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f/6.3 reducer spacing for SCT

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#1 Scott Mitchell

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Posted 01 April 2010 - 01:21 AM

Hi everyone, I finally got a focal reducer for my LX90 just in time for galaxy season! I took it out tonight to give it a try for the first time and I seem to be getting WAY more reduction that I'm supposed to. Using CCD Calc to compare my actual field of view with it's estimated field of view (using the Leo Trio as my example) I seem to be getting about a .4 reduction. I've attached my setup. My T-adapter has a 2 inch extension which is what I've been using successfully on my WO 66SD. This is the first time I've tried it on the SCT.

In addition to having a really wide field of view (too wide for my intended purpose), the vingetting is severe and the stars out in the corners look more like crescent moons than stars.

So the real question is... Is my sensor too far away from the focal reducer? Any input is appreciated.

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#2 ldesign1

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Posted 01 April 2010 - 02:28 AM

Hi Scott:

Which DSLR are you using? Which reducer are you using?

The distance from the Focal Plane (sensor) to the lens Flange on a Rebel XT = 44mm.

To achieve a 0.63 reduction, the optimal distance needed from the Sensor to the front of the reducer lens is as following:

Antares f/6.3 has focal length of 220mm and a diameter of 40mm = (82mm)
Meade f/6.3 has a focal length 260mm and a diameter of 40mm = (95mm)
Celestron f/6.3 has a focal length 284mm and a diameter of 40mm = (105mm)

The further out you are from the sensor, the greater the reduction and vignetting. The closer in you are from the sensor, the least amount of reduction will be achieved and also less vignetting.

If you aren't using one of the listed reducers, which one are you using? What is it's focal length and lens diameter at the stop.

My guess is that you between 120mm-130mm from looking at your photo.
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#3 Scott Mitchell

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Posted 01 April 2010 - 05:54 AM

Sorry, I guess I was a little short on specifics. I have a Canon XSi and the Meade focal reducer (4000 series if it matters).

#4 Steve OK

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Posted 01 April 2010 - 06:26 AM

ldesign1 said:

"To achieve a 0.63 reduction, the optimal distance needed from the Sensor to the front of the reducer lens is as following:"

So is this diagram appropriate for the Antares unit?

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#5 Scott Mitchell

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Posted 01 April 2010 - 05:53 PM

OK, I just measured the extension and it looks like it is just about 6 cm. So assuming the 44 mm setback is accurate for the XSi as well that puts me right around 104 mm. That doesn't seem that far off. Can that really explain the extreme reduction that I am seeing?

Can anyone with a similar setup (Meade LX90 or other SCT, Meade f/6.3 focal reducer, and Canon XSi) share what adapters they use to get the proper spacing?

#6 mclewis1

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Posted 01 April 2010 - 06:53 PM

It is too far ... your spacing is correct for the Celestron unit and some of the Meades but many Meades require the shorter 82-85mm spacing. You'll likely need to reduce the spacing by removing the 25mm spacer. You can fine tune the spacing using other T thread spacers like these

#7 Scott Mitchell

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Posted 01 April 2010 - 08:18 PM

Thanks Mark, I will try a combination that comes up with the 82-85mm spacing and see how that does.

#8 ldesign1

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Posted 01 April 2010 - 08:52 PM

Steve:
Your diagram is correct for the Antares unit.

Scott:
Is 104mm the distance to the first lens of your Meade Reducer? If so, then 104mm would give you a 0.6 reduction. What did you do to figure out that you are getting 0.4 reduction?

Here is a link to a Focal Reducer Calculator:

Focal Reducer Calculator

#9 Samir Kharusi

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Posted 02 April 2010 - 12:12 AM

Could be easiest to handhold the camera with the telescope horizontal pointing at the horizon. You can then move back and forth to gauge what looks "right" across the frame in the viewfinder. And then you buy/fabricate an adapter of the right length. My attempt at solving the spacing issue on a C14, here. By the way, a cheap set of extension tubes for macrophotography can also come in handy for varying the spacing.
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#10 Scott Mitchell

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Posted 02 April 2010 - 05:46 AM

Hi Ralph, I was measuring from the very back of the reducer. I guess that means my 104 mm is understated by a bit. I'm not at a point where I can measure right now, but I'd guess it's more like 130 mm or more.

