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Position of Focal Reducer

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#1 BobW55

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Posted 14 August 2019 - 02:11 PM

I have a ES 102mm refractor that has a Focal Length of 714mm.

 

I plan to add a stellarvue 0.8 mm focal reducer.  I know this should reduce the focal length to approximately 571mm.

Question is, where in the light path train do I add it.

 

1) Should it go at the 714mm distance?

2) Should I move it closer to the objective lens by the reduced 143mm ?

 

I know what the reducer is, I understand what it does, but I have not found a single youtube video, or any thing on Google that explains WHERE to place it.

 

This has NOTHING to do with back focus.  I know what that distance needs to be and have the proper spacers for it.

I think #2 in my crude picture is the correct location.

 

FocalDistance (2).jpg

 



#2 PirateMike

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Posted 14 August 2019 - 02:18 PM

Put it on the end of the focus tube, then add the rest of you imaging train. You should be able to reach focus.

 

 

Miguel   8-)

 

.


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#3 kathyastro

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Posted 14 August 2019 - 04:44 PM

The FR has to go inside the scope's focal length.  So the distance from the objective to the FR will be less than 714 mm.  How much less depends on the design of your focuser.  Usually, you put it where the diagonal would have been if you hadn't removed it.


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#4 BobW55

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Posted 14 August 2019 - 10:32 PM

Guess I'll find out if it is too far forward or back when I can not get the camera to focus.  And no I am not talking about back focus



#5 BobW55

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Posted 15 August 2019 - 09:18 AM

I figured it has to go in before any diagonals (Not using) and before the camera.  All the videos explain what a FR is, and how to set the back focus to your imager.  None of them explain the optimal place to put it in the light path.



#6 the Elf

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Posted 15 August 2019 - 12:25 PM

Three things to distinguish: flattener, reducer, flattening reducer (= reducing flattener? Don't know if this term is used.)

Flattener - as the name suggests - flattens the field. The scope's sharp image is on a sphere but the sensor is flat. A flattener corrects for that. A (pure/non reducing) flattener does only that, the focal length remains the same. A reducing flattener flattens and changes the focal lenght. Now the important point: flattener, reducing or non, must be exactly (within a mm or less) in the specified distance to the sensor. If you move closer or further away, the correction fails, the stars in the corners become bad.

Reducer (precisely non flattening reducer) - as you for sure have guessed - only changes the focal length while the image is still curvy. These things may be placed in different distances to the sensor, within a range of up to 50mm without loss of image quality. This is the commen way to work with Ritchey-Chretien telescopes. Here you have the choice: when you put the reducer relatively close to the sensor it reduces only a little bit. The camera moves closer to the scope. If you put more spacers between reducer and sensor it reduces more and the camer moves further towards the scope, often quite a lot. That in turn means often the reducer is either threaded inside of the focusers draw tube (I do that) or slipped in like an eye piece. With a (non flattening) reducer you don't need to be afraid not to come into focus. Just move it a bit closer to the sensor and you gain some distance. Example from my Setup: adding 3mm between sensor and reducer makes the camera move about 10mm towards the scope.

 

Now, would you use a flattening or non flattening reducer? If your sensor is small, the field curvature is not a big problem. If your scope is slow it is even less of a problem. The larger the sensor or the faster the scope the better you are off with a flattening reducer. If your refractor is f/7 and with 23mm chip diagonal (ASI294) you can probably live without flattening.

Bottom line: first you have to decide if you want/need the flattening. If yes, there is only one propper spacing. If no, you have options with a given range.

If a non reducing flatener is an option for you, this is the low cost standard item TS offers:

https://www.teleskop...-Anschluss.html

Mind the description: for your focal length you need 111mm between reducer and sensor

This is the "standard" flattening reducer for refractors. Find the spacing in the description, again it depends on focal length bit is far less than the above, 56mm in your case.

https://www.teleskop...-Anschluss.html

Here is a calculator for the distance to move in:

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

Entering the 56 and f/7 with the WO 0.8 (which I guess is similar to the TS 0.79) you move in by 15mm.

Hope that helps.


Edited by the Elf, 15 August 2019 - 12:29 PM.

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#7 BobW55

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Posted 17 August 2019 - 09:39 AM

Found out I had to use both of the supplied extension tubes, and my focuser racked out about 10mm.  I could probably get rid of one of the extension and rack the focuser out more.  Focuser has 45mm of travel.



#8 james7ca

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Posted 17 August 2019 - 11:26 AM

The reducer changes the effective focal length by a factor of 0.8 and that means that the final focus will converge as if it was coming from a system that had a focal length of 0.8 x f/7 = f/5.6. So, if the backfocus spacing is 55mm (as for a TYPICAL Stellarvue 0.8X reducer) that means that the normal f/7 convergence would be 1/0.8 x 55mm ≈ 69mm (since the f/5.6 beam converges sooner than would the f/7).

