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Found 78 results

  1. I'm aiming for the best possible field illumination for my Explore Scientific PN208 f/3.9 imaging newtonian. The scope is 5 years old now and while doing maintenance I wanted to do something about the design flaws as well. With the current secondary holder design it's impossible to center the mirror beneath the focuser. My question is: is this really an issue when the entire primary is visible? I read about optimal field illumination when the secondary is centered, but this isn't achievable with this telescope. I attached an image showing the (very bad aligned and already degrading) secondary at the moment. This is as far down as I can go, I would need to replace the holder screws with longer threads to go further but I don't know if it would make any difference for field illumination? Thanks in advance, Michael
  2. Hello, I am having a problem with a relatively expensive flat field eyepiece in a combination with a newtonian reflector. I am unable to achieve proper focus with that eyepiece because when I focus it on axis, the edges are blurry. When I focus it at the edge, the center field is out of focus. I don't want to mention the concrete type of this eyepiece because I received it with a spot on the bottom barrel, therefore I cannot be sure whether it has been dropped. This eyepiece has very good reviews on this forum. Optically it looks good and there is no rattle. My question is whether these flat field eyepieces are usable with newtonians as these telescopes doesn't generally suffer from the field curvature. Many thanks for help.
  3. I'm selling my my 70mm secondary mirror I used with my 150mm F4 newtonian for 100$ (final price). Shipping expenses (from Beirut, Lebanon) on me for free provided I use LibanPost for shipping. We can discuss using other shipping methods and maybe splitting the cost. The mirror is brand new, still have the original package and box, bought the telescope a month ago and used only twice. No scratches, not even dust (images attached). Excellent mirror, no issues whatsoever, 94% reflectivity and optimized for the offset of f / 4 Newtons, but it can be used up to f / 5. It's from TS, made in Taiwan, original price is 134 euros (including VAT) excluding shipping and custom fees. Link below: https://www.teleskop-express.de/shop/product_info.php/info/p10673_TS-Optics-Newton-Fangspiegel-D---70-mm-mit-Halter.html It does not come with the tilt screws and center screw though unfortunately. I can also sell you the spider vanes (1mm thickness) for 6 inch newtonian for 25USD. Also brand new but without the tilt screws and adjustment screws. https://www.teleskop-express.de/shop/product_info.php/info/p11056_TS-Optics-Spinne-fuer-6--Newtons---164-mm-bis-180-mm-Tubus-Innendurchmesser.html Let me know if interested. Clear Skies! Anthony <private email address removed>
  4. Having previously got some reasonable images by mounting a smart phone to the eyepiece. I thought I would try for a more sophisticated set up. So last night I tried to get some pictures of the moon using a Canon Eos D450 connected to a Skywatcher 130p Newtonian via a T2 connection on the eyepiece holder. As the camera has automatic focusing built into the lens I thought I would have to adjust the focuser on the eyepiece holder to manually get a sharp image. Basically the telescope acting as a manually focused lens for the camera. But no joy, I just got a bright light which seemed to fill the camera view finder. I tried various settings on the camera, adjusting ISO and aperture etc, I also had some extention rings for the camera lens so tired fitting those to extend the focal length but no better. I sure there are many palms being slapped against foreheads reading this but as you can tell I have no idea, although I do have some of the gear. Any pointers and/or advise would be gratefully received.
  5. Hi, I recently looked into telescope's & dso's again due to an interesting sight in the night sky . I wanted to get into telescope's etc... but my budget and other things got in the way. This time there's no budget limit (that doesn't mean I'm bill Gates though). My main concern is size since I will carry the telescope with my bare hands (bag). I don't have a local astronomy club or telescope shop around so I don't know if an 8" Newtonian is too bulky/big. Can someone post comparison pictures, if you have an 8" telescope (preferably newtonian, width and length)? Secondly I want to see dso's in the future (firstly beginner objects etc...) so the 8" is more adequate than the 6", but if an 8" is too big then I am okay with a 6" as long as it's much more portable.
