This is the best utility Ive seen of the many out there. To make this simple calculation, you need two numbers: the primary mirrors focal ratio (f) and the distance from the secondary mirror to the focal plane (L). I am building a 12.5 in f/5 telescope, and I did my calculations for my secondary mirror size and I got a size of 2.14 inches would be the closest and best secondary size. Quite easy to swap it out and re-collimate in the daylight. No scope can do it all, the only gap in the 8" f/9 performance envelope is very wide field low power viewing. That 225mm distance is only 66.25mm longer than the radius of your mirror. - Mel Bartel's diagonal Calculator This is the one I use, it's simple. Instead, Id encourage you to consider a more real-world approach. If you want to calculate the offset, use this simple formula: Offset = (secondary size)/ (4*focal ratio). Just to get it to work whit the smallest possible secondary. Gee.It simply is a no way you get a stiff secondary solution there. Given I would basically be using this for planets, I would not need to be object hopping all night and repositioning the ladder all the time. You obtained this 12 mirror on the surplus market. But it does mean you would be more or less committed for a given night. To calculate the obstruction by diameter, the values of the secondary mirror diameter and aperture must be known. That's really tight--literally 50mm shorter than on my own 12.5" f/5. If I was going special purpose like that, I would be tempted to have the fully illuminated field about the same size as the field stop of my favorite planetary eyepiece. Thanks to Amazons associates program, doing so costs you nothing yet helps keep this site up and running. Typically 24 - 36" wide. Will you be using it for wide-field astrophotography, or exclusively for viewing? If you make the I.D. The seeing is steady enough for 400x around here, winter can be very hit or miss. for optimal performance? On budget. Hi guys, does anyone know what difference it makes when it comes to secondary mirror size in a 10" newt? I used to use it but one has to be careful interpreting the results. I had a choice on my 12.5" f/5 and went with 2.6". That scope becomes my primary scope for planetary and double star observing. However, choosing a secondary mirror for a given size of fully illuminated field is not the best strategy and can often lead to choices that are not optimal. So how big should it be? View as Grid List. Myself, I have a nice Starsplitter Dob with 13.1 inch F/5.5 Royce mirror. I'd be concerned about my ability to build one accurately, but it wouldn't hurt to give it a try I suppose. It can help you learn and understand the whole thing better by manipulating the variables and seeing the results of the changes. This is the telescope's focal length.To find the minimum secondary size, simply divide L by f. In the case of an 8-inch f/6, L is often about 9 inches. That is there is little to gain and lots to lose if one goes smaller than 20%. Since the edge of the mirror is beveled to prevent accidental chipping, the actual useful surface might be less than the size given. To understand what this phrase means and why it is an important consideration, lets first look at how a reflector telescope works. It's the hanging on to the ladder with one hand while operating the scope with the other, leaning over. Wow! Tube Inserts. It requires a larger secondary, something he is willing to compromise on. ", Secondary mirror in reflecting telescope (attached to the optical tube with "spider"). Both partial and full offset conditions give good collimation. The Newtonian reflector has many strengths, not the least of which is that it consists of just two elements: a precisely shaped paraboloidal primary mirror and a flat diagonal secondary mirror. Distance between secondary and primary is focal length minus focal plane to secondary. Going 1 size up will not cause you any noticeable loss of performance. The DG-2 has 6 evenly spaced contact points, for optimal thermal distribution. You must have the focal plane to secondary distance already because you have worked out the diagonal size already. 0.3 magnitudes is better. A minimum-size secondary mirror allows you to take full advantage of your primary mirrors light-gathering and resolution capabilities but only at the center of the field of view. You will be most of the time just running around the scope, trying to catch with the OAZ position. The size of the secondary mirror depends on the diameter of the primary mirror and focal length. This calculator computes the minimum number of necessary samples to meet the desired statistical constraints. Ggp@6aW)*'r]mAU--zL>ORpr9&U;0Ljj1+Qat++*./%0YbG[Z)d})pK9d`)5H#FI&1%/R75;b"[Wo6E+Zr!We9[,0eQ]`h=>@7c Q%5{|&Lw|xt%}#+"nJX@>o4t,a{p ?z o3? &8scljA(T y4& Yo`]2cW,@]qlSw_phSX~,"(]e D$\ Specification Reviews It sounds like you would be using the very edge with a 2.14". Namely, for most observers, the size of the fully illuminated field is less important than how much dimming they will see at the edge of the field of view. However, if your have an f/8 scope, you will see more dimming even if you choose to fully illuminate a good portion of the field of view. I don't have metal fabrication tools, I have woodworking tools, so building a steel chassis is not possible. - Newt for the web: This is on the Stellarfane site. When I enter the corrected figures, still using your intercept distance of 225mm, I see a fully illuminated field 8mm wide, but the edge illumination is still OK for the 20mm T2 Nagler. A good place to start in your quest for the ideal secondary mirror is by figuring the absolute minimum diagonal size that will catch all the converging light rays from the telescopes primary mirror. Cleanliness my Obsession 12.5 inch, f5 scope was delivered (in 2004 ) with a 2.14 inch secondary and it works ok for me. In turn, the secondary mirror reflects the image out of an opening at the side of the front end of the main tube and into the eyepiece. Gee. In the article he states.."For example, all other things being equal, the same 1-inch secondary mirror will serve well for 6-inch reflectors from f/4 to