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127Mak Focal Length

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#1 Eric63

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Posted 13 September 2016 - 07:31 AM

Many here on CN have noted that the focal length of their 127Mak is greater than what is advertised.  I recall reading a particular post where the user noted that his 127Mak's focal length was in the 1700mm range as opposed to the listed 1540mm.

 

I therefore decided to do a very informal analysis of my own 127Mak using the Telescope view in Stellarium.  I have a 2012 Skywatcher with a 2" diagonal.   I used polaris (because it doesn't move) and centered the star as best as I could in the EP.   I then compared the view with the Telescope view option in Stellarium  using the same EP as in my scope. I then changed the focal length of the scope in Stellarium until I got a very close match at the EP.  I used a 20mm 70 degree EP as well as a  15mm 66 degree EP.   What I found is that a 1750mm focal length in Stellarium most closely resembled the view at the EP.  I expected a longer focal length because my scope used a 2" diagonal, but I was surprised at the 14% increase in focal length.  This may also explain why some claim that the sky is darker with better contrast when using their Mak.  The extra magnification is darkening the sky.

 

Anyway, I found this little analysis of mine interesting.  I'm curious if anyone has done a more rigorous analysis using star drifting.

 

Eric

 

 



#2 Peter Ceravolo

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Posted 13 September 2016 - 09:23 AM

Some years ago I analyzed the Celestron version of this Chinese instrument, worse than FL variation is that the actual, effective, aperture is about 118mm not 127mm. The 127mm beam is clipped in two places, the rear surface of the corrector (it's undersized) and the primary mirror, also undersized.

 

I tested a I-Optron 6" and found it to be 5.5" aperture. The only Mak scopes I tested whose apertures were as advertised were my 7" Meade, 6" Intes and 3.5" Questar.

 

Peter



#3 Eric63

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Posted 13 September 2016 - 11:21 AM

Some years ago I analyzed the Celestron version of this Chinese instrument, worse than FL variation is that the actual, effective, aperture is about 118mm not 127mm. The 127mm beam is clipped in two places, the rear surface of the corrector (it's undersized) and the primary mirror, also undersized.

 

I tested a I-Optron 6" and found it to be 5.5" aperture. The only Mak scopes I tested whose apertures were as advertised were my 7" Meade, 6" Intes and 3.5" Questar.

 

Peter

 

I measured mine in the 118 mm to 120 mm range with a 48 mm CO (due to the secondary Baffle).  This creates a 40% CO, but to be honest the views are still amazing and very close to my treasured 150F5 Reflector with a re-figured primary and a 33% CO.   The true focal length is now what I am trying to figure out.

 

Eric



#4 Marcsabb

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Posted 13 September 2016 - 02:23 PM

The Bresser / Omegon Mak 127 and soon to be Meade ETX125 (same tube, different rear end and coatings) is actually a 125mm diameter. But that is another scope with a longer 1900mm f/15 focal length.



#5 Asbytec

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Posted 14 September 2016 - 10:27 AM

Eric, here's my star drift data. I find my FL to be closer to 1950mm at f/13 (depending on the eyepiece/diagonal used and focus travel) rather than the 1800mm f/12 as advertised. I would not be surprised if your's was longer than advertised, too. And it will change slightly as you move the primary mirror relative to the meniscus/secondary during focusing. 

 

Actually, now that I remember it, those figures above gave f/15. I was having difficulty with finding the data on the field stop on all my eyepieces and an accurate AFOV. So wasn't real confident I was getting a good measurement for the magnification (leading me to the focal length I had to operating at and I seriously doubt I was observing Jupiter at 280x...more like 240x. So, the focal length had to be shorter. 

 

So, I measured the moon's diameter on the focal plane (using a transparent tape) and used this equation to solve for focal length: s=FL x tan(angle radians), where 's' is the linear diameter of the moon on the focal plane in millimeters (so the answer is also in millimeters) and the 'angle' (in arcseconds converted to radians) is the moon's apparent diameter that evening. This gave me a more comfortable f/13. So, I went with that...lol

 

Attached File  Book2.xlsx   17.67KB   67 downloads


Edited by Asbytec, 14 September 2016 - 10:46 AM.


#6 Eric63

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Posted 14 September 2016 - 11:26 AM

Eric, here's my star drift data. I find my FL to be closer to 1950mm at f/13 (depending on the eyepiece/diagonal used and focus travel) rather than the 1800mm f/12 as advertised. I would not be surprised if your's was longer than advertised, too. And it will change slightly as you move the primary mirror relative to the meniscus/secondary during focusing. 

