Very impressed with the optics and build quality and it looks great. Usually, with my 2 inch dielectric diagonal, I enjoy it's wide field views to scan the area southern skies over Long Island and enjoy viewing the DSO's around Sagittarius. I use either a unit (red dot) or my ES 50mm optical finder scope. There's also a. If you use any of the below telescopes, then make sure your back focus spacing is accurate. I have Meade infinity 80mm and I have problem Focusing with T-ring and T-adapter on my Canon 60D. The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power.A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light. In both cases the cameras have fixed-focus plastic lenses, but the Instax Mini has more sharpness and detail in spite of a much smaller print size. Hi, thank you very much for this very clear article. For example, the Baader MaxBright II binoviewers have a straight-through path length of 110mm. [4], Learn how and when to remove this template message, A 300mm f/2.8 photographic lens with the "APO" designation, tested for chromatic aberration, A 300mm f/2.8 photographic lens without the "APO" designation, tested for chromatic aberration, "An inexpensive photographic zoom lens with the "APO" designation", https://en.wikipedia.org/w/index.php?title=Apochromat&oldid=969074342, Articles needing additional references from October 2009, All articles needing additional references, Creative Commons Attribution-ShareAlike License 3.0, This page was last edited on 23 July 2020, at 07:18. Sometimes we get lucky and capture beautiful sunrises and sunsets with blood red skies, and other times we are stuck with a clear, boring sky. The AT102ED represents an amazing bargain for a 4" class ED doublet. Move the focus ring until you can clearly see the defined shape of the moon and simply turn off Live View you are done. So, while 55mm is usually correct for most setups, we always recommend double checking the manuals and diagrams for your corrector/reducer or telescope (if it has a corrector built in) to be sure. Focusing with T-ring and T-adapter on my Canon 60D. Remember, our planet constantly rotates, and since we are shooting from a tripod that is fixed in one position, we really have to be careful about timing each exposure, as we need to keep stars as dots in our shots. Here are some selected early quotations and interpretations on the topic. For any practical f-number, the added focal length is insignificant in comparison It must be in stock at the time of Price Match for us to make a guarantee. To calculate how much back focus spacing you need to add, take the thickness of the filter and divide it by 3. For a camera focused on this distance, [7] He uses the term Depth Constant for H, and measures it from the front principal focus (i. e., he counts one focal length less than the distance from the lens to get the simpler formula), and even introduces the modern term: This is the maximum depth of field possible, and H + f may be styled the distance of maximum depth of field. [1] The residual color error (secondary spectrum) can be up to an order of magnitude less than for an achromatic lens of equivalent aperture and focal length. Sometimes it means driving several hours away from where you live. Fit, finish and mechanics are far above the modest sale price of $499. Or you can use an extension tube with a small piece of translucent paper taped to the far end of the tube. Is additional back focus adjustment needed due to the shift caused by the camera glass window? The scope now comes with a retractable dew shield. First, lets look at the back focus of a telescope and guide scopes. Thank you. There is no indication that the 5mm was pushing the scope. I've had a few viewing sessions with this beautiful telescope and I'm pleased to say it was money well spent. First using similar triangles hatched in green. You do this in specialized software that aligns the stars (without rotating the foreground) before blending the images together. , and we see that the range of distances acceptably in focus will run from just half the hyperfocal distance to infinity. Hi there Im confused as to who is right. Should I be able to just thread the OAG onto the SCT threads and the ASI2600MC-P onto the OAG to achieve the 55mm back focus you mention in the article? That means DSLR cameras will not work with this instrument, and there is virtually no room for many other optical accessories. The other hardware is very nice, too. {\displaystyle f} I have to concentrate on seeing color fringing visually on very bright objects and we are talking moon level bright objects here at 200+ X. I have done some imaging with this scope and find the views very well color corrected, however I have yet to image with a bright object in the field of my sensor. About rule of 500; what should be the length if we use 15-30 lens? {\displaystyle H} The correct back focus spacing for that reducer/flattener is 55mm. Both optical correctors and focal reducers are designed to work best when the focal plane is at a set distance away. Back Focus of Optical Components for Astrophotography, 3.1 Required Back Focus of Field Flatteners, Focal Reducers, and Coma Correctors, 3.3 The Back Focus of an Astronomy Camera and Flange Focal Distance, 6.1 William Optics 50mm f/4 RotoLock Guidescope, 6.2 Small Petzval Refractor, DSLR, and a NO Focal Reducer, 6.3 Small Petzval Refractor, DSLR, and a Focal Reducer, 6.4 Small Refractor, Astronomy Camera, Focal Reducer, and Filter, 6.5 Schmidt-Cassegrain Telescopes and f/6.3 Focal Reducer, William Optics 50mm f/4 guidescopes with RotoLock, ZWO 5-position 2 electronic filter wheel, Choosing the Best Telescope for Beginners 2022, Top Night Sky & Astronomy Events in August 2022, Top Night Sky & Astronomy Events in July 2022, The Agena AstroProducts Buyers Guide to iOptron Mounts, Selecting a Baader Planetarium Star Diagonal, A Guide to Choosing Astronomy Eyepieces for Binoviewers, Choosing a Magnifying Finder Scope for Your Telescope, Sharpest image across the field of view (a flat field), An optimized and even illumination across the sensor, For many imaging applications, its very useful to have, Rotators, used for camera orientation control, should typically be placed between the telescope focuser and the corrective optical accessory, but not between the optical