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A too-short Meade f/6.3 focal reducer

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#1 MadBulgarian

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Posted 01 March 2017 - 12:39 AM

After reading many discussions on the Meade vs Celestron SCT focal reducers, and the confusion about what backfocus spacing is required, I decided to run a simple test (see pictures below).

 

At the bottom is a Celestron f/6.3 reducer/corrector; in the middle is its Meade counterpart, also labeled as an f/6.3 reducer/corrector; and at the top is the Meade f/3.3 reducer. The bright point at right is the rising Sun, good for the infamous “firestarter” method of focal length measurement. Hey, it was a simple setup, and I hadn’t made coffee yet.

 

It’s immediately apparent that this particular Meade f/6.3 unit is not the same as the Celestron. The Celestron is right at specs, with a ~230 mm focal length. The Meade f/3.3 also clocks in close enough to specs, with an ~80 mm focal length. But this Meade f/6.3 unit being tested shows a focal length of just ~130mm. That places it at around 0.4 reduction! (All you optics PhDs will have to run the math for me – no coffee, remember?)

 

Many observers have noted that sometime around 2005 to 2010 Meade’s f/6.3 reducers seemed short. Some have called it a defect; others have speculated that Meade wanted to provide clearance for fork-mounted SCTs to swing vertical. I find this second explanation farfetched.

 

But it also doesn’t seem like a defect to me. I’ve used this exact Meade “f/6.3” unit for many years with great results – big flat fields, nice stars to the edge of a KAI-2020 sensor through a C-11, good enough for some ambitious DSO mosaics. It’s a fine lens – just a much shorter-than-labeled focal length. By far.

 

If you own one of these “short” babies, I’d keep it. It’s a great compromise between the regular f/6.3 and the scary short f/3.3 that’s only good for very small sensors. But always ask before buying one online, and use the numbers here to determine which version you’re being sold.

 

Did Meade and their Chinese manufacturer get their signals crossed? I’d think that the extra curvatures in a 130 mm unit would be harder to figure. Maybe someone out there has the inside scoop on what happened.

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#2 mclewis1

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Posted 01 March 2017 - 09:19 AM

There was indeed a run of Meade f6.3 SCT reducers built with shorter than spec'd focal length.  Newer units from the past few years are apparently back delivering the original spec focal length. From anecdotal information it looked like these shorter fl reducers occurred when the production first shifted from Japan to China. So older units marked Japan are supposed to be fine, as are the units marked China built in the past few years.



#3 evan9162

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Posted 01 March 2017 - 09:43 AM

I have one of these.

 

So did a fellow club member of mine.  He mentioned using it, and how he couldn't focus his SCT with it in place.  I mentioned the issue, he did some research, figured out a test, and confirmed it had too short of a focal length.

 

He badgered Meade about it, and they eventually relented and sent him a new one that works properly. 

 

This was only a couple of months ago, on an FR he got years ago, so it looks like there's hope that you can get a replacement if you're persistent.

 

I'm considering trying the same.  I would think that all 4 elements in the FF/FR are designed to work together to flatten the field without introducing other aberrations.  It seems like a deviation in part of the design (reducing element focal length) would introduce issues elsewhere.

 

I can get it to work as an 0.63x reducer, but with very close spacing - with my DSLR, I had to use a zero-length SCT-T2 adapter to get close to the proper spacing, but the edges of the field weren't very nice with an APS-C sensor (perhaps that's the best I could expect anyways).  It looks like the proper spacing with this "bad" one is about 55mm from the front of the reducer to the focal plane.


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#4 MadBulgarian

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Posted 01 March 2017 - 10:01 AM

Yes, you're right - you can get to focus, barely, with a very short adapter. And then you get more like f/5 reduction because the SCT mirror has to move almost to the end, changing the entire focal length. There was a very entertaining discussion regarding this topic a while back - optics folks take this stuff very seriously!

 

I've had great results with a KAI-2020 chip (~15 mm in diameter) and have pushed it with the KAI-4020 (~21 mm diameter) but got some vignetting. On the 8300, I just can't get the focus right, and have trouble getting the pick-off mirror on the autoguider to intercept the light cone. I'll need some shorter spacers...



#5 evan9162

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Posted 01 March 2017 - 10:11 AM

I've used it with my C6 SCT at around F/5 with a ASI224mc (6mm), with decent results, but there was some bad overcorrection starting to creep in due to how much focus range was used.  I wouldn't be surprised if some aperture reduction happened as well.


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#6 Rickster

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Posted 03 March 2017 - 03:12 PM

Thanks for starting this thread MB. 

