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Thread: Measuring Newtonian Reflector Back-Focus Distance

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    Default Measuring Newtonian Reflector Back-Focus Distance



    This topic has come up numerous times recently, so I'm posting it in the hopes of providing a simple means of measuring your telescope's back-focus distance.

    If you have a Newtonian reflector (or a refractor), here is a procedure that will help you determine the total available "back-focus" distance.

    Back-focus is something that you need in abundance if you're going use your telescope for astrophotography/imaging.

    Unfortunately, many telescopes, including some refractors and many Newtonians, have very limited back focus and are designed for visual observing only.

    The only telescopes that have near infinite back focus are SCT's, which use movement of their primary mirrors to obtain primary focus, like the commercial Celestron and Meade Schmidt-Cassegrain instruments.

    For every inch of primary mirror travel, an SCT obtains the equivalent of about 6" (150mm) of back-focus distance, depending on the instrument's focal ratio (f/#).

    To measure the available back-focus of your Newtonian reflector, including a Dobsonian mounted reflector you can use the following procedure (this also works for refractors):

    - You will need to understand how to use your focuser first, including the eyepiece lock screw, draw-tube lock, etc. specific to your focuser

    - Obtain a small machinist's scale or similar fine measuring scale, ideally graduated in millimeters. A small measuring "flat" will also be helpful - this can be any flat, stiff item.

    - Locate your shortest focal length eyepiece, and longest focal length eyepiece.

    - You will need to locate a distant object to focus on, or have a focusing mask available.

    - Rack (move via the knob or manually slide) the focuser draw-tube all the way in (toward the telescope tube).

    - Then rack (move via the knob or manually slide) the focuser draw-tube out 1/4" (6mm). ** See note

    - Insert your longest focal length (lowest magnification/power) eyepiece into the focuser draw tube, allowing it to bottom out on the top of the draw-tube (i.e. seat the eyepiece fully in the draw-tube).

    - DO NOT LOCK the eyepiece in place in the focuser's draw-tube.

    - Slide the eyepiece up/out from the focuser draw-tube, *WITHOUT* moving the focuser's draw-tube position until you achieve focus at the eyepiece.

    - Once sharp focus has been achieved, lock the eyepiece in place in the focuser draw-tube with the thumbscrew *WITHOUT* moving it. Re-check the focus, but make sure not to move the draw-tube.

    - Using a small scale, measure the distance from the top of the focuser's draw-tube to a flat laid across the top of the eyepiece, or held level with the eye-lens at the top of the eyepiece.

    - Record this distance.

    - Remove the eyepiece from the focuser's draw-tube - do not move the draw-tube in the process.

    - Repeat the steps above using the shortest focal length eyepiece you have available, and record the measurement.

    - Average the two values, which provide your total available back-focus distance.

    ** Note: The 6mm distance is critical. This represents a "buffer", or safety margin because not all components are created equally. This "spare" 6mm of inward travel represents a reasonable margin of error that may arise due to measuring precision, dimensional differences in components, center-to-center spacing of optical components, etc.

    Many Newtonian reflectors have back-focus distances of as little as 1" (25mm) to 2" (50mm). Some more relaxed designs, or those with low-profile focusers may have values in the 3" (75mm) to 5" (125mm") range. A few that were designed with imaging in mind may have as much as 6" (150mm) of total back-focus distance.
    Last edited by AustinPSD; 01-16-2010 at 05:20 PM. Reason: added note
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    By use of the outlined procedure, you now know your telescope's available back-focus distance. So what does that tell you, and how does one make use of the data?

    As I've mentioned in previous posts, there is a dearth of manufacturer data provided for not only a telescope's back-focus distance, but how much of that precious, limited back-focus distance will be consumed by various components of the optical train.

    Everything that gets inserted into the optical train "aft" of the focuser consumes back-focus distance. This includes things like Barlow lenses, focal reducers, correctors, T-rings and camera adapters, filter wheels, adapter tubes, and so on.

    The physical length of these components is the important factor in computing the back-focus distance required for any specific optical train configuration, and comparison of this total length to the telescope's available back-focus.

    As an example, lets use a common, simple optical train configuration for imaging:

    - 2" T-ring/bayonet adapter
    - Canon 20D camera body

    To compute the required back-focus, we need to know:

    - the total length of the 2" T-ring/bayonet adapter
    - the distance from the front flange of the Canon 20D camera body to the image sensor

    The sum of these lengths is the total back-focus distance requirement for this specific configuration.