#11 mclewis1

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Posted 02 April 2010 - 08:56 AM

Ralph,

Any chance you could do us all a favour and shorten the link in your post using the embedded URL BBB code or one of the TinyURL services?

Please

#12 Steve OK

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Posted 02 April 2010 - 10:08 AM

The link worked fine for me...just sayin'!

#13 mclewis1

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Posted 02 April 2010 - 01:22 PM

It's not that the link does or doesn't work (it does for me too) ... it's just so long that it causes the the browser screen to scroll sideways and that's kinda annoying.

#14 ldesign1

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Posted 02 April 2010 - 07:20 PM

Scott:

At 130mm you would get a 0.5 reduction. What's cool about this calculator is that I will also tell you how much of your image will not be vignetted. If you know the size of your sensor that would be helpful. Here is another link that gives the size of various deep sky objects in arc minutes. As well as a calculator to determine what will fit on you sensor depending on your setup.

DSLR Calculator for Astrophotography

#15 Scott Mitchell

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Posted 02 April 2010 - 07:48 PM

Is there any way to know the focal length of my reducer. I've now heard from a couple of sources that there are at least two different models with different focal lengths. The single page of info that came with it doesn't mention anything useful in that regard (no big surprise there).

#16 ldesign1

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Posted 02 April 2010 - 08:46 PM

Scott:

I listed three of them in my first post. I thought you said you had a Mead f/6.3 reducer? If you don't have one of the three I've listed, try Googling (Focal length of _______ reducer). That's how I found out what these three were. Hope that helps.

Antares f/6.3 has focal length of 220mm.
Meade f/6.3 has a focal length of 260mm.
Celestron f/6.3 has a focal length of 284mm.

#17 Samir Kharusi

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Posted 02 April 2010 - 10:39 PM

There were rumours going around at one time that Meade had released some 0.63x reducers with the wrong lens elements (like for their 0.33x). If you happen to have received one of these then the easiest way to be sure is to measure the focal length; awkward because the focal length is negative. Offhand the easiest method that comes to mind is to use a pair of reading glasses. Measure the focal length (eg simply by focusing the Sun on the ground) with and without the focal reducer and then use the formula:
Combined 1/f = 1/f1 + 1/f2
Hope I recall that correctly from 50 years back in high school... f is the combined focal length, f1 the reading glasses on their own, and f2 the reducer. f2 should come out negative. No matter what, that distance between the reducer and the DSLR sensor in your photo looks excessive.

#18 RoundStars

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Posted 03 April 2010 - 05:35 AM

Ralph:

Please explain the math here:

To achieve a 0.63 reduction, the optimal distance needed from the Sensor to the front of the reducer lens is as following:

Antares f/6.3 has focal length of 220mm and a diameter of 40mm = (82mm)


I don't understand how you're manipulating the figures for FL, diameter and distance to sensor to get 82mm. Are you saying that the extension tube should be 82mm long? How are you calculating this?

Are you saying that the distance to the sensor should equal the FR's focal length?

How does the FR's diameter enter the calculations? Or by diameter do you mean the FR's front-to-back length?

Sorry for being slow on the uptake.

-Joe

#19 ldesign1

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Posted 03 April 2010 - 06:17 PM

Joe:

The optimal distance numbers aren't my calculations. I got them from inputting the Reducer focal length, Reducer lens diameter, and the distance from the sensor. See my second post that has a link to the Focal Reducer Calculator.

The reason you need the reducer diameter helps determine how much vignetting you will have and various reduction ratios.

Give it a try.