accessory and the camera, Flip mirrors are difficult to use after field flatteners and coma correctors because of their long optical path length, Some corrective optical elements have threads for filters. Image size was about that of a marble so very reasonable. There is a slight bit of color on the lunar limb, but it is more that made up for by the quality glass figure and ED glass, which provides crisp detail for a 4" scope. Its actually comprised of two individual frames, stitched together: The image quality from this method is extremely high. Focal Range. To me it's worth twice the price. If the back focus of the component is 55mm, then the camera sensor must be placed 55mm away from the back of the cell of the corrector. Lenses designed to correct for two wavelengths are called achromatic, while photography lenses designed to be highly corrected for red, green, and blue wavelengths are referred to as apochromatic. The William Optics 50mm f/4 guidescope, for example, has a back focus of just 6.2mm. The imaging camera is a ZWO ASI2600MC Pro. Binary stars that I've been able to view (very rainy summer) have resolved nicely.I do get veeerryy slight fringing on the moon at high magnification (+200X) but it something you have to look for, it isn't generally noticeable. 1 D In some cases, the scopes focuser may not be able to accommodate this change. Back Focus of Telescopes and Guide Scopes 2.1 Back Focus Specification of Telescopes. When looking into the visual use of this telescope, it is best to view in a somewhat "darker" place with not as much light pollution such as suburbs of the city of Chicago, etc. Thus if a lens which is focused for infinity still gives a sharp image for an object at 6 yards, its depth of field is from infinity to 6 yards, every object beyond 6 yards being in focus. Now lets see how this all works for a small refractor, for this example, the Askar FR400 f/5.6 76mm Petzval refractor and a Canon DSLR. Still, it almost always appears under similar circumstances, like dark foliage against a blown-out sky. The depth constant and the hyperfocal distance are quite distinct, though of the same value. But it all relates to making sure that the optical accessories you use focal reducers, field flatteners, and coma correctors are positioned in the optical path at a location where they work as their designers intended. I believe that should have been included or shipped ASAP so the scope would have a "finished/complete" look, and not appear like some of those "generic" achros being sold on Ebay. Once you see it in crisp focus either via the LCD screen or the view finder, you have your focus at infinity. 2 The E & F stars in the Trapezium were easy. In the same lens, the focal range will depend upon the size of the diaphragm used, while in different lenses having the same apertal ratio the focal ranges will be greater as the focal length of the lens is increased. The whole scene was illuminated just by moonlight! This "mathematically" observed relationship implies that he had a formula at hand, and a parameterization with the f-number or intensity ratio in it. Piper (1901) calls this phenomenon "consecutive depths of field" and shows how to test the idea easily. Adjacent to Table I in his appendix, he further notes: If we focus on infinity, the constant is the focal distance of the nearest object in focus. Its typically close to about 5 inches or 127mm from the rear SCT thread. and if yes, from what I have to measure it ? Here, an object at H forms an image with a circle of confusion indicated by the brown ellipse where the lower red ray converging to its sharp image intersects the blue line. f This type of back focus is a little more challenging to grasp, and its often the bane of new astrophotographers. Like the 2" eyepiece holder, the 1.25" adapter also uses a non-marring soft brass compression ring to hold 1.25" star diagonals and accessories in place. An object at distance H forms a sharp image at distance x (blue line). But in moving the mirror, the telescope delivers a huge range of focus travel which enables focus with optical components like diagonals or binoviewers. I never use AF in astrophotography and I do use iOptrons Sky Guider Pro mount. Welcome back AT102ED! Hi c F/4.1 means very short exposures, and a long back focus means compatibility with a variety of camera gear. Both values are for the circumstance when no Baader glass-path corrector (GPC) is used. While many lenses particularly fast and poorly corrected designs will produce a small amount of purple fringing by default, post-processing can make it particularly noticeable. Beautiful crisp wide views, great sturdy build quality, nice smooth focuser, easy to adjust and customize to my needs. This article explains the back focus of telescopes and optical accessories like field flatteners and focal reducers, and why back focus is important for visual observers and especially for astrophotographers. Detail was beyond expectation. Weighs just 12.7 lbs. Now that I see many good reviews on this scope. For example, in astrophotography, its very common to get purple fringes around stars if youve focused a bit too far. Kind regards Andy (very, very new to DSLR photography). Just wanted to leave a better description of my experience. All of the guys in my club were impressed. C. Welborne Piper may be the first to have published a clear distinction between Depth of Field in the modern sense and Depth of Definition in the focal plane, and implies that Depth of Focus and Depth of Distance are sometimes used for the former (in modern usage, Depth of Focus is usually reserved for the latter). I am due to go to Finnish Lapland in search of the Aurora Borealis, and wondered what is the best lens for capturing these wonderful dancing night lights. Perfect for the visual observer who wants near color-free views without a high price tag. But most general-purpose refractors are designed to have a sufficiently long back focus to accommodate star diagonals for visual observation. Purchase your high-quality Meade electronic telescope today at High Point Scientific. That has a flange to sensor distance of 20mm. Although I did not know a whole lot about Telescopes he was very impressed and said that it works excellent and loves it.
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