    How do we get these values? Two possibilities; the manufacturer/vendor provides them as part of their specification, or we must measure them. In the former case, ideally we need this data before buying the particular component, so that we can choose the most appropriate part... sadly, this information is rarely provided.

    Let's examine the information for the 2" T-ring/bayonet adapter for the Canon camera. There are two sources for this component; the OEM (CNC Parts Supply), and a distributor (ScopeStuff):

    http://www.telescopeadapters.com/true2.htm

    or

    ScopeStuff

    Examining the OEM web-site, we find *no* dimensional information *at all*.

    Examining the distributor web-site, we find partial dimensional data, i.e. the barrel length is 1.25" (31.75mm).

    We can do one of the following:

    - contact the OEM and/or distributor and request dimensional data
    - purchase the component and measure it after the fact

    In my case, I happen to have one of these, and its total length is 1.57" (40mm).

    Canon does not make the necessary dimensional data available at all, so we must resort to measurement. From the face of the body flange to the front of the sensor, the depth is 37mm (1.45").

    Using the data we now have, the total back-focus requirement for this particular configuration is:

    40mm + 37mm = 77mm or approximately 3.03"

    This suggests that our telescope must have at least 77mm of available back-focus distance in order to bring this particular camera and adapter combination into focus.

    Next in this short series - how do I increase the available back-focus distance on my telescope?
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    So now you know how to measure your reflector's back-focus distance, and make use of that information in selecting optical train components like cameras and T-ring/adapter tubes.

    But what if my telescope's back-focus distance is too short, and I can't find any shorter components for the optical train I need to configure?

    Short of getting a new telescope, there are some solutions that can be applied in isolation, or in combination with each other.

    If no off-the-shelf components are available that are short enough in stock form, first consider modifying, or having custom made the necessary short adapter. This will almost always be less expensive and risky than any other alternative. It is possible to purchase adapters, and using the services of a machine shop, muffler repair shop, or hydraulic shop cut down the length of a tube adapter. If one has a dremel or similar rotary tool, bandsaw, lathe, and the patience and skill it is also possible to perform modification of an adapter tube yourself.

    Some of these solutions involve modification of the telescope and/or its components. Some are bolt-on/add-on modifications, while others involve opto-mechanical alteration of the telescope. Some are "free", other than the time and skill required to perform the modification, while others can be relatively expensive - as much or more than the telescope's original cost in some cases.

    It is up to the individual telescope owner to weigh the cost-benefit of modifying an existing reflector (or refractor) telescope vs. purchasing another instrument with sufficient back-focus to utilize any specific set of optical train components.

    The solutions, at summary level, in order of simplicity:

    - Obtain an optical path compensator which inserts into the optical train (add-on)
    - Replace the existing/OEM focuser with a low-profile focuser (bolt-on w/possible OTA modifications, i.e. mounting holes)
    - Replace the existing secondary mirror with a larger dimensioned mirror (bolt-on with possible spider/secondary mirror mount modifications)
    - Move the primary mirror forward, or "up" in the OTA (typically the most complex modification, and difficult to impossible to reverse)

    Let's examine each of these solutions individually.

    The optical path compensator... Baader-Planetarium, an optical component provider that manufactures eyepieces, imaging support components, adapters, and bino-viewers manufactures an item known under the trade-name "Glass Path Compensator". This device is intended for use in supporting a binocular viewing attachment with a Newtonian reflector, and artificially increases the available/apparent back-focus distance without any modification to the telescope or focuser. It is available in 1.25" and 2" diameter, and with a compensating value of 1.7X (i.e. it increases the available back-focus distance by a factor of 1.7X).

    The device is throated on one end like an eyepiece or Barlow lens, to fit into the draw-tube of a 1.25" or 2" focuser. The opposite end has standard T-threads like a T-ring camera adapter. To use this component, the telescope must have a minimum of 32mm (1.25") of available back-focus. In this case, it would provide 1.7X the "base" back-focus distance, increasing the available total to almost 55mm. The Glass Path Compensator ranges from $250 to $350 USD depending on size and vendor.