f/10, in all cases producing essentially the same edge-of-field illumination _ one of the more important parameters to consider when you are selecting a secondary mirror." Planetary observers tend to be fanatical about keeping the secondary mirror as small as possible and dont mind if the edge illumination drops to zero. Did the scope you used have a 112 inch focal length? Choosing a secondary that is just big enough means that you are counting on that mirror being perfectly flat right up to the very edge. I did my calculations graphically, that is drawing it all out in MS Visio then measuring the width of the cone where the diagonal needed to be based on the width of the UTA. It is used to bounce the light gathered by the primary mirror into an eyepiece and to extend the path of the light. In use, when looking at star fields, I've never felt that the edge of the field of view was dim or lacked contrast. How do you know where, along the light cone, it equals the 2.6" m.A. The point at which this loss of image contrast becomes noticeable is a matter of considerable debate, but most telescope experts agree that as long as the secondary mirrors diameter is less than 20 percent that of the primary mirror, its effects should be all but impossible to see. That said, if you're saying there's no meaningful visual difference between F/6 and F/9, then F/6 would obviously make more sense to aim for. Most of the time, a 12.5" would have done better in my seeing conditions. But that shouldn't change the results appreciably. On the other hand, a secondary mirror that is too large will also adversely affect the telescopes performance. Edited by CrazyPanda, 10 January 2017 - 02:39 PM. The plan is to build a 12.5" F/9 planetary dob similar in style to conventional truss dobs. This is a lot more than required for visual observations, for which typically 12 mm (0.5") is recommended. To choose the optimal size of the secondary mirror, we need to know for what purpose the telescope will be used. Edited by CrazyPanda, 08 January 2017 - 11:34 PM. This is "take no prisoners" territory. I again used the full moon and after obtaining focus checked to see if the enterance window to the Amplifier / Corrector was fully illuminated, it looked to me that it was and that's good enough for me. Hello all, this week I'll be ordering a new secondary mirror for my 10" f/5 dobsonian. What I'm most unsure about it is how to calculate the values of what seems to be an interrelated system: But I feel there's some nuance there in terms of designing the optical path to account for the field stop of the eyepieces you're using, and accounting for exit pupil etc. Will you be using it to precisely gauge the brightnesses of variable stars, or will this telescope see more duty as a high-resolution lunar and planetary instrument? Population Size: Leave blank if unlimited population size. A secondary mirror is located directly on the path where light enters the primary mirror. The size of the secondary mirr or is Choosing the right size is very important: It is essential to find the right adjustment. . No more heat ropes! Certainly, the second of these is the greater evil of the two. If so, consider using this link the next time you shop at Amazon.com. You don't want the scope stopped down by the size of the secondary. . Mirror Coatings (not applicable for Refractors) Choose standard or enhanced mirror coatings (88%-95% reflectivity) as applicable to your telescope. The edges of secondaries are generally lacking in accuracy and should be painted black about a 1/16th of an inch all the way around. I haven't noticed what sort of mount is envisioned. All secondary mirrors are interferometrically tested for surface accuracy using a Zygo interferometer during manufacture and prior to shipping. The minimum net clear opening width shall be 20 inches. Dividing 9 by 6 gives a minimum secondary size of 1.5 inches. Secondary mirrors are not that expensive. If you use an Astrosystems secondary holder, there is also a lip around the entire edge of the secondary. A secondary mirror is usually attached to the optical tube by a specially shaped construction, named "spider. Note: This is an updated and expanded version of an article I wrote for the August 2000 issue of Sky & Telescope magazine. The atmosphere always has the last word. Our Locations Corporate Office : 14 Mackenzie Lane, Plainsboro, NJ 08536, USA R&D office : A-40, iThum Tower-B, 5th Floor, # 506, Sector-62, Noida-201301, Uttar Pradesh, India Call Us +91-8383069918 +1-732-369-6885 We are social It's easy, free and just a click away Not unreasonable but perhaps a bit small at 17%. But be sure to make the UTA I.D. What it means is no space between mirror edge and UTA and eyepiece or paracorr will be inside UTA. In this case, a 5.5" f/3.6 Schmidt-Newtonian with a night vision eyepiece. I use a low profile focuser and a 2.6" secondary in my 12.5" f-4.8. One way to check is to use a clean length of wood or a wooden yardstick, if it is long enough. To calculate the obstruction by diameter, the values of the secondary mirror diameter and aperture must be known. A diagonal that is too large will block incoming light and exaggerate image-harming diffraction effects, while one that is too small will fail to deliver all the light from the primary mirror to the eyepiece. A secondary mirror is a small mirror element in reflectors and catadioptric telescopes. A short focuser body is around 1.5". The diagonal mirror simply intercepts the converging cone of light and diverts it out the side of the tube, where the image can be viewed or photographed. This means that, depending on mirror size, it causes more or less obstruction. This is just a word of caution. On 23/10/2012 at 09:19, Gina said: I would have thought you'd want to go for 2" optics and therefore a secondary mirror at least 2" x 3" but I'm no expert on scope making. Must be way better to has 20% and a great mirror and feet on ground. The 12.5" Obsession has a very small opening for the UTA, so the secondary-to-focal plane distance is short, but 2.14" would be a very small secondary for a 12.5" f/5.

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