 

Actually, now that I remember it, those figures above gave f/15. I was having difficulty with finding the data on the field stop on all my eyepieces and an accurate AFOV. So wasn't real confident I was getting a good measurement for the magnification (leading me to the focal length I had to operating at and I seriously doubt I was observing Jupiter at 280x...more like 240x. So, the focal length had to be shorter. 

 

So, I measured the moon's diameter on the focal plane (using a transparent tape) and used this equation to solve for focal length: s=FL x tan(angle radians), where 's' is the linear diameter of the moon on the focal plane in millimeters (so the answer is also in millimeters) and the 'angle' (in arcseconds converted to radians) is the moon's apparent diameter that evening. This gave me a more comfortable f/13. So, I went with that...lol

 

attachicon.gifBook2.xlsx

 

Hey thanks Norme, I'll give that I try.  Thanks also for the spreadsheet.   Sometimes I'm just not in the mood to observe anything but playing around with my scope and trying different techniques to better understand how it functions is lots of fun.  The Geek in me comes out then lol.

 

Eric



#7 Asbytec

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Posted 14 September 2016 - 11:33 AM

As it does me, as well. Prpblem is when our inner geek stays on the loose. :)

Yea, sometimes I just explore my own scope, too. Its fun.

#8 Mike I. Jones

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Posted 14 September 2016 - 05:50 PM

Can you do a set of short exposures on the Pleiades, just long enough to image the brightest stars?  The attached picture I made gives most of the angular separations between the individual stars.  You measure the number of pixels between stars with whatever IP software you prefer.  Then using the pixel size of the camera, multiply by the number of pixels to convert to millimeters.  The focal length is then

 

EFL = D / ( 2 * tan(Θ/2) )

 

where D is the linear distance in millimeters between any two given stars in the image and Θ is the angular separation between the two stars.

 

Example: From the chart, the angular separation between Merope and Electra is 0.370035206º.  You image the Pleiades, and your software analyzes that the images of these two stars are separated by 2076 pixels.  The camera pixels are 5.6µm square, thus the linear image separation between Merope and Electra is 2076 * 0.0056 = 11.6256 mm.  The EFL is then 11.6256 / ( 2 * tan(0.370035206 / 2) ) = 1800.086 mm.

 

Once you have the image, you can statistically calculate the EFL by measuring each of the distances between the Pleiades members and applying the formula, then averaging the results.

 

I chose the Pleiades to be FOV-compatible with a 35mm DLSR and the EFL you estimate in your post.

 

Hope this can be of help,

Mike

Pleiades with star separations 750x543.jpg



#9 gfamily

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Posted 14 September 2016 - 06:14 PM

My understanding is that, if you're using different eyepieces, by the simple fact of having a movable primary mirror (whose position depends on which eyepiece you're using), there is not a 'fixed' focal length, as the geometry changes every time you change the eyepiece.

 

(Guess who read a webpage about 'magnifications' over the weekend) 



#10 Eric63

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Posted 14 September 2016 - 08:25 PM

Can you do a set of short exposures on the Pleiades, just long enough to image the brightest stars?  The attached picture I made gives most of the angular separations between the individual stars.  You measure the number of pixels between stars with whatever IP software you prefer.  Then using the pixel size of the camera, multiply by the number of pixels to convert to millimeters.  The focal length is then

 

EFL = D / ( 2 * tan(Θ/2) )

 

where D is the linear distance in millimeters between any two given stars in the image and Θ is the angular separation between the two stars.

 

Example: From the chart, the angular separation between Merope and Electra is 0.370035206º.  You image the Pleiades, and your software analyzes that the images of these two stars are separated by 2076 pixels.  The camera pixels are 5.6µm square, thus the linear image separation between Merope and Electra is 2076 * 0.0056 = 11.6256 mm.  The EFL is then 11.6256 / ( 2 * tan(0.370035206 / 2) ) = 1800.086 mm.

 

Once you have the image, you can statistically calculate the EFL by measuring each of the distances between the Pleiades members and applying the formula, then averaging the results.

 

I chose the Pleiades to be FOV-compatible with a 35mm DLSR and the EFL you estimate in your post.

 

Hope this can be of help,

Mike

attachicon.gifPleiades with star separations 750x543.jpg

Thanks Mike.  Something else for me to try on another one of my scope testing nights :) I love these various suggestions.

 

Eric  ​



#11 Eric63

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Posted 14 September 2016 - 09:14 PM

Eric, here's my star drift data. I find my FL to be closer to 1950mm at f/13 (depending on the eyepiece/diagonal used and focus travel) rather than the 1800mm f/12 as advertised. I would not be surprised if your's was longer than advertised, too. And it will change slightly as you move the primary mirror relative to the meniscus/secondary during focusing.