    Here is a link to one representative vendor/distributor:

    Binoviewers

    Replace the existing/OEM focuser... There are several aftermarket focuser manufacturers/dealers world-wide, including Jim's Mobile (JMI), Moonlight, and so on. The objective here is to replace the existing/OEM focuser with a "low-profile" unit. A low-profile focuser has a shorter base/housing, and a shorter draw-tube, which together reduce the height of the focuser, and move its inward field stop closer to the secondary mirror. These are designed mechanically to allow maximum inward, as well as outward travel. Outward travel can be increased if necessary by addition of a draw-tube extension, which is a widely available, standard component for both 1.25" and 2" tube diameters.

    In some cases, the focuser design is also changed, most commonly to a helical draw-tube travel as opposed to rack-and-pinion or Crayford style unit. This allows the focuser designer to minimize the overall focuser height.

    There are a variety of strategies available with respect to replacing the focuser, which are in part specific to the telescope itself, and in part subjective decisions by the telescope's owner. As an example, the telescope's OEM focuser may be a 1.25" unit. The scope's owner may elect to replace it with a 2", low-profile aftermarket focuser to gain the benefit of the lower profile height and back-focus increase, as well as the benefit of being able to use both 1.25" and 2" hardware with the replacement focuser.

    The specific process depends on the telescope OTA, how the existing focuser is attached, and how much difference there is in the new/replacement focuser's mounting pattern, including bolt holes, the opening in the OTA, etc. Unless a specific replacement exists for a particular telescope, it may require drilling new mounting holes, fitting a new tube plate, or enlarging the opening in the tube if moving from a 1.25" to a 2" focuser.

    The amount of back-focus gain depends on the focuser's design, the height difference between the OEM focuser and the replacement, and the thickness of any OTA tube adapter that may be require to match the OTA's outer wall curvature.

    This particular modification be be combined with the use of the Baader Glass Path Compensator, increasing the overall back-focus distance gain.

    The cost depends on the aftermarket focuser quality, cost of any modification that the end-user/owner is unable to do, and may range from less than $100 USD to as much as $400 USD.

    In the following group of solutions, the telescope's fundamental optical characteristics are altered, and the necessary opto-mechanical changes to support these changes may result in irreversible modifications, damage, or complete failure/destruction of the instrument. For many individuals, the scope of these changes, the risk, skills, etc. may be well beyond that individuals ability and comfort zone. Because the specifics of each modification are entirely dependent on an individual telescope's design and optical parameters, only the general aspects of the solution are provided.

    **WARNING** The following modifications fundamentally alter the telescopes optical and opto-mechanical specification and construction. They are difficult, if not impossible to reverse. If done incorrectly, the may render the telescope unusable, and permanently inoperative.

    Increasing the size of the secondary mirror... By replacing the existing/OEM elliptical secondary mirror with an aftermarket mirror, we can permanently "move" the telescope's focal plane, the point at which the image is formed, further away from the focuser's outer flange or "lip" - i.e. the focal plane is moved away from the telescope OTA.

    This depends on a number of underlying factors: The telescope's aperture must be sufficient to accommodate the larger secondary mirror, a means of removing the existing spider assembly, mirror support, the mirror itself from the secondary mount/support must be available, and a suitable replacement mirror must be available, either off-the-shelf, or designed and produced on a custom basis.

    If the existing components are carefully removed, and retained, this modification may be reversible if the need arises.

    One drawback this solution creates is a reduction in apparent image contrast and brightness, as the consequence of enlarging the secondary mirror is increased central obstruction of the main mirror/OTA. This is usually not a significant issue for imaging, but it almost always is for visual use. Aside from the increase in central obstruction, it also increases the telescope's focal length, with no corresponding increase in aperture. In turn, this increases the telescope's focal ratio (f/#). This has a manageable impact on imaging (requires increasing exposure duration and/or ISO), but nothing can be done for the visual observer.

    This solution path requires optical design, using a support tool like MODAS, Newt, OSLO, or a similar design tool, along with ideally full-scale ray tracing of the proposed change/increase in secondary size. One needs to look for light cone obstruction, increases in aberration, vignetting, and calculate the new focal length and focal plane distance. It may require increasing the focuser opening diameter from 1.25" to 2" on some configurations to avoid vignetting.

    This solution can be combined with the use of the Glass Path Compensator and the low-profile focuser replacement to increase total back-focus significantly.

    Moving the primary mirror forward/"up" in the OTA... This modification ranges from moderately difficult to very difficult, and almost always fundamentally alters the telescope in a way that is impossible to reverse.

