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Measured focal length of 6.3 Meade Reducer 210mm

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#1 AntMan1

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Posted 15 April 2018 - 08:41 PM

I am trying to figure out the amount of spacing i need for my Meade 6.3 reducer. I have read that some have a 240 mm focal length & others 110 mm .. but mine just measured about 210 mm How does my measurement help me measure the correct spacing for my camera? I know about the calculator in link below but my focal reducer can't be 6.3 if it measures 210 mm can it? And that calculator does not give me the spacing just the calculated focal ratio. I swear i used to know this but i am getting older. 

 

The scope is a de forked 8 inch lx90

The camera is a QHYCCD168C with a back focus of around 18mm

 

 

http://www.wilmslowa...rmulae.htm#FR_b



#2 Martin Lyons

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Posted 16 April 2018 - 04:47 AM

How did you measure the focal length?

It may sound like a silly question, but your method can make a huge difference.

The fact that ....edited due to misreading OP....there is a difference, suggests that there could be an error in method.


Edited by Martin Lyons, 16 April 2018 - 04:51 AM.


#3 AntMan1

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Posted 16 April 2018 - 06:46 AM

How did you measure the focal length?

It may sound like a silly question, but your method can make a huge difference.

The fact that ....edited due to misreading OP....there is a difference, suggests that there could be an error in method.

Very good question..I used the distant object method similar to this https://www.youtube....h?v=4hZwgtuovgA

From what i have read they are all over the place due to the lax chinese manufacturing.


Edited by AntMan1, 16 April 2018 - 06:48 AM.


#4 Thrifty1

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Posted 16 April 2018 - 02:58 PM

Here was a thread on the difference in Meade 3.3 and 6.3 focal reducers and a Celestron focal reducer.  It talks about the 2 versions of the Meade f6.3 reducer.

 

https://www.cloudyni...na#entry7734366

 

 

I also have both of the Meade 6.3 focal reducer versions and can verify there is a big difference in the focal length whether it was intentional by Meade or an error.  The first one I had would not obtain focus as it should and Meade sent me the 2nd one as a replacement (never asked for the first one back).  

 

I have not used them in a while so I don't recall which one (longer or shorter focal length) worked under certain conditions. I have an 10" LX200R and an ETX-105.  I believe one worked better on the LX200 and one worked better on the ETX (but not 100% certain of that).  

 

I will have to get them both out again and try them when it warms up.  


Edited by Thrifty1, 16 April 2018 - 03:01 PM.


#5 AntMan1

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Posted 16 April 2018 - 11:13 PM

Anyone? 



#6 Thrifty1

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Posted 18 April 2018 - 07:04 AM

Doesn’t that calculator provide something like “in focus required” which is the distance you are looking for?

#7 AntMan1

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Posted 18 April 2018 - 02:38 PM

Doesn’t that calculator provide something like “in focus required” which is the distance you are looking for?

I don't think so. What i am trying to figure out is if the reducer is a 6.3f to begin with as The measured focal length is 210mm. So even if i ran a plate sove is the correct spacing at 6.3 or different based on the true focal length.



#8 nitegeezer

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Posted 18 April 2018 - 03:01 PM

I think what you will have to do is a plate solve.  You know what focal length you want, with a couple of plate solves you should have a good idea what spacing is needed.



#9 OzAndrewJ

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Posted 18 April 2018 - 04:38 PM

Gday Antman

When you measure the 210mm you are using parallel input rays.

When its used in the scope, the input rays are converging.

( Thats my full limit of optical knowledge lol.gif )

Meade specified the reducer to sensor distance for these 230mm units

as 105mm ( IIRC ) for F6.3, but a plate solve will quickly confirm that.

Lots of threads on this topic

https://www.cloudyni...pacing-for-sct/

https://www.cloudyni...-focal-reducer/

https://www.cloudyni...ttener-spacing/

 

Andrew Johansen Melbourne Australia


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#10 AntMan1

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Posted 18 April 2018 - 08:58 PM

I think what you will have to do is a plate solve.  You know what focal length you want, with a couple of plate solves you should have a good idea what spacing is needed.

That is what i am trying to ask. Does my measured focal length of 210 impact where the reducer should be reducing to? Sorry i am confusing everyone. 

 

Gday Antman

When you measure the 210mm you are using parallel input rays.

When its used in the scope, the input rays are converging.

( Thats my full limit of optical knowledge lol.gif )

Meade specified the reducer to sensor distance for these 230mm units

as 105mm ( IIRC ) for F6.3, but a plate solve will quickly confirm that.

