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Found 8 results

  1. I have encountered a unique problem last night while trying to image using my GSO RC6. The scope is well collimated using a long Cheshire as well as a Howie Glatter single beam laser for verification. CCD inspector also gives me good results when checking with an artificial star. However, when I check using a collimation cap the innermost shadow looks offcentered as well as when I pointed the scope on a real star yesterday (Alp Libra) as well as Spica, it showed a crescent shaped star. This I found to be really weird. Anyone have any idea whats happening here? I live in Bangalore, India, so there's virtually no cool down time required. The temperature gradient between outside and inside is very little if any.
  2. The Newtonian telescope design is both simple and remarkable. It is capable of producing a perfect image on axis, but off axis, the image quality degrades mainly due to an optical aberration called coma. Modern fast Newtonians and Donsonians of F/5 and below have a surprisingly small diffraction limited spot (just 2mm across in an F/4.5), where the image is not disturbed by coma. The Astro-Tech (also sold under the Altair Astro and GSO brand labels) coma corrector has been designed to cancel out this aberration to give a flat, wide field with high resolution from edge to edge. It is manufactured by Guan Sheng Optical (GSO) and was developed by Astro-Tech from a high quality, modern optical design by Roger Ceragioli My corrector came in a nice box and consists of two parts, the coma corrector itself and a 2" eyepiece adaptor which screw together with a 48mm (2" filter) thread. The eyepiece adaptor has two screws and a brass compression ring and is marked ALTAIR ASTRO 2", Coma Corrector, Made in Taiwan. At least I knew I had the right part, but no other documentation was supplied and I had to search the web for information on how to use it. Unfortunately the corrector is not ready for visual use as supplied, because of inadequate eyepiece spacing. The proper spacing is not critical and a compromise spacing to cover your eyepieces can made up with 2" extender tubes, such Hyperion fine tuning rings or empty 2" filters. You do not need a turntable like that of the Tele Vue Paracorr. With the spacers installed, the assembly which is now about 70mm long just slides into the focuser tube like a barlow. In this arrangement the focal point is moved in by a small distance of about 10mm (see photographs below). The corrector acts as a very slight barlow, enlarging the image by just about 10%. The lenses are nicely coated and reflect pale green. The aluminium housing is cleanly finished in satin black and the combined unit weighs about 350 grams. Once set up properly in a collimated telescope, the corrector works just as you would expect to give a clean, flat image. The view feels quite different, much more like a refractor, with pin point stars from edge to edge, but no chromatic aberration. Objects can be allowed to drift across the view of wide angle eyepieces with little or no visible loss of sharpness. The removal of coma can be clearly demonstrated by doing a star test on and off axis without the corrector installed and then with it. Any loss of contrast due to the extra corrector glass (two doublet lenses) in the light path is undetectable, I think. The coma corrector is now a permanent fixture in my focuser except on occasion when viewing planets with my 200mm Newtonian which now has a motor drive. It seems to me that a coma corrector should be a standard accessory for all fast Newtonian telescopes and particularly for larger Dobsonians with no tracking. This model is an effective, affordable example and I strongly recommend it. The first issue is actually finding one in stock. Supply has been patchy over the years and at the time of writing, it is listed by Astronomics (Astro-Tech brand at $135, including T-mount, but out of stock), Agena (GSO brand at $130, including T-mount, but out of stock), Ian King (Altair Astro brand at £88) and Telescope Service (GSO brand without visual adaptor at 61 Euro). There is then the issue of setting it up properly and most of the remainder of this review is devoted to showing how this can be done, but first there is a little information about Newtonian telescopes and coma. Newtonian telescopes are all designed with a single figured mirror in the shape of a parabola rotated on its axis, a paraboloid. All mirrors of a given focal length are the same shape. If you have a fast mirror, it is easy to to create a slow one of the same focal length, just by blanking off the outer part of the mirror. It is the outer part of the mirror