    There are a number of ways to accomplish this, depending on the design of the existing OTA:

    - solid tube instruments that have movable/positionable mirror mounting cells are sometimes the most straightforward to modify in this fashion. The existing mirror cell support ring and internal tube stops are removed, and repositioned in the tube, moving the mirror forward. Every inch of forward movement translates into 3" to 6" (approximately 75mm - 150mm) of increased back-focus, depending on the aperture and focal ratio of the mirror.

    - solid tube instruments can be "sectioned", and mechanically joined. This is brute force, and requires cutting out a section of the OTA between the primary mirror end of the OTA and the open/focuser end of the OTA.

    - truss tube instruments can either have the trusses shortened (i.e. cut down), or replaced with shorter truss sections

    In all cases, this solution path requires optical design, using a support tool like MODAS, Newt, OSLO, or a similar design tool, along with ideally full-scale ray tracing of the proposed relocation of the primary mirror. One needs to look for light cone obstruction, increases in aberration, vignetting, and calculate the new focal length and focal plane distance. It may require increasing the focuser opening diameter from 1.25" to 2" on some configurations to avoid vignetting. This may also require resizing of the secondary mirror.

    The telescope will require collimation in all three major cases including a low-profile replacement focuser, enlargement of the secondary mirror, or moving the primary mirror forward.
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    Hi AustinPSD,
    I just tried your procedure on my 8" Newt on a cell phone tower about 1 mile away. My 1.25" 32mm eyepiece had to be slid out about 4mm from the top of the focuser to get the tower in focus. Adding this to the length of the 32mm eyepiece gave me~97mm back-focus distance. Similarly for the 6.4mm eyepiece the distance above the focus tube was 10mm (73mm distance). Therefore, my average calculation is 85mm back-focus distance.
    Would I need to purchase a 1.25" telescope extender tube to attach a camera (with t-adapter and ring)?
    Regards, E-Ray
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    It would depend upon the length of the T-ring and bayonet adapter assembly, and the distance between the camera body flange and the sensor.

    There are a variety of T-ring/bayonet adapters, some are one-piece and consume minimum back-focus distance, while others are relatively long, and consume significant back-focus distance.

    Without knowing the specifics of the camera you are planning to use, and the T-ring/bayonet adapter, its hard to say.

    In most cases, it would be unusual to require an extender - in fact the opposite is most often the problem, lack of sufficient inward focuser travel to allow the combined path length of the T-ring/bayonet and flange-to-sensor distance to find focus.

    The solutions in order of cost/complexity include one-piece, minimum distance T-ring/bayonet adapters, a replacement/aftermarket low-profile focuser, and from there it gets non-trivial and expensive as described in the "sticky".
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    Hi. Are you saying that if I move my primary mirror up 1" toward my focuser on my Newtonian telescope that I don't gain 1" further out foucus but rather approx. 3 to 6 inches of further out focus. Reason is for my canon to work compared to my DSI camera I needed about 2 inches. So I moved my mirror up 2 inches and it seems I gained way more then 2" of focus. I am confused here as I thought that if my mirror is an f8 focusing at 48" then if I move mirror up 1" then it still equals 48" for focus just that I have brought the focal point 1" out from focuser. Did I move mirror to far? Should I have only gone 1" forward with Mirror rather then the 2" forward that I did?
    Thank you
    Don

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    Quote Originally Posted by emflocater View Post
    Hi. Are you saying that if I move my primary mirror up 1" toward my focuser on my Newtonian telescope that I don't gain 1" further out foucus but rather approx. 3 to 6 inches of further out focus. Reason is for my canon to work compared to my DSI camera I needed about 2 inches. So I moved my mirror up 2 inches and it seems I gained way more then 2" of focus. I am confused here as I thought that if my mirror is an f8 focusing at 48" then if I move mirror up 1" then it still equals 48" for focus just that I have brought the focal point 1" out from focuser. Did I move mirror to far? Should I have only gone 1" forward with Mirror rather then the 2" forward that I did?
    Thank you
    Don
    No. On an SCT, a change in mirror position is magnified by the secondary, which is why they don't have back focus issues. On a Newtonian, a 1" move of the mirror will make a 1" change in the back focus because the secondary does not magnify.

