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Rocket_the_Raccoon

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Everything posted by Rocket_the_Raccoon

  1. ...could be related to seeing, not the optics?
  2. How about using the objective lens if a cheap 8x21 or 10x25 binoculars to make a symmetrical eyepiece?
  3. Thanks for the clarification. Both Kaspereit and Erfle are early 20th century wide field designs. "Rank" is the last name of Dr. David Rank who developed the RKE. I have a Celestron Circle-V 6mm Ortho. which gives superb contrast and sharpness despite its short eye relief. These classics, like yours and mine, are truly gems.
  4. I have two of these sets sitting around, so I decided to have some fun with them... :-) Many entry-level telescopes (Celestron, Orion, Skywatcher) include eyepieces like the above, economical 3-element of unknown designs. Most people simply called them "modified achromatic", "modified Kellner", or just "Kellner". So what actually are they? A "true" Kellner consists of a singlet plano-convex field lens and a achromatic doublet plano-convex eye lens, it has an AFOV of 40 degrees: (by Tamaflex, used under CC-BY SA 3.0) High quality classic Kellners are widely available in the used market with some being semi-collectables. At least one online dealer is still selling brand new ones: http://www.stjarnhusetonline.se/prod/Sky-Watcher/ kellner/6101525 Eyepieces.html The name "Modified Achromatic" actually came from the MA series of eyepieces from Meade which has been discontinued. They resemble the original Kellner with the eye lens replaced by a bi-convex doublet. They have AFOV of 40 degrees. It seems that Meade aimed for improvements in image qualities rather than increasing AFOV. The RKE from Edmund Scientific reverses the orientation of the field and eye lenses of the Kellners, replacing the eye lens with a bi-convex singlet, and field lens with a concave-convex doublet. The AFOV increases to 45 degrees with image quality praised by many. Another 3-element design, unknown to many, is actually the original Königs, surficially resembles the RKE but actually a close relative to the Abbe Orthoscopic. AFOV of this version of Königs is 55 degrees: (by Tamaflex, used under CC-BY SA 3.0) Well, how about the entry-level set mentioned at the beginning? Let's disassemble one, the 10mm in particular, to find out its "secret formula"... *lens edges blackened by myself using fine tip felt-pen As shown above, the eye lens is a singlet plano-convex with the field lens being a thick plano-convex doublet. It looks like a Kellner in reverse but in fact, the field and eye lenses are of the same size and more important, they almost touch each other. Thus, this 3-element Celestron stock 10mm is actually a slight modification of the Königs. Moreover, from what I have read from the Internet, this is in fact the same design of the Celestron SMA series from the past.
  5. From the album: Rocket's Album

    - Celestron Astromaster 70AZ (70mm f/13) - Celestron 3-element Stock Eyepiece - Orion Astroview Equatorial Mount - Kendrick Astro Solar Filter with Solar Finder
  6. From the album: Rocket's Album

    - 70mm f/13 refractor, 20mm eyepiece, cellphone camera afocal *All three images were handheld. First two were taken by myself, the third is courtesy of my friend as he has super steady hands. :-)
  7. Ingredients: - eyepiece from an old 10x50mm (5-degree FOV) binocular - 35mm film can (...the 1.25" eyepiece barrel) - rubber cup from another old eyepiece (...installed in reversed) - superglue, masking tape, Tamiya XF-1 flat-black model paint Using my Celestron stock 3-element stock 20mm as a reference, this DIY eyepiece has a focal length of approximately 18mm and a AFOV of around 45~50 degrees. Image quality falls slightly behind the Celestron but it is not very obvious. However, it makes a good general purpose eyepiece for public viewing.
  8. From the album: Rocket's Album