  6. Helios 8" Newtonian telescope OTA. Equivalent with Skywatcher Explorer 200P.Has some cosmetic markings, but otherwise in excellent condition.Included:- 1.25" and 2" eyepiece adapters- 9x50 finderscope- long Vixen style dovetail Price: £130 (negotiable)Celestron Astromaster Kit is available for £20 extraMount not included.Pick up only in Oxford.
  7. Hello, Is anyone here using the TS Photon 6" F4 newtonian? I'm about to purchase it but I have some doubts and questions: 1- Does it hold collimation well, at least in a single session? 2- Is it impossible to balance in DEC due to its small dovetail or is it possible but harder? 3- Is the focuser rigid enough or does it introduce tilt? 4- Will collimating it be a nightmare? 5- I'm really picky when it comes to coma, should I expect some coma on edges even while using the Skywatcher Aplanatic F4 CC? 6- All in all, do you advice me to buy it or have some other option in the same price range. Cheers! Anthony
  8. Hi everybody - I'm humbled that some of you are following the thread on the 800mm telescope. I thought it was worth starting a sub-topic specifically related to the mirror making. So as the works on the mount got halted by the lockdown we had some time to virtually meet-up and discuss the optics for this project. The onset was pretty straightforward with an 'if we are doing this we are making the mirror'. And that sorts out the make vs buy, I suppose. As of now, the only two things decided so far are the diameter -at 800mm- and the f/3.3 - of course we can accommodate some variance. Some aspects of the making are pretty unchartered territories for our club so I'd like to seek some good advice from anybody in terms of direct experience or rather point us at some resources/threads. Back-ground: to make it short, our senior member had three 500mm f/5 done years ago. He did of course faster optics up to f/3 among many other mirrors - I actually never asked him how many, uh! He has always worked with full tool and has no experience with slumped glass. Btw we do have some experience in slumping glass but -weirdly enough- not in machining. During our initial discussions we boiled the scenarios down to 3: float glass, 35/40mm thickness considering 15mm of sagitta. This is a thermal challenge with its big outward mass, it's going to make the machining more of a challenge and the stabilization time longer float glass, 25mm (which seems more of a commercially available), slumped. Trading the thermal challenge with the slumping borosilicate, 25mm, slumped. Tbh this is just a better version of the previous point at a cost that is not prohibitive What are the sources of glass in Europe for thickness over the bog standard 25mm? Disclaimer: this is surely an ambitious endeavor and by no means we are underestimating that. Not only the bigger diameter is a step-up; the fast optics is a challenge too. To start with we have some questions about slumped glass. It looks like an attractive, modern approach to mirrors that exceed a given diameter. I think I saw already some threads specifically about the slumping itself - that's golden. However it's the grinding/finishing/parabolizing that is puzzling us. Are there specific techniques or is it the same as the flat back glass? Also, how do you support the mirror? Would a support that replicates the telescope mirror cage be appropriate - a 27-point in our case? Or is it a matter to build a concave support that holds the back of the mirror? How accurate/solid should that support be? Any experience out there?? I reckon that's enough as a start - thanks everybody in advance for your contribution. Stay safe! Michele
  9. Hi guys, firstly - sorry if this is posted in the wrong place. I've just ordered a Canon 600D to start doing some astrophotography and it arrives tomorrow - I'm stoked. I've been wanting to get into astrophotography for a while now and I'm so happy to finally be pulling the trigger. The last item on my shopping list is a solid beginner telescope which would couple well with my Canon 600D. The research I've done is all pointing towards a newtonian reflector. My primary goal here is to take some pictures of stars and planets with the Canon 600D attached to the telescope. I've been looking at the Sky-Watcher 114/900 as it's relatively cheap, which I think is great for someone just starting off - but was wondering what your opinions were seeing as you've all been through this before. Do you have any other recommendations? I'd like to keep my budget up to £300, and that's stretching it really. Thanks for taking the time to help!