that generates coma, which is zero on axis but which increases linearly the further from the axis you get. At the focal surface, the amount of coma is independent of the mirror focal length so a single corrector will work for any Newtonian. In practice, a perfect corrector is not attainable so the designer will aim to produce the best result he can for a specific F/ ratio, F/4.5 for this model I understand. However, the corrector will give good results for mirrors that are somewhat faster than this and for all slower mirrors. Coma correctors would actually be better called Newtonian correctors, because the designer is looking to produce the smallest attainable spot size for a point source, so will also be looking to reduce the other lesser Newtonian aberrations, field curvature and astigmatism. To do this, he will have in mind a particular focal length, around the longest that is commonly used (so about 2000mm or slightly less), because these aberrations are less in longer telescopes and it is wise not to over correct significantly. Newtonian telescopes are perfect on axis, but coma damages image quality at even a modest distance off axis. At the focal plane, about 1mm off axis, in an uncorrected F/4.5 Newtonian, the image is just at the diffraction limit and the strehl of even a perfect mirror has fallen to 0.8. In a 250mm scope, this gives a coma free, sharp field of about 6 arc minutes across, about 1/5 of the apparent diameter of the moon. For comparison, the field stop of a 9mm orthoscopic eyepiece is about 6mm so only the central 1/3 (1/9 of the area) of the view is free of coma in an F/4.5 scope. Coma increases sharply with the speed of the telescope, at the focal surface inversely with the cube of the F ratio. Collimation is the business of lining up the coma free sweet spot with the centre of the eyepiece axis. The tolerance for collimation is perhaps 1/4 (though some would say 1/6) the size of the sweet spot so that it covers the centre of the eyepiece. So far as I can tell, this tolerance also looks good for a telescope fitted with a coma corrector. To set up the GSO coma corrector properly, the total back focus (distance from the last lens to the focal plane) has to be about 75mm. The designer says that it is not critical and from 65mm to 85mm will produce a good spot size. This distance will be made up somthing like mine below, added to the height of the eyepiece focal point height above the eyepiece shoulder (or subtracting the height below the shoulder). 1.25" My 2" 2mm 2mm Spacing from last coma corrector lens to the shoulder 45mm 45mm 2" adaptor spacing 11mm .... 2" to 1.25" adaptor (if any) 19mm 19mm Spacers (Hyperion 14mm ring + empty 2" filter) 77mm 66mm Total (excluding eyepiece distance) My one 2" eyepiece has a focal point above the shoulder, and my 1.25" eyepieces are all within -12mm/+8mm of nominal, so are all fine. Tele Vue is unique in publishing the height below the shoulder of the focal point for all their eyepieces. For other users, you are going to have assume the focal point is close to the shoulder or measure the position. First, locate the prime focus by taping a piece of tracing paper to the top of the focuser and focusing on something. This does not have to be at night and can anything sufficiently distant so that it comes into focus, such as a church spire or distant tree. It does not depend on the telescope so using a refractor with a graduated focus scale is very convenient. You then measure how far in (plus) or out (minus) you have to move the focuser for each of your eyepieces in turn. For users only intending to use 2" eyepieces, a single 28mm Hyperion tuning ring might be fine. If you do not like the idea of finding empty filter rings, or more likely buying cheap ones on eBay and removing the glass, some suppliers (in particular Telescope Service) have spacing rings with the right 48mm thread, in a few sizes such as 10mm and 20mm, but these are generally expensive. Variable spacers are also available but these are not going to sink into your focuser tube. When I first set this up I had to remove a 2" to 2" adapter to allow the unit to go all the way into the focus tube. This left too little out focus so I made a plastic washer (from a yoghurt tub, see photo below) to prevent the corrector slipping all the way into the focuser and providing the necesssary out focus. One correspondent who uses only 2" eyepieces has done away with the eyepiece adaptor and has simply added enough extender rings to screw the corrector to each eyepiece as he uses it. I hope that this will is enough information to set up this corrector properly but I would welcome questions, and of course comments and correction.