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    Thanks to all who responded. I finally put the movement of my primary mirror to the test last night. My reason to move mirror up 2" was so that my focal plane would reach the sensor on my DSLR. This amount of movement would also allow the focal point to fall approx. in the middle of my focuser travel to allow some forward and backward play. My focuser middle point is 0.900" have a total max travel of 1.800" So...focusing the Moon using Live View via computer and the Canon software my camera came to focus at 1.067" I was very happy! Though I did not focus on a star as the cold got the best of me, just using the Moon is pretty close to infinity and that when I do eventually focus on a star, the travel on my focuser will easily reach accurate focus.
    So all is good.
    Thanks
    Don

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    Default Re: Measuring Newtonian Reflector Back-Focus Distance

    I'm wanting to measure the back focus on my 12" truss-style Dob, and I was very appreciative to find this excellent posting on the subject. The primary concern is for AP but I'm looking at a different visual back where this might also be an issue. However, I have a few questions I was hoping you could help with.

    Is this method meant to be an accurate measurement? Since it's based on random numbers to calculate this it doesn't seem like it would be. I say random because any focal length EPs are used. For example, an individual with the same scope could use the same two EPs for the shortest focal length and a different one for the longest. For example, a 6mm might be used as the shortest by two individuals whereas they separately use 32mm and 40mm for the longest. Wouldn't the separate efforts provide noticeably different numbers when averaged that I would imagine would be different by .5" to 1"?

    Another question I have is why I can't use the focuser to rack out to find focus with the EPS and then measure the distance extended vs leaving the focuser tube stationary and moving the EP? Using the focuser seems much easier and more accurate. For error compensation as suggested, the 6mm could be added to the measurement.


    Thanks for your help.

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    Default Re: Measuring Newtonian Reflector Back-Focus Distance

    Quote Originally Posted by shannong View Post
    I'm wanting to measure the back focus on my 12" truss-style Dob, and I was very appreciative to find this excellent posting on the subject. The primary concern is for AP but I'm looking at a different visual back where this might also be an issue. However, I have a few questions I was hoping you could help with.

    Is this method meant to be an accurate measurement? Since it's based on random numbers to calculate this it doesn't seem like it would be. I say random because any focal length EPs are used. For example, an individual with the same scope could use the same two EPs for the shortest focal length and a different one for the longest. For example, a 6mm might be used as the shortest by two individuals whereas they separately use 32mm and 40mm for the longest. Wouldn't the separate efforts provide noticeably different numbers when averaged that I would imagine would be different by .5" to 1"?

    Another question I have is why I can't use the focuser to rack out to find focus with the EPS and then measure the distance extended vs leaving the focuser tube stationary and moving the EP? Using the focuser seems much easier and more accurate. For error compensation as suggested, the 6mm could be added to the measurement.


    Thanks for your help.
    The procedure described here is not meant to provide an 'exact' measurement of back-focus, rather it provides a figure of merit close enough to be of use in estimating how much back-focus a particular instrument has, without requiring an optical bench, interferometer and other tools beyond the reach of the "average Joe". Unfortunately, manufacturers (should, but) don't provide this spec with their product (at least not consumer-grade telescope manufacturers).

    This procedure is independent of the design of a focuser, be it rack-and-pinion, helical, or Crayford, hence fixing the draw-tube at a particular reference point, and moving the eyepiece. It is not 'random' for a particular instrument and eyepiece pair, but I supposed could be considered 'random' in the sense of eyepiece focal length and barrel length over a wide range of individual instruments and eyepiece pairs (but that is more or less irrelevant, as we're not interested in computing the back-focus over a collection of instruments...).

    Moving the eyepiece in the focuser draw-tube barrel will result in more accurate results for rack-and-pinion, as well as almost all helical focusers - this is due to mechanical slop in the draw-tube movement mechanisms, and consequent variance in the end position (i.e. lack of repeatable positioning) in these focusers. It will likely be at least as accurate, if not more so (if one uses patience and care) for a Crayford focuser - variance in the friction mechanism can create lack of repeatability for poorly adjusted or inexpensive focusers...
    CGEM 800 HD, NexGuide, Orion XT8 Limited Edition, Oberwerk BT-100, Canon 20D/20Da/T3i/60D/5D Mk III, various eyepieces, adapters, geegaws, widgets, and tiddlybits

  15. The Following User Says Thank You to AustinPSD For This Useful Post:

    shannong (08-09-2012)

 

 

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