  10. Observations of the Sculptor Galaxy ( NGC 253 ) by William and John Herschel ......... Part 2. Observations of "Caroline's Galaxy" by Sir John Herschel, 1830's Sir John Herschel, the only child of Mary Baldwin and Sir William Herschel, was born in 1792 when his father was in middle age and already famous as one of world's leading astronomers. Having excelled in school, and no doubt inspired by his famous elders, John Herschel decided upon a career as a 'man of science' and set out to pursue a wide range of interests; with one particular focus being a continuation of the study of the heavens commenced by his father and aunt, Caroline Herschel. In 1820, with the assistance of his father, John Herschel supervised the construction of a new telescope at Slough in England. As described in the extract below ( from a paper presented to the Royal Society in 1826, titled "Account of some observations made with a 20-feet reflecting telescope ... " ), the telescope had a polished metal mirror with clear aperture of 18 inches, focal length of 20 feet and was modelled on the same design created by his father. It is this telescope, in the 1820’s and early 30’s, following the death of his father and the return of his aunt Caroline to Hanover, that John Herschel used to 'sweep' the night sky and extend the catalogue of nebulae and clusters of stars that was published by his father ( see W. Herschel's Catalogue of One Thousand new Nebulae and Clusters of Stars ). On the 1st of July 1833, having complied sufficient observations, John Herschel presented to the Royal Society an updated list of the positions and descriptions of the Nebulae and Clusters of Stars that he had thus far observed. As noted in the introduction to the paper published in the Philosophical Transactions, he had planned to wait before publishing until he had complied a fully comprehensive general catalogue of objects visible from the south of England. However, due to his expectation of “several more more years additional work” needed to complete the task and his assessment that he now was in a position to address, at least in part, the then current “... want of an extensive list of nebulae arranged in order of right ascension ...”, he elected to present his list, “ ... simply stating the individual results of such observations as I have hitherto made ... “. It was not until October 16, 1863, some thirty years later, that Sir John would deliver to the Royal Society his General Catalogue of Nebulae and Clusters of Stars. As well as introducing many objects that had not previously been recorded, Sir John’s list of 1833 included a re-examination of, and in some cases a small correction to, the positions of many of the deep sky objects observed by his father and noted down by his aunt. One of these re-visited objects was, unsurprisingly, the large and bright nebula discovered by Caroline Herschel in 1783 and recorded in Sir Williams’s catalogue as V.1 / CH 10 ( object number one, of class five ( very large nebulae ) / Caroline Herschel #10 ). In total, John Herschel records around 2500 observations of nebulae and clusters of stars in his 1833 paper; with observation #61 being V.1, the “ Sculptor Galaxy “ . The measured position of V.1is given in RA and the angle from the north celestial pole ( all reduced to epoch 1830.0 ). The description can be interpreted by reference to the legend in the paper. Thus, “ A vL mE vB neb “ becomes “ A very large, much extended, elliptic or elongated, very bright nebula “. He also notes that in addition to this observation, #61, noted down from sweep #306, V.1 was also observed in sweep #292, “but no place was taken”. The figure to which he refers , figure 52, is included towards the back of his paper and is a sketch he made of the Sculptor Galaxy. to be continued ...
