the Petzval curvature which is independent of element spacing and astigmatism (if present).
If astigmatism is zero then for a flat field the Petzval curvature must be zero.
If astigmatism is non zero then asuitable mount of astigmatism can be balanced against Petzval curvature to flatten the field.

Thanks for this discussion,

 

John



#5 BGRE

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Posted 25 March 2020 - 05:04 PM

https://wp.optics.ar..._Distortion.pdf
may help a little.
However there's no real substitute for raytracing for exploring the dependence of field curvature etc on various parameters

Field flattener Video:
https://www.coursera...flattener-pyAYP

#6 MKV

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Posted 26 March 2020 - 01:08 AM

...I'm wondering if there is a simple way to construct your own field flattener?  I've wondered about using the individual elements from an air-spaced (short focal length) achromat or even the rear group of an SLR lens.

I am afraid there's no simple way to procure a field flattener for your optics. BGRE has already outlined the main reasons. You may wish to look at this popular source as well

 

https://www.telescop..._flattener.htm 

 

For optics such as the Schmidt camera (a two-element anastigmat, where only the mirror contirbutes to field curvature), a field flattener is an easy solution. One could use a plano-concave lens, or just a curved film holder turned to a proper radius of  curvature on a lathe.

 

For configurations other than those specifically designed for a flat field the (i.e. Wright, Baker, Slevogt, Linfoot, etc. cameras), a field flattener must be designed separately as an addition to the light train. Nowadays, this is best done using raytracing optimization solutions, and will usually consist of several glass elements.

 

For digital cameras, field flatteners may not be in contact with the imaging sensors but located some distance away. This further complicates the design.

 

My understanding is that some people offer "universal" field flatteners. I would do a thorough critical review research before buying any of those. I seriously doubt there is such a thing as a "universal field flattener" in optics, but if there is one it's probably priced for well-funded institutions. 


Edited by MKV, 26 March 2020 - 05:08 AM.


#7 BGRE

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Posted 26 March 2020 - 03:02 AM

A truly universal field flattener isn't possible despite claims to the contrary.
Its not even possible to design a field flattener that will work well with all F/4 telescopes.


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