 

I recently purchased a Meade f/6.3 FR from Focus Camera on Ebay for the discounted price of  $68 shipped.  I just now checked and it is the 230mm version.  So...If anyone is wondering where to buy one of these (or not), here is a link to the ebay page.  http://www.ebay.com/...=STRK:MEBIDX:IT

 

I haven't had a chance to try mine yet.


Edited by Rickster, 03 March 2017 - 03:14 PM.


#7 MadBulgarian

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Posted 03 March 2017 - 06:26 PM

If anyone out there has used the short version of the Meade f/6.3 reducer (with the off-label 130 mm focal length), please share your experiences - I'm curious to know how usable it was for you, and if you were able to produce good results with it. (Like I said, I'm keeping mine.)



#8 evan9162

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Posted 03 March 2017 - 07:54 PM

C6 SCT, T4i DSLR, zero-length T-adapter, significantly cropped

http://www.astrobin.com/141082/



#9 wa5dxp

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Posted 09 January 2018 - 06:19 AM

I seem to have gotten one of these Chinese focal reducers bought off CN, but was not informed by seller of the shortened focal length.  I had one heck of a time getting the thing

to focus, it would not work with any of 3 diagonals.  (I had not seen this thread and was

working blind trying to figure out if I got flim-flammed).  Finally was able to construct

a very short adapter between rear of lens and the little webcam and got the results in these

captures of moon 1/6/2018.

1) Moon at 400mm through Orion ST-80.

2) Moon through Meade f6.3 focal reducer - Meade 2045 SCT

3) Moon at F10 prime focus, Meade 2045 SCT.

 

Thanks for the information in this thread, it had me puzzled and that image of the

"firestarter" test explained it all.  I wrote to Meade and will see what explanation

they will give.  I had written to them before purchasing the reducer to make sure it

would be compatible with the Meade 2045, and they told me it would work on any

F10 SCT.  I wrote them again when I could not get any eyepiece to work with the thing,

as everything had a black spot of the central obstruction and they told me it was my

eyepiece and my eye pupil that was the problem.  I tried many focal length eyepieces and

got same central spot obstruction in all.

 

As you can see, I'm a beginner, so you can see why I was confused.  I did find out that if

you put the eyepiece almost touching the rear of the focal reducer you can get a

correct image, but it's very awkward putting your eye almost on the rear of the cell.

So with the reducer it's impossible to use a diagonal for normal eyepiece

viewing due to the defective short focal plane not back far enough to make it through

the diagonal.. 

 

Images taken on a cloudy night with $6 ebay special webcam

 

 

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#10 Luna-tic

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Posted 13 January 2018 - 12:13 PM

After reading many discussions on the Meade vs Celestron SCT focal reducers, and the confusion about what backfocus spacing is required, I decided to run a simple test (see pictures below).

 

At the bottom is a Celestron f/6.3 reducer/corrector; in the middle is its Meade counterpart, also labeled as an f/6.3 reducer/corrector; and at the top is the Meade f/3.3 reducer. The bright point at right is the rising Sun, good for the infamous “firestarter” method of focal length measurement. Hey, it was a simple setup, and I hadn’t made coffee yet.

 

 

Very nice expose', good information to file away for future use.

 

Now, let's see a video of the "firestarter" test, and see which one of those puppies lights up first. shocked.gif lol.gif 



#11 E Sully

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Posted 24 January 2018 - 02:01 PM

wa5dxp, have you heard back from Meade?  I have done a little searching, and there are the rumors Meade reduced the back focus length to shorten the imaging train, but no real word.
I have one of the Japan 6.3 FR/FF from 2001.   I did a quick "fire starter" test.  I am not sure where to start the measurement from on the FR, front, center, or back, but it was roughly 230mm focal length like the Celestron. 

Does anyone have the paperwork supplied with the short Chinese FR?  I would like to see if the description differs, I can post mine later.


Edited by E Sully, 25 January 2018 - 11:52 AM.


#12 E Sully

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Posted 28 January 2018 - 03:24 PM

Here are a scans of my original paper work.  They do say it can be difficult for 2" diagonal due to back focus.

Just curious if recommendations changed with the short focal length.

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#13 photoracer18

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Posted 23 March 2018 - 05:59 PM

As far as I know the Japanese made Meade and Celestron 0.63x focal reducers have the same specs because they were made in the same plant. I was not aware of the Meade change in design when they went to a Chinese maker. I have been using the Meade and Celestron ones interchangeably for a couple of decades or more before then.