  11. The Discovery of the Sculptor Galaxy by Miss Caroline Herschel in 1783 On the 23rd of September 1783, sitting before her telescope in the field behind the house she shared with her brother William in Datchet near Slough in the south of England, Miss Caroline Herschel "swept" the sky searching for new comets and never before seen star clusters and nebulae. On this occasion, way down in the sky, not far above the Southern horizon, Miss Herschel saw and noted down a very bright and large nebula where one had never before been recorded and that was later recognised by her brother, Sir William, as the discovery by Caroline Herschel of the nebula he listed in his catalogue as H V.1. ( circ. 1825-33, Sir John Herschel, beloved nephew of Miss Caroline Herschel ) Today we know this 'nebula' to be, not as some thought then, a swirling mass of stars and gases within our own galaxy, but rather, a galaxy not unlike our own but way more distant than the outer reaches of of own Milkyway galaxy. Given various names, Silver Dollar Galaxy, Sliver Coin Galaxy or simply by its number in the New General Catalogue, NGC 253, it is most commonly called the Sculptor Galaxy and we owe its discovery to the first female professional astronomer. Caroline Herschel ( 1750 - 1848 ) ... ( link ) ( 1782 - 1783 ) ... ... ... H V.1 Observed ( by WH ): 30 Oct 1784 128 minutes, 17 seconds following and 1 degree, 39 minutes north of referenced star Description: - cB: "confidently bright" - mE: "much extended: - sp: "south preceding" - nf: "north following" -mbF: "much brighter middle" - size: 50' x 7 or 8' from: ( link ) ............................... The location reference to H V.1 ( NGC 253 ) in William Hershel's catalogue is in relation to a star found in Flamsteed's Catalogue, 18 Pis. Aust., which is #18 in Piscis Austrainus or Epsilon PsA, the 4th magnitude star HD214748 ( HIP111954 ) ( source ) ( Plate from "Atlas Coelestis" by John Flamsteed, 1646-1719 ) ------------------------------------- William Herschel found favour with the King and was granted a position as Royal Astronomer to George III in 1782. Shortly after, William and Caroline moved from Bath to Datchet ( near Windsor ) and took up residency in a rented house which, whilst somewhat delapadated and damp, had ample accommodation and fields for William to construct and deploy the large telescopes he wished to build. It was in these grounds that Caroline set up her "Sweeper" to look for comets and doing so also discovered a number of 'nebulae' including ( in 1783 ) what was later to become known as the Sculptor Galaxy. ( The Herschel house at Datchet near Windsor ) ( The Lawn, Horton Road, Slough ( Datchet ) - Google Maps ) ............. Caroline Herschel's "Sweeper" was a 27" focal length Newtonian telescope that was supported in a kind of altitude-azimuth mount consisting of a rotating table and a small gantry and pulley system that was used to effect altitude adjustments by sliding the tube up and down against a board used to provide stability. There has been some conjecture as to the exact details of the construction, however the image below, even if perhaps not the actual instrument, gives an indication of the overal design philosophy. Late in her life Caroline Herschel recorded details of her telescope in a booklet titled "My little Newtonian sweeper": In her memoir, Caroline Herschel describes the performance of her observations as the conducting of "horizontal sweeps"; from which one might assume the task consisted of setting the altitude in accordance with the plan for the night's observing and then slowing rotating the top of the table in azimuth as one observed and noted down the objects that passed across the view in the eyepiece. However, with the arrival of this new "telescopic sweeper" in the middle of 1783, Caroline Herschel added the new method of sweeping in the vertical, as noted below in an extract from her observing book ( source for both extracts: "Caroline Herschel as observer", Michael Hoskin, Journal for the History of Astronomy, 2005 ) .... The achievement of her discovery of the 'nebula' in the Sculptor constellation was remarkable in so many ways; not the least of which being the low path in the sky that the Sculptor galaxy follows when observed from Datchet in southern England - which on the night of her observation would not have exceeded 12 degrees or so above the horizon. Today, 234 years later, and blessed with 21st century luxuries and conveniences, I write on my IPAD and flip over to my planetarium application, SkySafari, and model the sky as it was seen by Caroline Herschel from near her house on the 23rd of September, 1783 ... ( SkySafari by Simulation Curriculum )
  12. Right now i am imaging with a Star Adventurer and a 300mm telelens on a dslr. I am planning to upgrade to the eq6-r pro and currently i am searching for a telescope. i want to take pictures of nebulae, galaxies and planets. My first idea was to get the Skywatcher 250/1200, then i read that it is too big for the eq6, i was recommended a shorter tube like the orion 10" at f/3.9 or a SCT, due to my budget i can't get higher than the edgeHD 8 unfortunately. what would you recommend? i am also open for different suggestions, thank you :)
