Flattener Theory

[NickMorris]

Hi,

I was wondering if anyone could answer a question on flatteners for me....

Obviously the spacing from the rear of the flattener to CCD chip plane is crucial but what is the effect of moving the flattener position in the telescope's light cone?

Does it matter if the flattener is placed directly behind the focus drawer tube or actually inside the focus drawer tube?

Also does this have an effect on vignetting?

Many thanks

Nick 

[sharkmelley]

The flattener is designed to sit in a particular position within the light cone.  Since the focuser has limited travel, you will almost certainly find that you are only able to achieve focus if you position the flattener where it was designed to be - which may either be behind the focus drawer tube or actually inside the focus drawer tube.

If you have a particular scope and flattener in mind then maybe someone here can give some specific advice.

Mark

[NickK]

The Pentax SDP flattener sits inside the focuser tube itself.

[D4N]

I'm not sure moving it relative to the light path is even possible, since you have a set distance from the flattener to the sensor you will just end up adjusting the focuser to place the flattener back where it would have been in the first place.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

[NickMorris]

That's in essence the question, does moving the reducer up or down the light cone move the final focuser position?  Does coma correction change by its location....?

[Merlin66]

Nick, moving the flattener away from the optimum design position will probably have little effect on the final focus position (it really depends on the optical design of the flattener) BUT could have a great negative effect on it's performance.

Why use a flattener in a position where it doesn't do its job????

[NickMorris]

The flattener(s) in question are Teleskop Service TSFLAT2 and TSFLAT2A.  They seem to be a "universal design" if thats possible and refer to CCD spacing for different FL / F ratios.  They do not specify where in the light cone they should be placed and therefore the question.....

hope that clarifies,

Nick

[NickMorris]

Sorry meant TSFLAT2 and TSFLAT25....

[Merlin66]

Nick,

The TS2 version quotes a nominal spacing to the CCD chip of 109mm

""

Recommended distances between the M48 thread of the flattener and the sensor of the camera:

For most refractors the optimum back focus distance is 109mm to 129mm from the M48 thread to the camera's sensor. Practical experience has demonstrated that the flattener has a large tolerance for deviations from the ideal distance. 5% deviation does not cause any deterioration of the image quality in an APS-sized sensor (DSLR). For smaller sensors (CCD) somewhat larger deviations may be acceptable, but please keep in mind the higher resolution of many CCD cameras which in turn suggests that the distance should be kept as precisely as possible. The main tests have been made with a Digital SLR camera with a 15.1mm x 22.7mm sensor.

Suggested Distances from the M48 thread of the flattener to the camera sensor:

The distance depends from the focal length of the telescope:

-- Focal Length < 450mm: 128mm
-- Focal Length 450-490mm: 123mm
-- Focal Length 500-550mm: 118mm
-- Focal Length 560-590mm: 116mm
-- Focal Length 600-690mm: 113mm
-- Focal Length 700-800mm: 111mm
-- Focal Length from 800mm: 108mm

""

and the TS2.5 version around 100mm

""

With the TSFlat25A adapter set (see accessory section below), we offer a complete adaptation for your telescope / camera set.

General rules:

The shorter the focal length, the longer the distance between flattener and focal plane must be (and vice versa).
Focal lenghts / back focus:
< 500 mm: 106 mm
510-600 mm: 101 mm
610-700 mm: 96 mm
710-800 mm: 91 mm
810-950 mm: 88 mm
> 1000 mm: 87 mm
(Distances measured from the back end of the corrector)

Hope this helps.

[NickMorris]

Nick,

The TS2 version quotes a nominal spacing to the CCD chip of 109mm

""

This surely refers to the flattener => CCD chip distance, not the position of the flattener in the light cone???

[NickMorris]

Hi,

I was wondering if anyone could answer a question on flatteners for me....

Obviously the spacing from the rear of the flattener to CCD chip plane is crucial but what is the effect of moving the flattener position in the telescope's light cone?

I understand the effect of incorrect flattener to CCD chip distances....

[Merlin66]

Nick,

Imagine the prime focused cone of light sitting on the CCD plane.....

If the corrector is positioned 100mm from the CCD then it will be 100mm into the light cone...

or am I missing something here???