Edited by photoracer18, 23 March 2018 - 06:00 PM.


#14 wa5dxp

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Posted 23 May 2018 - 02:58 PM

I never did hear back from Meade after requesting the correct spacing between FR and focal plane.  The big confusion is where the measurement is taken?  Some say the FRONT of the FR, but if you look at the FR, the first lens is recessed quite a distance inside the front of the FR.

 

The REAR  FR lens is almost at the rear of the cell with very little spacing between the two.  That would seem the logical place to measure from.

 

I now have an APS-C camera and would like to do some kind of test but don't know how to do it, as the sky here is Bortle 9 and there is no way to see a bunch of stars across the field, as seeing a magnitude 2 star here is lucky.



#15 mclewis1

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Posted 23 May 2018 - 06:29 PM

Try using the open cluster M11 which will be well placed over the next few months as a suitable target to evaluate optical quality (and don't forget to keep the gain turned up to watch for fainter aberrations from these stars). With a big sensor in a DSLR you'll probably want to position the cluster towards each of the corners. In the winter I'll often use the double cluster the same way.

 

The "proper" position to measure a focal reducer from is within or the middle of the lens elements. That can be difficult to approximate on many reducers. Usually with the SCT reducers you can use the shoulder at the top of the male threads to approximate this position (within a mm or two).  It appears however that Meade originally used the bottom of the f3.3 reducer as a reference point so some of the spacing numbers some folks use is based on this measurement point.

 

One number that isn't in dispute is the actual focal length of the focal reducer (this number does however appear to vary a bit between different examples of the reducer). Proper spacing is half this focal length measurement referenced from the middle of the lens elements to the sensor. 

 

So measuring the focal length (using an infinite distance source such as the moon or bright star) and then positioning the camera sensor at the halfway point will get you the correct spacing for your particular example.


Edited by mclewis1, 23 May 2018 - 06:33 PM.


#16 E Sully

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Posted 24 May 2018 - 09:38 AM

I never did hear back from Meade after requesting the correct spacing between FR and focal plane.  The big confusion is where the measurement is taken?  Some say the FRONT of the FR, but if you look at the FR, the first lens is recessed quite a distance inside the front of the FR.

 

The REAR  FR lens is almost at the rear of the cell with very little spacing between the two.  That would seem the logical place to measure from.

 

I now have an APS-C camera and would like to do some kind of test but don't know how to do it, as the sky here is Bortle 9 and there is no way to see a bunch of stars across the field, as seeing a magnitude 2 star here is lucky.

This is from another thread on the Forum regarding Meades f6.3 FR/FF  https://www.cloudyni...ng#entry8532127

 

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.

    Attachment fr (1).jpg



#17 Adun

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Posted 04 August 2018 - 05:28 PM

Hi all.

 

Several months ago I got a C6 OTA, a basically new EVO 6 from someone who just wanted the EVO mount.

 

I got it to be my "grab and go" scope, on a manual mount, and I ordered a Meade F6.3 reducer from telescopesplus.com to make all-around-use easier, TFOV larger and exit pupils brighter. It has worked quite well, I basically love this scope+reducer combination, together they have overtaken my 10" dob as my most used scope.

 

Now I'm about to add an RA drive to my EQ3 mount to try using the C6 for EAA and I decided to measure the focal length of my reducer using the Sun, like was done on this thread. Lo and behold my caliper showed 130mm from reducer lens to the focused sun point.

 

So these shorties are still selling new out there, at least on Hayneedle/telescopesplus.

 

I've had no problems with mine for visual (I use the celestron prism diagonal that came with the scope) and I know it reaches focus with my RT224 (one night I was imaging the planets, and after recording videos of Jupiter, Saturn and Mars I realized I had left the reducer on).

 

What I wonder now is: I need to get down to between F3.3 and F4. Will I have trouble if I get an antares 0.5x to stack it with the F6.3 like I've read several people do here on CN?

 

I also wonder what would happen if I stacked it with another F6.3 shorty. Errol did it here with 240mm FL Meades made in Japan and he said the FL was halved.


Edited by Adun, 04 August 2018 - 05:30 PM.


#18 mclewis1

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Posted 06 August 2018 - 09:39 AM

Adun,

 

The measured focal point is half the specified focal length of the reducer. So getting a number just over 100mm is normal  (the exact number will depend on where you measure the focal point from on the reducer's body so you'll see numbers like 110, 120 or 130mm).

 

The "shorty" reducers were usually branded Meade and had a focal point of 85mm or less.