  13. Update: 3rd June Re-processed to remove slight magenta tint caused by the non-uniform removal of light pollution by the DBE process ( it was being fooled by the very bright image centre ). The globular star cluster Omega Centauri ( NGC 5139 ) in Centaurus ( please click / tap on image to see larger and sharper ) A full size image can be found here. original below ..... A newly captured ( May 2018 ) image of the great southern globular star cluster, Omega Centauri ( NGC 5139 ) Omega Centauri ( NGC 5139 ) in Centaurus - ( please click / tap image to see larger and sharper ) A full size ( ~ 6000 x 4000 ) image can be found here ....... This image is an attempt to look deeply into the mighty Omega Centauri star cluster and, by using HDR techniques, record as many of its faint members as possible whilst capturing and bringing out the colours of the stars, including in the core. Image details: Resolution ........ 0.586 arcsec/px ( full size image ) Rotation .......... 0.00 deg ( up is North ) Focal ............. 1375.99 mm Pixel size ........ 3.91 um Field of view ..... 58' 20.9" x 38' 55.1" Image center ...... RA: 13 26 45.065 Dec: -47 28 27.26 Telescope: Orion Optics CT12 Newtonian ( mirror 300mm, fl 1200mm, f4 ). Corrector: ASA 2" Coma Corrector Quattro 1.175x. Effective Focal Length / Aperture : 1470mm f4.7 Mount: Skywatcher Eq8 Guiding: TSOAG9 Off-Axis-Guider, Starlight Xpress Lodestar X2, PHD2 Camera: Nikon D5300 (unmodified) (sensor 23.5 x 15.6mm, 6016x4016 3.9um pixels)\ Location: Blue Mountains, Australia Moderate light pollution ( pale green zone on darksitefinder.com map ) Capture ( May 2018 ) 8 sets of sub-images with exposure duration for each set doubling ( 2s to 240s ) all at ISO 250. Processing: Calibration: master bias, master flat and master dark Integration in 8 sets HDR combination Pixinsight May 2018
  14. Hi all, have decided to join this forum after reading lots of different posts and have learnt a lot, know much more now then I did in the beginning. So I’d thought I would create a thread so that you would be able to help. So I’ll start to tell you abit about me and what I’m looking for. I’ve been interested in space and the science side of it for about 4 years now and every time Most of it blows my mind even if I don’t understand all of it . I have been using some binoculars to get me started as I’ve never owned ( or even looked through a scope before) but can’ really see much as they’re fairly low powered. Now I’ve read that the dobsonians are the best bang for your buck when it comes to light gathering. However I would like one but concerned about the weight I was looking at the 250px and seems abit heavy here are some things that I’ve noted down that I would like from a telescope and hopefully this might help you. I’m not looking for goto as that seems to up the price and I’ll get less. I’m a bit of a weakling so nothing to heavy. Storage Will be storing it in the garage though it will be a tight fit but don’t have room in the house I’ll have to walk down only one step to the garden from the garage but don’t want to walk with something too heavy. Where will I be using it? Mainly garden but as it’s south facing I can’t really see things behind the house and but have a wide view of the south and some of the east/west so maybe something a bit portable so I can walk to near by fields ( no car) What do I want to see? as stated in title most of the DSOs as that’s what I’m most interested in examples: Planetary nebulae,Emission nebulae ect Galaxys globular clusters I live in the countryside but between two main towns however my exact location light pollution isn’t bad. I’ve tried to put down as much info as I can to help Thanks for all the help in advanced sorry for the long post
  15. Hi! I just remembered I had made a short gif of Neptune this summer over two nights. My fast reflector has a huge field of view, so you can't really make out any detail on the planet's surface. But you can still definitely make it out, and see that Neptune did indeed wander over the time of a few nights - a planet indeed! Enjoy, and let me know what you think!
  16. It is a solid tube scope made in the UK by Darkstar, probably in the mid to late 1990s. It comes with a Telrad finder and I can throw in a finderscope if you prefer. Focusser is a Skywatcher dual speed crayford. The build is that of a traditional dobsonian with a plywood rocker section which provides a solid foundation and smooth movement. The mirrors could do with re-coating, but are still quite usable. This scope could be well described as a light bucket; the optics are not great (maybe a half wave?) and are really only suitable for low power views. However, it is pretty good for deep sky viewing. From my pretty good location I have seen tricky objects such as three of Stephan's Quintet, the Flame nebula, M51's arms, NGC 5053, C17 and much more. The previous keeper to me used it for galaxy hunting and amassed an impressive catalogue of observations. The scope is just too big for me to use frequently and I tend to get out the ten inch instead. However, I hope it could make a good instrument for someone else. Obviously this is too big to post, so it is pick up from mid Wales (Llanwrtyd Wells) or Penarth. £250 or offers/trades.