Lots of threads on this topic

https://www.cloudyni...pacing-for-sct/

https://www.cloudyni...-focal-reducer/

https://www.cloudyni...ttener-spacing/

 

Andrew Johansen Melbourne Australia

Hi Andrew, I just used Astrometry.net and cant figure out how to translate it to what i am looking for. Per above since my reducer is measuring 210 instead of 230 should the reducer still be reducing to 6.3 or a different F value? Can someone hold my hand here and walk me through how to plate solve my way out of this. 

 

This is what i just got: Size: 66.4 x 44.1 arcmin

Radius: 0.664 deg
Pixel scale: 0.805 arcsec/pixel
Orientation: Up is -58.4 degrees E of N


Edited by AntMan1, 18 April 2018 - 09:00 PM.


#11 OzAndrewJ

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Posted 18 April 2018 - 10:06 PM

Gday Antman

 

The focal reducer has approx 210mm focal length when measured against a target at infinity.

You are actually placing it into an (approximately) F10 converging ray pattern, and this shortens the distance behind the reducer where the focal plane needs to be.

As per above, and IIRC, you need about 105mm spacing from reducer to CCD to get the correct F6.3

 

As to using astrometry data, all you need is the calculated arcsec per pixel and the pixel size of your camera.

After that it is simple trigonometry to calculate the working Focal length

ie FL = Pixel size / tan( ArcSecPerPix )

lots of online calculators out there will do this for you.

 

Andrew Johansen Melbourne Australia


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#12 AntMan1

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Posted 18 April 2018 - 10:13 PM

Gday Antman

 

The focal reducer has approx 210mm focal length when measured against a target at infinity.

You are actually placing it into an (approximately) F10 converging ray pattern, and this shortens the distance behind the reducer where the focal plane needs to be.

As per above, and IIRC, you need about 105mm spacing from reducer to CCD to get the correct F6.3

 

As to using astrometry data, all you need is the calculated arcsec per pixel and the pixel size of your camera.

After that it is simple trigonometry to calculate the working Focal length

ie FL = Pixel size / tan( ArcSecPerPix )

lots of online calculators out there will do this for you.

 

Andrew Johansen Melbourne Australia

My God man! Thank you! My head was stuck & you just slapped it and got it working again! How many times have you saved my rear end now? Wish you were in NY i would seriously make you the best Italian dinner you ever had!



#13 OzAndrewJ

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Posted 18 April 2018 - 10:13 PM

oooh forgot

Have a squizz at

http://webpages.char.../flr-notes.html

He made a gizmo up many years ago to allow the lenses of a focal reducer to be fitted into a 2" camera nosepiece and adjusted "relative to the camera sensor", thus removing the effects of focusser movement.

As part of that, he also did a detailed test on spacings required and their effects.

 

Andrew Johansen Melbourne Australia



#14 AntMan1

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Posted 19 April 2018 - 12:15 AM

oooh forgot

Have a squizz at

http://webpages.char.../flr-notes.html

He made a gizmo up many years ago to allow the lenses of a focal reducer to be fitted into a 2" camera nosepiece and adjusted "relative to the camera sensor", thus removing the effects of focusser movement.

As part of that, he also did a detailed test on spacings required and their effects.

 

Andrew Johansen Melbourne Australia

This is ingenious. Thank you again for all your help my friend. If you ever need a hand with anything in the states i am your man.



#15 OzAndrewJ

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Posted 19 April 2018 - 12:36 AM

Gday Antman

Yep, very ingenious.

Also note that he discusses that there are really 2 requirements to assess when using a focal reducer

1) Its optimum distance from the backplane of the OTA

2) Its optimum distance from the CCD sensor

If you put the FR on the rear SCT thread then fit the micro focusser, then camera,

the act of micro focussing changes the FR to sensor distance, and you may also get too much distance between FR and CCD.

If you put the FR behind the microfocusser, then correct spacers to camera, you are placing the FR too far behind the backplane and hence can ( possibly ??? ) change its characteristics relative to design.

Which one is worst ????? No idea. As i said, i am a mug at the details here.

The Agos solution at least allows the camera to CCD distance to be fixed and then the nosepiece can be inserted to get the FR as close to the backplane as possible.

 

Andrew Johansen Melbourne Australia



#16 AntMan1

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Posted 21 April 2018 - 11:34 PM

Gday Antman

Yep, very ingenious.

Also note that he discusses that there are really 2 requirements to assess when using a focal reducer

1) Its optimum distance from the backplane of the OTA

2) Its optimum distance from the CCD sensor

If you put the FR on the rear SCT thread then fit the micro focusser, then camera,

the act of micro focussing changes the FR to sensor distance, and you may also get too much distance between FR and CCD.

If you put the FR behind the microfocusser, then correct spacers to camera, you are placing the FR too far behind the backplane and hence can ( possibly ??? ) change its characteristics relative to design.