#19 Adun

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Posted 06 August 2018 - 11:09 AM

The measured focal point is half the specified focal length of the reducer. So getting a number just over 100mm is normal  (the exact number will depend on where you measure the focal point from on the reducer's body so you'll see numbers like 110, 120 or 130mm).

 
Wouldn't that mean that Celestron's dont match their specification?
 
In the OP's test, the distance from "focal point to the reducer's body" is 240mm for the Celestron reducer (bottom row), which do match it's specs:
 
post-20602-0-19512900-1488346400_thumb.j
 
 
If the measurement was only half the FL then the Celestron specs would be wrong.
 
 
What I got from my experiment looked just like the second row of the OP's post (~130mm from the edge of the corrector to the focused sun spot). If his was a "shorty" Meade, then mine is too. ¿Or was his (and mine) a normal Meade reducer?


Edited by Adun, 06 August 2018 - 11:11 AM.


#20 Adun

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Posted 11 August 2018 - 05:35 PM

Ok, I have to make a correction.

 

Today I made a test of stacking two reducers (my Meade F6.3 and a 2" Antares 0.5x I also have) to check their combined FL (I'm trying to figure out how to reduce my C6 for a 1/3" sensor).

 

I made the "firestarter" test somewhat better this time, to get a better measurement, and was able to check focusing the sun to a tinier point. The FL for the combined reducers seemed a little long, so I decided to re-do the test for the Meade, and this time I got ~200mm

 

I must have gotten it wrong in post #17, so I'm setting the record straight, in case anyone's interested in the newer Meade reducers.

 

Also if anybody is interested in stacking reducers, combining "back to back" the newer Meade F6.3 and the 2" Antares 0.5x (from scopestuff) on the firestarter bench yielded 67mm of focal distance.


Edited by Adun, 11 August 2018 - 05:46 PM.


#21 mclewis1

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Posted 12 August 2018 - 06:54 AM

Adun,

 

No, I believe that the specifications are still generally correct ... but I think I may have confused folks with some of the nomenclature. The "measured focal point" vs. the published focal length of the focal reducer. The published focal length of any focal reducer is about twice the required spacing. So Celestron publishing 240mm is in line with folks actually using 100-110mm of spacing or possibly measuring as much as 130mm in the test. Part of the small discrepancy (110 vs. 130mm) may be in the actual measurements ... the uncertainty of where to measure from (the focal length number is from the middle of the lens elements and that can be difficult to measure accurately). Part of that may also be a slight change in the manufacturing specs over time vs. the old published specs. 

 

In actual use the published focal length really has little or no value and in fact can be very confusing. In actual use it's all about that spacing between the reducer and sensor or that "measured focal point".  The "measured focal point" can be considered the maximum spacing used for a particular focal reducer. The actual spacing used by many folks (due to a variety of factors) is often less than that measured number.



#22 Thrifty1

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Posted 01 March 2019 - 05:22 PM

I finally got around to pulling out and comparing my 2 versions of the Meade f/6.3 focal reducer.

I set set up a high powered flashlight about 40 feet away for this test as it is cloudy today. 

 

As as you can see the 2 reducers have a much different focus distance. One is about 135mm to the male threads and the other is closer to 210mm. 

 

It’s been a while since I’ve used them but I have it noted on the case that the shorter one works better with my 10” LX200R and the longer one works better with my ETX-105.  I don’t know how the math works with these reducers so I’m not sure if the scope preference I have noted makes mathematical sense. 

 

I just wanted to share the specifics of the 2 variations of the Meade f/6.3 that are circulating.  

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#23 photoracer18

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Posted 04 March 2019 - 04:28 PM

I notice that the one farther back says "China" on it. Can't see the other one. This sounds logical, as far as I know the original Japan versions from C and M were made in the same factory and I have used them interchangeably in the past but have not used one recently or a China version.

#24 Thrifty1

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Posted 05 March 2019 - 02:15 AM

Both of these have “China” on them - all of the lettering is identical. I never found out if the shorter one was intentionality made that way or was a mistake. When I had trouble getting it to focus following their diagram on the instructions I was sent another one as a free replacement (the longer one).  From what I recall, it seemed like the short one was a production error but Meade never quite came out and said that. 


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#25 asanmax

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Posted 14 October 2019 - 03:38 PM

I hope someone could shed some light on how to use the Chinese reducer which has the focal length of 130mm.

I can focus with my DSLR no problem but it does not flatten the field, it makes it worse, the corners of the frame are severely out of focus.

I unscrewed the ring and double-checked that the two elements are placed correctly.

Just wondering if mine is just a lemon.

Thanks!




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