  17. Hello again, After many hours of researching and asking on forums I’ve decided not to go down the imaging road as I’ve come to realise it’s way out of my budget. Now after realising this I’ve narrowed down to three telescopes that I’m considering on getting purely for visual use.. Skywatcher 150p 150pl or the dobsonian 200p. What is the difference between the 150p and the pl? I’m after something that can let me see enough detail on planets that I’ll enjoy and also allows me to get views of deep sky objects. I have been talking to Martin from FLO as well and still can’t decide. I’m hoping you can help me make my decision. I’d also like to know how comefortable these three are as I’ll be most likely doing long sessions for sketching. My budget is £400 max. Thanks for for the help (again).
  18. Hello, I'm saving money to buy a professional telescope for visual observing (deep sky) and astrophotograpy. I was wondering whether someone here has experience with the following equipment: 250/1200 SkyWatcher Newtonian telescope on EQ5 or EQ6 Here is the link to the telescope. As I've never done astrophotography, any other suggestion would be appreciated! Adam
  19. Over the last couple of years, I've been rebuilding my Newtonian into a double truss (reusing the mirrors and focuser from an Orion SPX350, replacing the mirror mounts and building everything else from scratch). I've had some good help from a couple of mates who weld and machine! I’m still some way off completing the commissioning of this – the scope needs shrouds round the trusses to keep out stray light (sewing (argh!) and need to find the right material still), and I need to find myself a suitable micro-PC to mount *on* scope to reduce the mass of cables handing off it – this will tidy everything up very well. But, I had a quick stab on Sat night at trying my CCD on the rebuilt Newtonian for the first time (I have previously used the ZWO cam for lunar work, but this is a bit more demanding...). First off, it does come to focus now - hooray! The test was on M1, the Crab. This is 3x13min using Lum filter self guided with the second CCD in the ST2000XM, taken with an MPCC v1. Focusing went nicely using FocusMax and a quickly built V-curve, but I’d like to also make myself a Bahtinov mask. Image solving gave me the following info: Resolution ........ 0.963 arcsec/px Rotation .......... -4.179 deg Focal ............. 1585.40 mm (350mm dia -> f4.53) Pixel size ........ 7.40 um Field of view ..... 25' 40.4" x 19' 15.3" Image center ...... RA: 05 34 36.691 Dec: +22 00 46.97 I also need to sort out the secondary heater – I thought I may not have too many issues with this, but from subframe examination, it looks as though my secondary started to dew up fairly soon after using it - there was a light misting over it from inspecting. The result is some very odd background issues, see the attached image – all but the first (trailed!) subframe start to show the shadow which I assume is due to the coma corrector… odd that this is off center. Any thoughts? Star shapes look OK. PI gives me 3.3px FWHM, which I think is reasonable given the local seeing: see AberrationInspector output below. There are a few stars bottom right near that massive shadow region that have very long diffraction spikes – may have a bit of cleaning to do there if there’s something on the lens/filter causing extended diffraction. Had to use some judicious background correction in the processing (using DBE twice), and I didn’t use any flats here. Had a little bit of deconvolution and Local Histogram Enhancement. Flats are going to be another challenge and will need the shrouds to do well (definitely required for this camera), but I have an idea on what I may do. Other than that, here it is, warts and all for the pixel peepers… Graeme Processed: Subframe towards end of run - dew on secondary (?) (stretched in PI): Corners, etc at full res from AberrationInspector in PI:
  20. Hey guys I've finally managed to get myself an NEQ 6. now I'm looking for a scope. After months of reading and searching on the net, I ended up with three decisions. I will use the scope for Deep-sky, but i like to do planetary sometimes. 1- Sky-watcher Quattro 8 f/4 (Good aperture, Good FL and Good Optics / collimation and bad built quality) 2- William Optics GTF-81 Five element APO (Good (built and optics) quality / limited aperture) 3- Sky-watcher MN190 Maksutov-Newtonian (Good for visual and AP / high weight and higher price) Its a dead end for me, each one has Pros/Cons of its own and i cant choose. So I hope you can help me out. I'm not trying to start the old war of Refractor/Reflector so don't fight each other I will add a long FL system (RC or SC) in a not-near future but for now it will be all i have. I will image with a Crop DSLR (Nikon D5500), I plan to get a CCD but it too belongs to the same not near future. Thanks in advance and I apologize for my Bad English.
