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Back focus for SCT

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#1 Jack160219

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Posted 30 May 2020 - 11:24 AM

I’m waiting for an asi camera to start imaging, I’ll be using a 6.2 focal reducer on my Celestron 9.25 sct which states is a back focus

of 105mm. Now I’m assuming the back focus has some leeway with the focus knob on the telescope, my question is how much? I’m trying to find out

what adapters to get. Thanks john 



#2 kathyastro

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Posted 30 May 2020 - 11:26 AM

The 105 mm "back focus" for the focal reducer means that the camera sensor must be 105mm from the back of the reducer.  There is +/- 1mm tolerance on this spacing.  With that spacing set, you just focus the scope as normal.


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#3 spereira

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Posted 30 May 2020 - 11:36 AM

Moving this to Cats & Casses, for a better fit.

 

smp



#4 barbarosa

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Posted 30 May 2020 - 12:49 PM

The 105 mm "back focus" for the focal reducer means that the camera sensor must be 105mm from the back of the reducer.  There is +/- 1mm tolerance on this spacing.  With that spacing set, you just focus the scope as normal.

AFAIK the 105 mm spacing is the spacing at which you get exactly .63x reduction. Although Celestron sometimes describes the f/63 reducer as a reducer-corrector,  it is a reducer an not a field flattener. Unlike a flattener a reducer does not have to be at a specific spacing.

 

With a reducer changing the spacing increases or decreases the reduction factor. For the f/6.3 the range is from about 0.7x at 50mm spacing to about 0.5x at 225mm spacing. That said the optimal spacing for best image quality is about 105mm.



#5 KNak

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Posted 30 May 2020 - 02:19 PM

 There is +/- 1mm tolerance on this spacing

 

Curious where this is from?

 

I got the 0.63x reduction on my C11 at 96mm from the very back of the reducer.  This thread may be of interest to you:  https://www.cloudyni...-925-sct/page-2

 

So takeaway would be to be as flexible as you can with choice of spacers to account for variance in your particular equipment.


Edited by KNak, 30 May 2020 - 02:21 PM.


#6 luxo II

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Posted 30 May 2020 - 09:26 PM

There is +/- 1mm tolerance on this spacing..

There is no such tolerance - and what’s more it is altered by the use of a compressor.

The nominal spacing of 105mm is correct for an SCT with camera alone (no compressor). If you do not alter the SCT focus and add a compressor with the nominal spacing between that and the camera, you’ll find the focal plane has moved quite significantly.

Hence the 105mm bit is merely a guide at best. In practice you can shift the focal plane + or - 50mm and it makes no difference whatever on image quality.

Edited by luxo II, 30 May 2020 - 09:32 PM.


#7 AhBok

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Posted 31 May 2020 - 09:44 AM

My chip is 105mm from the Celestron r/c on my C9.25 giving me a platesolved focal length of 1385mm as opposed to the expected 1480mm. I get a .63 reduction at 95mm spacing.

#8 donstim

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Posted 31 May 2020 - 04:24 PM

AFAIK the 105 mm spacing is the spacing at which you get exactly .63x reduction. Although Celestron sometimes describes the f/63 reducer as a reducer-corrector,  it is a reducer an not a field flattener. Unlike a flattener a reducer does not have to be at a specific spacing.

 

Celestron "describes" the f/6.3 reducer/corrector lens (part #94175) as a reducer-corrector because that is what it is! It is a combo reducer and field flattener. Although you won't get a perfectly flat field edge-to-edge, it provides a flatter corrected field than without it.

 

At 105 mm spacing, you get both the designed f.6.3 focal ratio and the flatter field. At other spacings, as you note, you get a different amount of reduction and you also deviate farther from a flat field.



#9 XM381

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Posted 31 May 2020 - 07:52 PM

I spent a lot of time trying to figure out the correct backspacing because I found a lot of conflicting and ambiguous info.

 

Then I found this page: https://astro.ecuado...-dslr-on-c9-25/

 

Mike W



#10 donstim

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Posted 31 May 2020 - 09:11 PM

Be careful about using someone else's chart (including mine) to determine the backfocus distance. There appears to be some individual variance between scopes/reducers. For mine, 105 mm from the back flange (basically the back of the glass) gives me exactly f/6.3, as determined from the image scale as analyzed by Astrometry.net. I recommend trying a spacing, then upload it to Astrometry.net and calculating the reduction for your scope/reducer. The Astrometry.net results page provides a value for the image scale in arc-seconds/pixel. The focal length of your scope with reducer then equals (206.265 times the camera's pixel size in µm) divided by the image scale. Divide that by your telescope's aperture in mm and you have your focal ratio. If it's larger than 6.3, you need more backspacing; if it's less you need less.



#11 MikiSJ

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Posted 31 May 2020 - 09:25 PM

FWIW, I have a C11 EdgeHD and added a Celestron OAG with a ASI294 imager and an ASI174 guider. I was able to use the included spacers from the OAG and I found focus roughly halfway at 14,815 steps using a Celestron focuser (0 - ±27,000 calibrated steps).

 

I offer this knowing that a C-9.25 is not a C-11, but the geometry may be similar.

 

Celestron claims the reducer does not significantly affect the focal plane when using their OAG (unfortunately, I cannot find where that claim was madefrown.gif )



#12 barbarosa

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Posted 01 June 2020 - 01:23 AM

Celestron "describes" the f/6.3 reducer/corrector lens (part #94175) as a reducer-corrector because that is what it is! It is a combo reducer and field flattener. Although you won't get a perfectly flat field edge-to-edge, it provides a flatter corrected field than without it.

 

At 105 mm spacing, you get both the designed f.6.3 focal ratio and the flatter field. At other spacings, as you note, you get a different amount of reduction and you also deviate farther from a flat field.

Yes Celestron does say the it has two virtues, focal reduction for "a wider field and much shorter exposures" and "reduces field curvature significantly so you get a flatter, well corrected field." It does appear to provide a degree of correction and it has more lens elements than a simple .5x doublet.  I don't know what can be inferred from the number of lens elements. Some also have 4 and some more.

 

The reason for what I wrote is that the field flatteners with which I am familiar, require very precise back spacing. The Celestron reducer-corrector manual does not give any specification for spacing.  On the web there are conflicting opinions as to just what it corrects. That suggests to me that it is in some way different from a flattener with a specified spacing.   I am a proponent of using the f/6.3 but my personal impression is that it is not as effective a field flattener compared to my refractors with dedicated flatteners. That said the lack of a specified spacing might be seen as flexibility giving the user a range of choices.




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