  3. GSO RC 8" f/8 is on order so apologies for incoming bad weather!! Looking forward to getting the Atik on it!!
  4. I purchased a standard GSO 6" RC telescope late 2016. As with other RC scopes, I've had my share of problems with the collimation. As many of you know collimation of the RC is a bit of a struggle. After reading much of the material online, it seems that the procedure boils down to multiple iterations with the simple Cheshire and something called as Hall of Mirrors aka multiple reflection effect. Below are the test images from tonight. Reference star is Canopus. All images are inside of focus, unless mentioned. The small rectangle shows the location of the star in the FoV. I also built myself an artificial star from Aluminum foil and LED torch and checked the collimation indoors. The indoor test is quite better as seeing conditions have not affected the judgement. As you may all notice, I've used Mire De Collimation exe file for final checking. I've not used a laser collimator or Howie, just a simple Cheshire and a real star, though I'd love to try again with a Howie Glatter.
  5. Hi everyone, I recently got myself a GSO 6" f/4 astrograph, but seems like the secondary mirror was quite off. I then tried collimation using Astro Baby's tutorials, and others around the net. I've had a few Nets during the past 20 years, but this one seems to be a bit of a pain to get right. The secondary holder is a bit stiff. Anyone here done any mods for the scope and have reasonably good performance? I'be attached an image with Mire De Collimation circles. Image shows the reflection of primary, followed by the edge of secondary and followed by the grey card placed behind the secondary, which ends with the focuser tube. Any help will be appreciated. Edit: If I were to move the secondary towards 2'o'clock position, could it solve the problem?
  6. Hello Everyone, I have a GSO 6" f/4 Astrograph, and there's always been this issue with elongated stars on one edge. I've checked collimation and it is perfect, but for the elongation on one corner. Could someone please throw some light on what's happening
  7. I am taking a slightly different direction with my observatory and imaging, and so I must regretfully offer my GSO RC scope for sale. The Carbon Fibre version, apart from looking very sleek, is much less resistant to changes in temperature, thus reducing the need for frequent collimation The telescope was purchased new by me in August 2012 and I purchased it with the Baader SteelTrack focuser upgrade. There have been no other modifications. The scope has been kept in my observatory – it has only been to three star parties as I recall. While in the observatory it has been kept capped and covered when not in use. The mirror is blemish free, apart from the usual collection of dust! The telescope was purchased new by me in August 2012 and I purchased it with the Baader SteelTrack focuser upgrade. There have been no other modifications. The scope has been kept in my observatory - it has only been to three star parties as I recall. While in the observatory it has been kept capped and covered when not in use. The mirror is blemish free, apart from the usual collection of dust! The specification is as follows: Aperture: 204mm Focal Length: 1624mm Focal Ratio: f/8 Included in this package are the following: Aluminum storage box (not pictured) Red dot finder 25mm and 50mm M90 extension tubes Collimation Tilt Plate Price: 725 GBP The following are also offered for sale, first refusal is given to the purchaser of the telescope Baader Steel Drive Motor Focuser + Handset *: 130 GBP AP 0.67 Reducer: 140 GBP ADM Losmandy Plate: 40 GBP ADM Vixen plate: 40 GBP Howie Glatter 635nm Laser + 2 reticules: 110 GBP StarSharp Bahtinov Mask: 15 GBP Gert Neumann Aurora Flat Panel inc power: 90 GBP However, if someone wanted the whole package then the price would be 1,275 GBP I would prefer the telescope to be collected, or I am happy to travel to meet the buyer. I will obtain courier pricing (insured delivery) if required, but the buyer assumes all responsibility More images can be seen at http://darrenjehan.me.uk/for-sale/
  8. The well known Triffid Nebula - M20. I took Luminance data using my GSO RC10 (reduced to f/5.6) & QSI683L. Unfortunately I didn't have time to capture color but I had these color data since 2011. When I shot this target using my old WO FLT98 and my lovely ATIK314L camera. Well this is why the target cropped at rectangular shape. Hope you like it Camera: Lum: QSI 683L, Color ATIK314L Telescopes: GSO RC10 Reduced to f/5.6 (Lum) WO FLT98 (Color) Mount: Astrophysics Mach 1 GTO + Skywatcher HEQ5 Filters: LRGB Location:Mt Parnon, Greece @ 1430m Full resolution: http://www.celestialpixels.com/Nebulae/i-85LQxdP/X2
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