Actually, now that I remember it, those figures above gave f/15. I was having difficulty with finding the data on the field stop on all my eyepieces and an accurate AFOV. So wasn't real confident I was getting a good measurement for the magnification (leading me to the focal length I had to operating at and I seriously doubt I was observing Jupiter at 280x...more like 240x. So, the focal length had to be shorter.

So, I measured the moon's diameter on the focal plane (using a transparent tape) and used this equation to solve for focal length: s=FL x tan(angle radians), where 's' is the linear diameter of the moon on the focal plane in millimeters (so the answer is also in millimeters) and the 'angle' (in arcseconds converted to radians) is the moon's apparent diameter that evening. This gave me a more comfortable f/13. So, I went with that...lol

attachicon.gifBook2.xlsx

I tried this tonight and measured the moon at 16.5 mm on the focal plane and the moon diameter at the time was 32'23.8" or 0.54 degrees. I used your equation but I had to keep the moon diameter in degrees and not radians for it to work.

So solving for FL: 16.5mm = FL x Tan (0.54) , I get FL=1750mm, exactly what I got using the Stellarium Method. Hmm it looks like my Stellarium Method was not that bad. So there you have it, my 127Mak with a 2" diagonal has a 1750mm focal length and is an F14.8 if I assume the actual 118mm diameter.

Thanks Norme
Eric

P.s. I also tried this with my 6" reflector and I got bang on 750mm (f5) as advertised.

Edited by Eric63, 14 September 2016 - 10:19 PM.


#12 Asbytec

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Posted 14 September 2016 - 11:33 PM

Thanks Eric. Thats good news for me, too. Your measurements give me confidence in mine.

Now go back and compute the magnification you've been using to observe Jupiter.

#13 Asbytec

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Posted 14 September 2016 - 11:33 PM

Thanks Eric. Thats good news for me, too. Your measurements give me confidence in mine.

Now go back and compute the magnification you've been using to observe Jupiter.

#14 Asbytec

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Posted 14 September 2016 - 11:33 PM

Thanks Eric. Thats good news for me, too. Your measurements give me confidence in mine.

Now go back and compute the magnification you've been using to observe Jupiter.

#15 Eric63

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Posted 15 September 2016 - 07:13 AM

Thanks Eric. Thats good news for me, too. Your measurements give me confidence in mine.

Now go back and compute the magnification you've been using to observe Jupiter.

 

Most of the time my best views of Jupiter were with a 10mm eyepiece, which means that I was observing at 175X and not 150X.  My best view for my seeing conditions was achieved in December 2012 using a 9 mm EP (194X).  I have also used an 8mm EP with good results in good seeing (218X), but the 9mm was still better.  So in the end I would say that the atmosphere here limits me to about 200X magnification on Jupiter or about 42X per inch with 37X per inch being the norm.

 

Eric



#16 Eric63

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Posted 15 September 2016 - 08:27 PM

For the heck of it I also used the moon technique on my C90 and found it to have a focal length of 1450mm  instead of the 1250mm advertised,  This means that my C90 is an F16.



#17 Asbytec

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Posted 15 September 2016 - 10:22 PM

Yea, interesting, eh? I was shocked, really, when I found out I was observing Jupiter at a higher magnification than I thought feasible. 



#18 Ed D

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Posted 17 September 2016 - 07:52 AM

Eric, I have attached my summary of laser and drift tests I did on my 127mm Synta Mak.  I originally scanned it as a .pdf but it was too large to load, so I converted it to a Word doc.  It's still readable.

 

I also attached a pic of the scope with and without the Astro-Zap dew shield showing the view down the baffle tube.  This was taken before removal of the secondary baffle.

 

I did the tests to post accurate magnifications on my sketches.  My findings are in the ballpark:  fl=1700mm and 118mm-119mm aperture.

 

Ed D

 

 

 

 

 



#19 Eric63

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Posted 17 September 2016 - 10:56 AM

Eric, I have attached my summary of laser and drift tests I did on my 127mm Synta Mak. I originally scanned it as a .pdf but it was too large to load, so I converted it to a Word doc. It's still readable.

I also attached a pic of the scope with and without the Astro-Zap dew shield showing the view down the baffle tube. This was taken before removal of the secondary baffle.

I did the tests to post accurate magnifications on my sketches. My findings are in the ballpark: fl=1700mm and 118mm-119mm aperture.

Ed D


Doc1.doc


127 Mak Primary Baffling.jpg


Thanks Ed!