 

Now, taking that difference you'd have 69mm - 55mm = 14mm and thus the reducer needs to be moved 14mm closer to the objective (approximately, and MAYBE, see below). Thus, if you marked a point in space that indicated the focus position at f/7 (which would be 714mm from the front objective), the new point of focus with the reducer would be 714mm - 14mm = 700mm and given the recommended backfocus of 55mm you'd have the spacing between the objective and reducer of 700mm - 55mm = 645mm.

 

Now as a check note that 645mm + 69mm (equivalent backfocus WITHOUT the reducer) = 714mm.

 

However, it's more complicated than this because the reducer isn't infinitely thin and thus it adds to the effective spacing between the lens and the sensor. Plus, you really don't know at what point you need to measure to on the body of the reducer itself (when measuring the spacing between the objective and the reducer). Also, the optical spacing may differ from the measured spacing since you are measuring the so-called metal spacing (the length of the metal tubing) rather than the effective optical path length of the reducer itself. So, it's probably difficult to know at what point you need to place the reducer, other than the fact that it has a recommended backspacing, which for the TYPICAL Stellarvue 0.8X reducer is 55mm.

 

So, just make certain that the distance between the reducer and the sensor is equal to the recommended spacing (55mm?) and then let the focus fall where it may (and hope that your focuser has enough inward focus travel to reach focus with the reducer, outward focus travel isn't much of a concern, since that can be handled with extension tubes).

 

Oh, don't remove one of the spacers, it's better if the focuser is extended as little as possible (for rigidity). The only case where you might want to extend the focuser further is if the front edge of the focuser tube (the part that is closest to the objective) is extended so far forward that it causes vignetting by having the restricting diameter of the tube too close to the objective.


Edited by james7ca, 17 August 2019 - 11:44 AM.


#9 pfile

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Posted 17 August 2019 - 01:26 PM

this should not be this hard. your initial post contains a misunderstanding: "this has NOTHING to do with back focus."

 

it has everything to do with backfocus. since you seem to know that the reducer needs to be 55mm from the CCD (or 66mm without the extension stellarvue provides), that's all you need to know. you put the reducer that many mm in front of the sensor... and done. there's not much wiggle room here; the reducer to sensor distance is part of the optical design of the reducer, and there is a narrow range of spacings over which the reducer will operate properly.

 

there's no other "optimal place in the light path" other than the 55mm/66mm in front of the camera's sensor.

 

now, if the reducer fits entirely inside your focuser drawtube, then there is no mechanical reduction of the available backfocus. if for some reason it does not, then you lose however mm that does not fit inside the focuser drawtube. of course you lose an amount of backfocus according to the reduction factor, since obvously now the focal point is ~143mm closer to the back of the telescope.

 

rob



#10 james7ca

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Posted 17 August 2019 - 01:42 PM

...of course you lose an amount of backfocus according to the reduction factor, since obvously now the focal point is ~143mm closer to the back of the telescope.

 

rob

No, it isn’t reduced by 143mm since the reducer only affects the convergence of the beam over the distance of the backfocus requirement (and that’s only 55mm, as previously reported). Everything prior to the reducer remains the same, so if the reducer intercepts the focus 69mm prior to the normal focal point then the optical system comes to focus at 0.8 x 69mm = 55mm (the backfocus requirement).



#11 bobzeq25

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Posted 17 August 2019 - 02:15 PM

Bottom line.  This isn't something anyone calculates.  Once you have the reducer to camera sensor distance correct ("backfocus" is somewhat ambiguous, which is why the answers are all over the map), it's figured out by trying it, and whatever will focus.  Usually the reducer goes as close as possible to the focuser, and it's non-critical.  You don't need any numbers, at all.


Edited by bobzeq25, 17 August 2019 - 02:19 PM.

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#12 pfile

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Posted 17 August 2019 - 02:29 PM

No, it isn’t reduced by 143mm since the reducer only affects the convergence of the beam over the distance of the backfocus requirement (and that’s only 55mm, as previously reported). Everything prior to the reducer remains the same, so if the reducer intercepts the focus 69mm prior to the normal focal point then the optical system comes to focus at 0.8 x 69mm = 55mm (the backfocus requirement).

yes you are right, the formula for the loss of backfocus is distance_to_sensor * ((distance_to_sensor*reducer_focal_length) / (reducer_focal_length - distance_to_sensor))

 

sorry, i only slept 5h last night :)

 

rob



#13 BobW55

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Posted Yesterday, 12:43 AM

this should not be this hard. your initial post contains a misunderstanding: "this has NOTHING to do with back focus."

 

it has everything to do with backfocus. since you seem to know that the reducer needs to be 55mm from the CCD (or 66mm without the extension stellarvue provides), that's all you need to know. you put the reducer that many mm in front of the sensor... and done. there's not much wiggle room here; the reducer to sensor distance is part of the optical design of the reducer, and there is a narrow range of spacings over which the reducer will operate properly.

 

there's no other "optimal place in the light path" other than the 55mm/66mm in front of the camera's sensor.