Which one is worst ????? No idea. As i said, i am a mug at the details here.

The Agos solution at least allows the camera to CCD distance to be fixed and then the nosepiece can be inserted to get the FR as close to the backplane as possible.

 

Andrew Johansen Melbourne Australia

Hi Andrew, the correct spacing wound up being 87mm Tried from 80 to 110 as i have the adjustable baader & plate solved every one. 87mm got me to f6.2 with a little tighter stars the f6.3 Makes sense only if you take into account the focal length i measured of 204 I set up a better test as originally i measured 210 but this test gave me best focus at 204. Here is the calculation..Look correct to you? If not, shoot me.

 

 

222.png 333.png

 

444.png


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#17 OzAndrewJ

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Posted 22 April 2018 - 12:09 AM

Gday AntMan

 

There is a reason you will very rarely ever see me posting about optics grin.gif

If your platesolving and fwhm say that is the spot for your reducer on your mount,

then i am not going to argue.

Reality always trumps theory.

 

Andrew Johansen Melbourne Australia



#18 AntMan1

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Posted 22 April 2018 - 07:13 AM

Gday AntMan

 

There is a reason you will very rarely ever see me posting about optics grin.gif

If your platesolving and fwhm say that is the spot for your reducer on your mount,

then i am not going to argue.

Reality always trumps theory.

 

Andrew Johansen Melbourne Australia

I thank you again sir... I would not have been able to get this spot on without you. 



#19 GlennLeDrew

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Posted 22 April 2018 - 06:18 PM

Here's what I do when assessing image scale changes. In the daytime I aim at some distant (say, 100m or farther) target having features amenable to taking on-screen measurements with a ruler. Starting from the native image scale as a baseline, other measurements when using a reducer reveal the reduction factor by simply dividing the reduced size by the native size. This is more than accurate enough for determining the optimal reducer-to-sensor separation.

 

The daytime is so much more convenient for this, and you're not wasting precious nighttime hours.

 

As to the reducer's own focal length, it's immaterial that the reducer is operating in the convergent light behind an objective. If you know the f.l. in parallel light (infinite conjugate), the working distance can be accurately derived, which is some fraction of the f.l. for the chosen reduction factor.

 

The problem in practice derives when one does not know where the relevant principal plane for the reducer is located. All lenses or combinations of lenses have two principal planes. Their location w.r.t. the lens depends primarily on the lens shape and overall form. One can easily be making a f.l. measurement from the incorrectly assumed reference plane for the lens.

 

If you derive the f.l. from the image scale formed by the reducer *alone*, this will be reliably accurate, and the relevant principal plane is perfunctorily located at precisely the f.l. distance from the image plane. Once thus located, it serves ever after as the reference for sensor spacings for all telescopes. Indeed, all makers of reducers should supply the location of the principal plane, ideally marked on the cell if (unlikely) it does not lie beyond the cell end.


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#20 AntMan1

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Posted 23 April 2018 - 02:14 PM

Here's what I do when assessing image scale changes. In the daytime I aim at some distant (say, 100m or farther) target having features amenable to taking on-screen measurements with a ruler. Starting from the native image scale as a baseline, other measurements when using a reducer reveal the reduction factor by simply dividing the reduced size by the native size. This is more than accurate enough for determining the optimal reducer-to-sensor separation.

 

The daytime is so much more convenient for this, and you're not wasting precious nighttime hours.

 

As to the reducer's own focal length, it's immaterial that the reducer is operating in the convergent light behind an objective. If you know the f.l. in parallel light (infinite conjugate), the working distance can be accurately derived, which is some fraction of the f.l. for the chosen reduction factor.

 

The problem in practice derives when one does not know where the relevant principal plane for the reducer is located. All lenses or combinations of lenses have two principal planes. Their location w.r.t. the lens depends primarily on the lens shape and overall form. One can easily be making a f.l. measurement from the incorrectly assumed reference plane for the lens.

 

If you derive the f.l. from the image scale formed by the reducer *alone*, this will be reliably accurate, and the relevant principal plane is perfunctorily located at precisely the f.l. distance from the image plane. Once thus located, it serves ever after as the reference for sensor spacings for all telescopes. Indeed, all makers of reducers should supply the location of the principal plane, ideally marked on the cell if (unlikely) it does not lie beyond the cell end.

Thank you for the information. I appreciate it. Hopefully others in the future having the same issue will find there way to this thread. Hard to believe in this day and age the manufacture still does not provide us with the specs or incorrect ones.