Your analysis is most likely more accurate than mine. I can only measure to within 0.5 mm on the focal plane and a 0.5 mm error is roughly equivalent to 60mm of focal length. Like you I found the true aperture to be 118mm and the CO 48mm. I'm glad I know the true focal length now, I only wish the manufacturers would provide this information. I guess their numbers assume no diagonal.

Eric

#20 Ed D

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Posted 17 September 2016 - 10:24 PM

Tonight I was observing the moon with my 127mm Mak and 7.5mm ED eyepiece.  Knowing the working specs I can figure out that my working magnification was x227 with an exit pupil of 0.53mm, as opposed to x200 and 0.625mm that the factory specs would yield.  To me the differences are significant for accuracy in recording observations.  To a casual observer probably not.  YMMV

 

Ed D



#21 elwaine

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Posted 18 September 2016 - 12:28 PM

So what's 2mm among friends?

 

Several years ago I owned a TEC 6" Mak - #50, from the first run of TEC 6" Maks. Selling it was one of my biggest blunders. As luck would have it, I'm delighted to say that I now have a brand new 6" TEC Mak. I never actually measured the aperture in #50 but it is stamped as having an aperture of 152mm. 

 

gallery_17233_3964_10768.jpg

 

 

My new Mak is stamped as having an aperture of "only" 150mm.

 

gallery_17233_3964_119765.jpg

 

 

I know Yuri made a few changes in the 6" Mak since the first run back around 2001 - 2002 or so. But those changes, AFAIK involved the removal of the knife edge baffles along the perimeter of the OTA to improve thermal equilibrium, and improved anti-reflection coatings on the optical elements. TEC #50 had a pyrex mirror. My new Mak has a quartz mirror. Would that account for the decreased aperture? Better question: is the aperture of my new Mak actually 150mm or is it 152mm? Best question: did I really care? No. Not until I read this thread, and curiosity got the better of me.

 

I decided to measure the aperture of my new Mak. With the primary mirror travel kept at approximately 1/2 way, I used an extremely bright flashlight (2,000 lumens) and shined the light straight down the optical axis with only the 2" visual back in place. The meniscus was 6 inches from a flat sheet of white paper. The projected light circle measured 152mm. A second set of measurements were obtained with a 13mm Ethos in the 2" visual back. Again, the projected light circle was 152mm.

 

What a relief!  :lol:

 

Next step is to measure the true f.o.v. via the star drift method.



#22 Asbytec

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Posted 18 September 2016 - 05:44 PM

Larry, the flashlight test should be done at infinity focus. 

 

"Selling it was one of my biggest blunders." How often do we hear the regrets folks have selling a good Mak. 

 

Best question: Am I envious? You bet. Congratulations on a fine scope. 



#23 elwaine

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Posted 18 September 2016 - 10:44 PM

Larry, the flashlight test should be done at infinity focus. 

:foreheadslap: Good grief. I forgot all about that. Thanks Norme.

 

I should have known Yuri would know a thing or two about the telescopes he builds. Re-measuring the aperture yielded a measurement of 150mm.  :blush:



#24 Vondragonnoggin

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Posted 18 September 2016 - 10:51 PM

My iOptron is really 141mm not 150mm according to a Sky and Telscope review. Not surprised, but it does skew measurements for FL and CO

 

Still gives some great views. Saw that thread on Saturn at 672x and was reminded that one night in some great seeing here in Southern California coastal area, I was able to view Saturn with no image degradation at around 575x in that 141mm iOptron. I have two of them and both samples have great mirrors. I use the 2" back on both.



#25 elwaine

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Posted 26 September 2016 - 04:35 PM

I used Sky Safari to match the f.o.v. I observed through a 13mm Ethos in my f/12, 1800mm, 6" Mak. From there it was easy to calculate that my actual, working fl=2200mm, f/14.7. I was surprised that I was operating my scope 22% above its nominal values. I thought I must have made a mistake somewhere, until I read a post by EdZ in which he calculated that his C5 was operating at a 23% increase over nominal values because of the amount of back focus he employed.

 

That led me to an article by C.J.R. Lord: Calibrating the Effective Focal Length of Catadioptric Cassegrains with Moving Primary Focussing. The math is way above my pay grade, but there is an interesting graph on the last page that shows the relationship of increasing back focus to an increase in actual focal lengths for a variety of SCTs/MCTs. 

 

According to a post written by Eddgie, "adding back focus would increase the spherical aberration of the system at the rate of 1/23d wave for every 25mm of back focus added." That's not very much and is likely way under what most of us lose from less than ideal collimation. Still, the most critical observers might want to limit the back focus of their system to a minimal amount in order to tease out every last bit of observable contrast. 

 

 




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