 

now, if the reducer fits entirely inside your focuser drawtube, then there is no mechanical reduction of the available backfocus. if for some reason it does not, then you lose however mm that does not fit inside the focuser drawtube. of course you lose an amount of backfocus according to the reduction factor, since obvously now the focal point is ~143mm closer to the back of the telescope.

 

rob

If you read my post:

This is about the position of the FR from the OBJECTIVE LENS.

It needs to be positioned so that I still have room to rack the focuser in and out and provide proper focus..

 

This is NOT about how far the camera CCD is located from the FR (Needs to be 55mm) as clearly stated in the camera literature.  This was obtained using the 55mm spacers provided by ZWO/ASI.

 

I should have asked where in the focal train, or how much closer to the Objective lens it needs to be placed.

ES supplies this scope with a diagonal, and  two 38mm long drawtube extensions.  One extension plus the diagonal will give you the 714mm FL, or light path distance.

Remove the diagonal, and add the second drawtube and you cannot achieve focus, you are about 10mm short in FL.  I lost my paper when I figured the Light Path distance through the diagonal.I think it was close to 60mm.  But I figured it out.  Needed BOTH extensions, then the FR.  and I can achieve focus with the focuser racked out 11mm.  I have not taken a tape measure to it to see.  Since I am using this scope 90% for AP I am happy with this setup.  I had quite the conversation with ES over all this, they could not even tell me how long the light path was through the diagonal. 

 

James is correct, it works out to about 65mm closer to the objective.

 

Don't confuse FL with Back focus.  Back focus only applies after the FR or FL and is the distance to the CCD or camera sensor.



#14 james7ca

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Posted Yesterday, 01:53 AM

I believe what pfile was getting at is that the only spacing you really need to be concerned about is the recommended backfocus (spacing) between the reducer and the sensor itself. Note what I said earlier in post #8:

So, just make certain that the distance between the reducer and the sensor is equal to the recommended spacing (55mm?) and then let the focus fall where it may (and hope that your focuser has enough inward focus travel to reach focus with the reducer, outward focus travel isn't much of a concern, since that can be handled with extension tubes).

Furthermore, it's actually this backfocus requirement that helps you determine how far forward you need to move the focus point. So, the 55mm of backfocus pretty much determines everything as suggested in post #8.



#15 bobzeq25

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Posted Yesterday, 10:13 AM

  Back focus only applies after the FR or FL and is the distance to the CCD or camera sensor.

"backfocus" is a general term, used for a number of things in astrophotography.

 

It can be the flattener to camera sensor spacing.  It can be the optical thickness of many accessories, such as off axis guiders (or various parts of OAGs) or filter wheels or....  It can refer to the capacity of a particular telescope to accept various accessories.

 

It would often be helpful in these discussions to use the specific description of what is under consideration, rather than the ambiguous term.  What's on someone's mind when they use the term may not be clear to all.


Edited by bobzeq25, Yesterday, 10:18 AM.


#16 pfile

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Posted Yesterday, 10:14 AM

i think the problem here is one of semantics. the term "backfocus" or "back focus" is unfortunately 'overloaded' as they say. on the one hand people say "what is the backfocus requirement for this focal reducer?", by which they mean "how far in front of the sensor is the reducer supposed to go?" people also say "how much backfocus does this telescope have?" and they mean "how far behind the back of the telescope is the focal point for this telescope?"

 

as james also said, the only "backfocus" number that needs to be precisely determined and set is the first one - the reducer to CCD spacing. the 2nd type is usually not framed as you have asked - not "where do i put the FR/FF?" but rather "will i still be able to come to focus after i insert this reducer into my optical train?" - in other words, #2 is a question of how much of the telescope's backfocus is "eaten" by the mechanical and optical properties of the reducer.

 

so #2 is best answered by knowing the original "telescope backfocus" and then adding up the length of all of the things you are trying to put in the train (rotator, focuser racked all the way in, camera adapters, any mechanical length of the reducer) and subtracting those from the original telescope backfocus and then subtracting the optical length of the reducer. if that number is > 0, then you can still come to focus. *where* the reducer sits is actually irrelevant. you can't change its position to make the telescope come to focus. it either does or doesn't - either the new, modified telescope backfocus supports all the junk in your optical train, or it doesn't, and the focal point is permanently in front of the sensor. there's no reason to finesse this unless for some reason even with the reducer inserted and the focuser racked all the way out, the focal plane is still behind the sensor. unless you are using one of these ultra-low throw focusers like an atlas, this is probably not a problem for you.

 

so: focus is possible if (telescope backfocus) - (reducer optical length) - (any reducer mechanical length) - (focuser racked-in length) - (rotator length) - (adapter lengths) > 0.

 

i say "any reducer mechanical length" because most of the time reducers are designed to fit inside a focuser like a camera nosepiece, in which case the mechanical length is almost 0. but sometimes a reducer is not designed this way, and it has some mechanical length that pushes the camera back more.

 

rob




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