    The Ingredients:- an 6x30 finderscope- a seemingly useless 90-degree Amici roof prism from the Celestron Astromaster 70AZ - 35mm film can (...the 1.25" eyepiece barrel) - 1 in. Copper Pipe "Cup x Cup" Coupling (...the optical tube)- superglue, electrical tape, Tamiya XF-1 flat black model paint
  9. The Ingredients: - an 6x30 finderscope - a seemingly useless 90-degree Amici roof prism from the Celestron Astromaster 70AZ - 35mm film can (...the 1.25" eyepiece barrel) - 1 in. Copper Pipe "Cup x Cup" Coupling (...the optical tube) - superglue, electrical tape, Tamiya XF-1 flat black model paint **Next Project: a new bracket to accommodate the shortened optical tube.
  10. Watched a solar eclipse for the first time in my life, stunning even it was a partial..:blob7:

  11. From the album: Rocket's Album

    3-element design with a 20mm field-stop, 58 degrees AFOV, a little germ covered in dirts
  12. From the album: Rocket's Album

    21mm achromatic objective lens from a pocket binoculars, ~6 degree of field.
  13. From the album: Rocket's Album

    the inside of it, showing how under-sized the Amici roof prism is
  14. The Celestron Astromastrer 70AZ is an entry-level 70mm f/13 long refractor on an altazimuth mount which resembles an over-sized photo-tripod with a fluid-motion pan-head. Celestron also offer an EQ model which comes with an "Astromaster Styled" EQ-1 German Equatorial mount. Other "manufacturers" that offer 70mm f/13 packages are Meade, Vixen, and Skywatcher (Capricorn 70 and 70/900 AZ3, not listed on the Skywatcher website but still being sold online). **Why I bought this? - I was searching for a light-weight, grad-and-go mount as I have been using an Orion Astroview equatorial mount with my little 70mm f/5.7 short-tube refractor, even for some quick observing. I came across this used Celestron Astromaster 70AZ from a moving sale, being sold at only $40 CAD! So, why not? Now I have an extra telescope which I intend to use it for public viewing and education. **Should you buy one? - No! While a 70mm f/13 refractor is a great economical choice for beginners, especially those living in a heavily light-polluted city, Meade offers a much better package while the Skywatcher Capricorn is also a good choice. PART 1 - THE MOUNT ------------------------- The fluid-motion of this simple altazimuth is way smoother than I expected. It is very usable for low to mid powers (< 60x). Occasional high power use also seems pretty acceptable though it starts bringing back my memories of using my 25+ years old 60mm f/11.7 on a Yoke-style altazimuth mount at 117x. However, there exists a problem: the setup is rear-heavy and thus the OTA is not balanced. This becomes more obvious when the scope is being pointed closer to the zenith. Adding a metal tube ring at the front helps a bit but not much. Due to the lack of mounting rings (the dovetail is directly attached to the OTA), there is no way to balance to tube. So how can a refractor with a glass objective with focuser, diagonal, eyepiece being all plastic be rear-heavy? Shouldn't it be front-heavy? I would say it is caused by the metal handle of the mount, which sits "above" the altitude axis, adding weight to the rear. A simple solution is to mount the OTA backward like the photo above. This should greatly reduce the free play when the scope is moving upward. A more permanent solution is of course to use a longer dovetail with a pair of mounting rings to truly balance the OTA. (Note that the O.D. of the OTA is slightly smaller then the I.D. of the Skywatcher 76mm tube-rings.) PART 2 - RED DOT FINDER -------------------------------- Called StarPointer by Celestron, this permanently mounted red-dot finder from the Astromaster series is the worst finder I have ever used. It is even worse than the 5x24mm optical finders on those so-called "department store telescopes”. The main reason is that it is almost impossible to find out how far away from the finder the eye(s) should be and it is extremely difficult to align even during daytime. Another shortcoming of this finder is the brightness of its LED not being adjustable. The simplest and most effective solution is to super-glue a Synta-type dovetail finder mount on the OTA and use an “ordinary” red-dot finder or a 6x30 finderscope. However, with the only two telescope shops in town closed down in the last two years, getting an extra dovetail finder mount means paying $15 shipping for a $15 item! After some readings