[sharkmelley]

The way it works is as follows.  Determine the back focus distance for your particular focal length - this specifies the distance between the flattener and the camera. Once you have fitted the necessary adapters to obtain this distance, you will find that there is only one position in the light cone where focus will be achieved.  You may next extra spacers/adapters to achieve this with your focuser - it depends on many variables.

Best contact TS and ask them what may be needed for your particular scope.

Mark

[NickMorris]

Thank you Mark and others,

I did contact TS and they sold me an adaptor stating that it 'should reach focus' with my equipment combination.  Sadly this is not the case and I do not have enough inward travel.  The solution is to get the adaptor machined so that I can place the flattener inside the focus tube and then have my 123mm to the CCD plane.  I suppose the question really should have been:- Does the absolute focus point change when you add a flattener to the imaging train versus without a flattener?

thanks for getting my head around this.....

Nick

[D4N]

It all seems irrelevant to me, you have a fixed spacing between the CCD and the flattener so there is only one place in the light cone that you can put the flattener to achieve focus. Whether you have it inside the draw tube and the focuser racked out or on an extension tube with the focuser racked in it will essentially be in the same place or you will be out of focus.

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[NickK]

I'm not sure moving it relative to the light path is even possible, since you have a set distance from the flattener to the sensor you will just end up adjusting the focuser to place the flattener back where it would have been in the first place.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

I forgot to say - the flattener itself sits inside the focuser tube but is suspended by an inner tube. Therefore the focuser moves and the flattener stays firmly fixed in position.

As the flattener shape is designed to undo the aberration caused by the shape of the forward lenses - it needs to be fixed in position. So if the lenses at infinity cause a specific aberration, the shape of the flattener is designed specifically for that aberration profile. Moving it or tilting it will cause the light cone to go out of specification.

[D4N]

Ok that's a different matter altogether then. In this case you just need to achieve focus for it to work?

Sounds a bit like how mine works as it is built into the scope.

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[sharkmelley]

  I suppose the question really should have been:- Does the absolute focus point change when you add a flattener to the imaging train versus without a flattener?

Nick

If the flattener only flattens (i.e. it does not act as a reducer as well) then the light cone emerging from it will subtend the same angle as it did without the flattener.  So broadly speaking the focal point should remain in roughly the same place as it was before the flattener, except that it has been pushed backwards slightly by the thickness of the optical components in the flattener.  I notice on the web site for the TSFlat2 there is a comment next to one of the sample images saying "We have been able to focus without any problems due to the fact that the flattener does not move the focal plane."

Mark

[Merlin66]

I can't understand your comment about the focuser moving an the lens staying "firmly fixed in position", unless you mean that the lens is fixed to the tube and the camera fixed to the other end and this "module" always maintains the distance between the lens and the CCD no matter where the focuser ends up...

A photo might help.

[NickMorris]

Thanks for all the help everyone, I now understand fully.  Much appreciated.....

NickK, I am also struggling to understand how your flattener stays fixed relative to the telescope and doesn't move when the focuser is adjusted?  This way your flattener to CCD distance would change.  Unless of course the telescope is a quad or quintuplet design and the therefore the flattener is incorporated into the telescope?

Cheers

Nick

[NickK]

Thanks for all the help everyone, I now understand fully.  Much appreciated.....

NickK, I am also struggling to understand how your flattener stays fixed relative to the telescope and doesn't move when the focuser is adjusted?  This way your flattener to CCD distance would change.  Unless of course the telescope is a quad or quintuplet design and the therefore the flattener is incorporated into the telescope?

Cheers

Nick

Yes - it's built into the scope. Many moons ago I took the SDP apart to clean it.. 

Here's the rear flattener cell (doublet):

The cell screws into a the central section of the scope so it doesn't move. The focuser then can move freely back and forth between the scope side and the flattener - as if a tube was slotting between to other tubes.

This is the focuser tube:

Sorry if I confused things

[NickMorris]

Cheers NickK,  All clear now,

To quote from a very old episode of Minder:-

"DO I MAKE MYSELF CLEAR INSPECTOR???"

"Crystal would be opaque in comparison, Sir"

All the best

Nick