  21. 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.
  22. With reluctance I am looking to sell my 200mm aperture f5 Helios Newtonian OTA. (Helios is a brand that became incorporated into Skywatcher - this closely resembles Skywatcher OTAs) The Helios is offered with single speed rack & pinion focuser, tube rings, 22 to 1.25" adaptor, dovetail bar and grab handle for £90 o.n.o. In good condition except that the dovetail bar appears to be discoloured by exposure to sunlight. I have not used the telescope for many months and with a persistent problem in my right arm my enthusiasm for moving heavy kit around has diminished. Buyer to collect from North Bucks. Offered as option with the OTA: EQ5 mount with RA drive + 6 volt sealed lead-acid battery +charger + standard and extra long Dec slow motion manual shafts. £150. Condition - used but working. If I sell the OTA I will be looking to dispose of its accessories: 9x50 Skywatcher straight finder - in excellent condition except that the small plastic dust cap is missing. Cheshire collimator, boxed, as new. Helical fine focuser, range 10mm, ID 1.25", screws into 2"adapter. 3x Datysun Barlow, as new, boxed. Widefield eyepiece, 22mm f.l., 30mm body (note not 1.25") with built in focus, works well with this telescope. Plastic collimation cap (no cost). This bundle would cost around £114 new - offered if bought with scope at £75 o.n.o.
  23. I am thinking on grinding my own lightweight mirror (first f4 16", later f4 24"). The lightest and cheapest option is to get a thin blank and slump it in a decent kiln. Anyone has longer term experiene with slumped mirrors? Overall doesn't seem to be more work than a normal (not pregenerated) blank. Do I have to grind the backside as in case of normal flat back mirrors to avoid astigmatism? How do people support a convex back while grinding? Does it make sense to grind a hole in the middle for additional support? I am remotely considering a convertible Newtonian/Cassegrain system anyway...
  24. Purchased the Orion Astroview 6 three years ago. Found out later that I could not attach my Canon EOS XTi and obtain focus. Everything I saw online required sawing and drilling the OTA. That makes the scope a single purpose telescope. After reviewing the primary mirror holder I created a holder that moved the mirror 25 mm. This way I can keep the scope ready for visual observing and sell later if I desire. If you're interested in saving a few dollars in this hobby check out the link below. How I Moved Prime Focus: Link to the detail. Photo from this telescope
  25. Hi guys, This is the first lights of my ASA 12"N f/3.62 (purchased in 2007) with my Moravian G4-16000 CCD camera using one KAF-16803 chip of 36mm x 36mm (4K x 4K). The adjustments, collimation, ASA OK3-Z pression screws, asked a lot of attention. For a precise collimation, i use the CATSEYE XL & XLKP kit. This is the first 3hrs of exposures taken the last weekend in one night. The second night was too windy unfortunately. Full Resolution image in 4K x 4K here : http://www.poigetdigitalpics.com/Fichiers_Divers/integration_NGC4438_DBE_3HDR_CompositeAplatie2_Spikes2-Modifier-Modifier-Modifier.jpg Full Resolution Annotated image here : http://www.poigetdigitalpics.com/Fichiers_Divers/NGC4438_new_image_Annotated.jpg All the information below : Enjoy ! Florent ?
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