online, I came across someone who used the StarPointer as a finderscope mount and put a small finderscope through it. So I decided do my own version of it. Here are the steps: Remove the StarPointer assembly by removing the two screws pointed by the red arrows. It is also a good idea to remove the battery as it is not needed any more. Then remove the two small screws circled in red. This will enable one to take out the two pieces of edged glass inside plus a metal retaining ring. The finderscope is a 5x20 from my 25+ years old 60mm f/11.7. I removed the single-element objective lens and replaced it with a 21mm achromat from a cheap pocket binoculars. The achromat has a shorter focal length than the original finder objective so I shortened the length of the tube to accommodate it. Using the separation between Castor and Pollux as a reference, this “new” finderscope has a field of a little more than 6 degrees. Finally, because there was too much free play when I put the finder scope through the now hollowed StarPointer, I added a small piece of self-adhesive felt to fill up the gap. This finderscope, thought small, is still up to the task of locating bright objects and works much better than any single element finder. Still, there is still one minor problem, the finder scope is mounted too close to the OTA. Not just every time I need to swing the eyepiece away from my face, a very small portion of the field is blocked because I cannot align my eye with the eye lens of the finder eyepiece. PART 3 - THE DIAGONAL ------------------------------ The diagonal that comes with the Astromaster 70AZ is a 90-degree Amici roof prism. This means it produces a fully corrected image but it is mainly designed for low-power terrestrial viewing. Except for some really high-end astronomical models on the market (e.g. Baader T-2), most corrected image diagonals house a under-sized, poor quality prism. This “spherical” one that came with many Celestron refractors is no exception (see photo on the left below). The only solution is to replace it with a mirror star diagonal. Since this telescope is merely a “bonus” from buying a grab-and-go mount and it was intended to be used for public viewing, I bought a $10 USD mirror diagonal (photo at the right) and a $19 USD 12.4mm Plossl (more on the eyepiece later) from an online store in the US that sells surplus from Meade. P.S. This seemingly useless Amici prism was later used to modified a 6x30 finder into a right-angle corrected image one which I will write another post later. PART 4 - EYEPIECES ------------------------- The scope comes with two eyepieces, a 20mm (45x) and a 10mm (90x). They are both 3-element modified Kellners. Judging from the look of them, they should be same as the Super 20mm and 10mm from Skywatcher. One might immediately wants to upgrade these seemingly low-quality plastic body eyepieces but I would suggest wait until they are fully utilized. The 20mm is a germ covered in dirts. Skywatcher claims it has a AFOV of 58 degrees which sounds pretty exaggerated. However, if one measure the field stop, which is ~20mm, and use it to calculate to TFOV and then use the magnification to calculate the AFOV, one would find the Skywatcher spec. is in fact correct which makes me suspect the optical formula is closer to a 3-element Konig than being a modified Kellner or RKE. This also means the TFOV is almost the same as a 25mm Plossl with a 50-degree AFOV, and would certainly be wider than a 25mm “traditional” Kellner. with an AFOV of 35~45 degrees. Performance-wise, this 20mm eyepiece is in fact pretty good partly because of the highly forgiving f/13 focal ratio. Only when it is reaching the outer 10% of the field, degrade in image quality starts becoming obvious, but this is a pretty minor shortcoming for a stock eyepiece that comes with an entry-level telescope. Contrast is also good given despite being a single coated 3-element design. The 10mm on the other hand has a bad reputation from many online reviews. However, when used on a f/13 system, it is not that bad. Sharpness and contrast are both acceptable, lacking slightly behind the Meade 12.4mm Plossl (Chinese-made Series 4000 equivilant) that I bought to replace it. The 12.4mm gives a magnification of 73x which I somehow found from experience significantly less susceptible to unstable seeing than 90x.