#21 AntMan1

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Posted 30 April 2018 - 12:19 PM

Meade finally chimed in about there reducer..... Notice the words "last lense" there is a 2mm gap from the last lense to where the spacer will be screwed into making the outside measured distance 87mm. My stars looked a little better at 88.

 

"The backfocus distance on this 6.3 reducer is pretty forgiving. The customer should use a 85mm back focus when measured from the last lens."

 

Best,

Gary Creason

Senior Product Development Manager

Meade Instruments Corp.

 

fr (1).jpg


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#22 AstroNikko

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Posted 22 February 2021 - 01:50 PM

Hey folks! Following up on the work laid out in this thread. Was hoping someone could check my math.
 
Had a chance to get out and test my setup last night. I'm using a Meade model 2080 8" f/10 (203.2/2000mm) SCT with a Meade f/6.3 FR/FF. Camera I'm using is a Fuji X-T100 with 3.9micron pixels.
 
I tried a few different working distances, and calculated the effective focal length and ratio from plate solved image scale values:

I used the equation for image scale to calculate the effective focal length (image scale = pixel size / focal length * 206.3).

 

Using the focal reducer calculator, the focal length of the Meade f/6.3 FR/FF appears to be 215.8mm. Based on these results, I think the working distance should be 91.5mm (0.628arcsec/px, 1280.16mm effective fl, f/6.3). Will test this the next chance I get.

I'm a bit underwhelmed by the image quality so far. Field curvature appears to be more extreme at 87.2mm working distance, while coma seems to be more obvious at 104.2mm working distance. I'm not entirely sure what to expect, though. Even at the optimal working distance.

 

At 0.629arcsec/px, my captures will be slightly oversampled. So I'm expecting images to be somewhat soft at best. However, I am hoping to see improvements to field curvature and coma at the optimal working distance.

 

Thanks in advance,

 

Nikko

 

---

 

Edit: I kicked the numbers. Fixed the observed and calculated values for plate solved images, and optimal image scale and effective focal length values.


Edited by wcoastsands, 22 February 2021 - 06:52 PM.


#23 nitegeezer

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Posted 22 February 2021 - 02:10 PM

At 87.2mm you were at f/5.8, and at 104.2mm you were at f/6.2. Therefore to get to f/6.3 you must go longer than 104.2mm, not shorter to 91.5mm!!

#24 AstroNikko

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Posted 22 February 2021 - 03:52 PM

Looking at it again now, I realize I don't understand how to use the Focal Reducers calculator. Tried using it to find the focal length of the FR/FF by trying to match the calculated values with observed values. Not getting consistent results between samples. So far unable to validate the FR/FF focal length and optimal working distance when using the calculator.

 

How would you calculate the working distance given the following:

  • Pixel size = 3.9 micron
  • SCT diameter = 203.2 mm
  • SCT focal length = 2000 mm
  • Effective focal ratio = f/6.3
  • Effective focal length = 1280.16 mm
  • Effective image scale = 0.628 arcsec/px

When a working distance of:

  • 87.2mm yields an image scale of 0.634 arcsec/px
  • 104.2mm yields an image scale of 0.679 arcsec/px

 

I haven't been able to figure out the math behind this yet. Any help would be much appreciated.

 

Thanks in advance,

 

Nikko

 

---

Edit: Corrected values due to mistakes made in previous post.


Edited by wcoastsands, 22 February 2021 - 04:12 PM.


#25 barnold84

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Posted 23 February 2021 - 03:02 AM

Hi,

 

There are two things to keep in mind with the Meade focal reducer. Although it's usually called "reducer", it's a reducer AND field flattener. (AND cannot be used with Meade's ACF optics).

For the reducer itself, the changing working distance essentially affects the focal length. So, if you don't match the recommended distance, you either get a faster or slower scope than the f/6.3. The closer you move the camera to the reducer, the slower the optics will be. Therefore, wcoastsands observation is correct that with the closer working distance, he gets a slower scope than f/6.3. The problem with the SCTs is that their focal length isn't constant when you turn the focuser. The calculator is therefore a good approximation. If you want to know the exact f-ratio, you'd better do plate solving.

 

But back to the flattener part: the working distance is important for the flattener. If you are not at the exact working distance, the flattener will not be able to perform at its best. If you are closer or further away, you will see distortions. 

 

It seems to me that the Meade reducer is also sold under different brands. For these, the working distance (the back focus distance) is given as 85mm.

However, according to Meade's documentation, they have an example, where you'd use Meade's SC-T2-Adapter to connect a DSLR. With the T2-DSLR-Adapters, you usually get a optical distance of 55mm from T2 to sensor (which is/was a standard value for photography) and the SC-T2-Adapter is 50mm long. In total that would be the 105mm that one usually finds as recommended back focus for the reducer/flattener.

 

Björn




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