#20 PatHolland

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Posted 21 April 2012 - 09:00 AM

Hi everyone, I finally got a focal reducer for my LX90 just in time for galaxy season! I took it out tonight to give it a try for the first time and I seem to be getting WAY more reduction that I'm supposed to. Using CCD Calc to compare my actual field of view with it's estimated field of view (using the Leo Trio as my example) I seem to be getting about a .4 reduction. I've attached my setup. My T-adapter has a 2 inch extension which is what I've been using successfully on my WO 66SD. This is the first time I've tried it on the SCT.

In addition to having a really wide field of view (too wide for my intended purpose), the vingetting is severe and the stars out in the corners look more like crescent moons than stars.

So the real question is... Is my sensor too far away from the focal reducer? Any input is appreciated.


Well Scott, I don't normally criticize images but I have to say that your image of your scope has a lot of noise in it. Have you tried stacking several images of your scope or adding a few dark frames? :lol:

Seriously though, with the Meade focal reducer, you want the center of the focal reducer to the focal plane (CMOS) to be 95mm for F6.3.
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#21 Suresh

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Posted 14 November 2015 - 12:25 AM

ldesign1 said:

"To achieve a 0.63 reduction, the optimal distance needed from the Sensor to the front of the reducer lens is as following:"

So is this diagram appropriate for the Antares unit?

Steve , for my ASA and TAK reducer the distance is calculated from the back surface (lens facing the camera chip) to sensor but in you r diagram for the antares youv e pointed to the front surface , how did you arrive at this. Im asking bcos i have an antares focal reducer myself and havent got good results ,

Suresh



#22 Thrifty1

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Posted 14 November 2015 - 08:27 AM

Scott - there were some "bad" or different Meade 4000 series f/6.3 reducers. I know as I bought one of the bad ones and had it replaced by Meade. They did not ask for the bad one to be returned so I have both.

 

If I hold them above a piece of paper and focus the moon (or a bright object) onto the paper the 2 focal lengths are very different.  I don't recall the actual measurements but one was around 110mm and the other much longer.  If needed I could dig them both out to get the measurements. 

 

i would do a quick measurement of the focal length or be sure you have one of the good ones. 

 

Edit: I pulled them out and did a rough estimate. One was around 120mm and the other around 170mm.  That was measuring from the rear male thread on the reducer to the point of focus using the sun. 

 

The shorter focal length one had a tighter focal point and the like longer one had a wider focal circle. 

 

They are both labeled the same and both have "China" on them. 

 


Edited by Thrifty1, 14 November 2015 - 09:15 AM.


#23 Thrifty1

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Posted 14 November 2015 - 09:23 AM

Based on the 2 focal reducers I have, which would you recommend I use? I have not had my scope out in the past couple of years and have just moved from the light polluted skies of Chicago suburbs to outside of Nashville with less light pollution. I am setting my 10" LX200R up again to do some astrophotography with my Canon Rebel XT.

 

If my memory is correct I was using the shorter focal length focal reducer in the past.  Now I am trying to get the set-up correct using all the adapters I have. 

 

Edit: just noticed the post from yesterday was responding to posts from a couple years ago. I'll ask my questions regarding my focal reducers on a new post. 


Edited by Thrifty1, 14 November 2015 - 11:08 AM.


#24 MadBulgarian

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Posted 04 July 2017 - 03:35 AM

Randy, you're right. You can see the comparison between the Celestron f/6.3, a "short" Meade f/6.3, and the Meade f/3.3 reducers here:

 

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

 

Clearlly there's a batch of Meade f/6.3 reducers that are more like f/4, explaining Scott's observation. Sorry it took 7 years to publish the experimental results!


Edited by MadBulgarian, 04 July 2017 - 03:36 AM.


#25 Lumix.guy

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Posted 17 March 2020 - 01:01 PM

Interesting ... I have a Japan-made Celestron 6.3 reducer/corrector (#94175) and I measure it's focal length to be about 225mm to the housing shoulder on the camera side.

 

That would suggest a back-focus distance of about 83mm; similar to the Antares.




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