  15. I once used a 3.6mm 4-Element Modified Kellner with my 70mm f/5.7 to get 111x for planets and doubles. Later on, I bought a Meade Short 3x Barlow and use my Omcon 10mm Plossl (~Celestron Silver-top Plossl) to get 120x. I found that the eyepiece plus barlow combination much better in image quality (slightly sharper, higher contrast), wider TFOV, and of course, longer eye relief. However, high power viewing requires good seeing and this doesn't happen every night. Sometimes I just need to stay at 67x by using my old Celestron 6mm Othroscopic. P.S. this is not a totally fair comparison as the Plossl formula performs better than the Modified Kellner by default.
  16. Thanks, Michael. I just read your "review" of the Antares from another threads. The OTA thar I am going to use the eyepiece on is in fact similar to yours, 70mm f/5.7 and my intended use of it is similar too. Worst case scenario, I can use it on a 50mm finder that I am building as you have mentioned the easy placement of cross-hair.
  17. Thank you everyone for your inputs! You are right, Ricochet. I forgot the fact that the background sky will become brighter as the magnification decreases. It seems that I will have to spend quite a bit more than originally planned to obtain a wider field as a wide field 25mm is more expensive than a basic 32mm/40mm Plossl. :-( Michael, may I ask how do you feel about your Antares W70 25mm? This is the only wide field 25mm that is within my budget. Thanks.
  18. Hi! Everyone, I have a grab-and-go short refractor with a focal length of 400mm. My lowest power eyepiece right now is an OMCON Plossl 25mm (~Celestron silver-top Plossl) which gives ~3 degrees of TFOV. As I live in a heavily light polluted city and the OTA only has a red-dot finder, I need the widest possible field for finding objects. Star-hopping with a 3 degrees field can sometimes be very difficult. So I am looking for an economical 32mm or 40mm, like the ones from Skywatcher, Orion, and GSO, which I believe they are all the same (just different brands). Then I came across some contradicting info. online. According to Orion: http://www.telescope.com/mobileProduct/Accessories/Telescope-Eyepieces/32mm-Orion-Sirius-Plossl-Telescope-Eyepiece/pc/3/c/47/8728.uts http://www.telescope.com/mobileProduct/Accessories/Telescope-Eyepieces/40mm-Orion-Sirius-Plossl-Telescope-Eyepiece/pc/3/c/47/8730.uts The field stop diameters are both 28.5mm. This gives a TFOV of (28.5 / 400) x 57.3 = 4.1 degrees. However, another online source did a "lab test" on the Skywatcher versions: http://astro-okulare.de/English/indexe.htm and found out the AFOV of the two (32mm, 40mm) are actually 43 degrees and 36 degrees respectively. These give TFOV of 3.44 degrees and 3.6 degrees using the forumla, AFOV / magnification = TFOV. I wonder which is true? Thanks!
  19. Thank you, Bobby. Below are the details. The Guiding Eyepiece : The Hybrid Diagonal : A Close-up of the .965" side of the diagonal, the "outer ring" is actually from the cap of a 5x24mm finder :
  20. Will the following help? http://www.bobatkins.com/photography/technical/Lens-scope-adapter.html Also, I think you might be able to make an adapter by using a 30.5-xxx step-up ring as connection.
  21. I found that these 5x24mm finders, though pretty useless for its intended purpose, offer many useful parts for mini DIY projects. Both my homemade guiding eyepiece and hybrid diagonal contain bits and pieces from these finders.
  22. Many people, including me, found out their telescopes cannot reach infinity focus (even with the focuser being fully retracted) after they purchased a hybrid diagonal in order to use 1.25" eyepieces. Most of the times, it is because the optical path (between the eyepiece field-lens and the focuser) being lengthened by the mechanical designs of both the 1.25" eyepiece and the hybrid diagonal. Without a feasible option of returning the item, I did some modifications to shorten the optical path. I used the top portion of a DWV plumbing 1 1/4" female trap adapter for the 1.25" end. The white plastic ring (a part of the adapter) is slightly elastic and does the job of securely holding the eyepiece. The .965" end, on the other hand, is made with the barrel of an old .965" H12.5mm eyepiece, the front cap of a 5x24mm finder, and some rubber. The whole setup shortens the optical path from the eyepiece field-lens to the focuser for ~10mm. P.S. As every OTA is designed differently, even for objectives with the same specification, there is no guarantee that this modification will always work.
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