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leonardo2012

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About leonardo2012

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    http://marklimbrick.co.uk

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    Suffolk
  1. Having spent time getting the telescope optics and GSO coma corrector aligned - it may be useful to anyone contemplating a Newtonian plus this corrector for astrophotography to note how it performed in practice. firstly - with the GSO CC in the optical train you have to provide an extra 35mm to reach the new focal plane. The CC itself also shifts the focal plane by its insertion length, about 50mm. I was able to fit longer bolts on the mirror cell to take care of this extra distance. If you just hang extensions on the focuser the secondary mirror size needs checking. secondly - don't expect 'razor-sharp' 'tack-sharp' (or any of the other phrases that get used subjectively) image quality from edge-to-edge of an APC sensor. Forget about using this with full 35mm frame sensors. The focus is softer in the centre of frame compared to an image without any CC. I estimate a star is 30 to 40 microns at best. I mean when done properly, measured 50% CCD Full Well, geometric diameter of blob. By the time you get to the corners of the frame it is a smear image of a star. I should have checked the few images on the web 'before-after GSO cc' with a forensic eye before buying this! My GSO CC also has a chromatic error due to one element being non concentric or with 'wedge'. I can only minimise the effect by trading off against other image faults that arise when trying to tilt the cc independently of the optical train axis. Roger Ceraglio, the designer of this coma corrector according to other blogs, only claims that it reduces coma in fast Newtonians. Maybe the GSO is only loosely based on his design 'type' and as I suggested earlier may not even have two groups of four elements. Poor Quality Control in manufacturing may be an issue. Shame he didn't feel able to just post his spot diagrams or indicate likely results empirically when asked on other forums a couple of years ago. Whether he fudged the answers because of manufacturing licence issues, only he can say. I wouldn't recommend a Newtonian with coma corrector for astrophotos that will be enlarged more than 10 times or so, say a screen image width of 1000 pixels. I say 'coma corrector' because so many contributors have subjectively praised or derided any one unit over others, whatever the cost. Lots of major errors go unnoticed or overlooked during visual use. Technically speaking, the camera does not lie - I took hundreds of test images, analysed star images and tweaked everything in the alignment as best as I could over several days. I have a perfect imaging scope DIY Crocheted (Corrected) Dall-Kirkiham 20" aperture f8, with a corrector which gives images 100 times better than this GSO. But its useless in my skyglow-polluted backyard with an unmodded DSLR. Over-sampled, too slow, and now an even more restricted sky view from the permanent dome now a neighbours tree has been allowed to grow!
  2. Chris - is that a drawtube extension piece - it's difficult to see any change in diameter of the focuser tube and whats added to it? So, the CC is inside that extension? That's what you called a 'holder'. And hence it has the thumbscrews to hold the eyepiece/2" to 1.25" adaptor etc.
  3. Thanks Chris. I meant is the GSO coma corrector inside the focuser drawtube and then the 2" spacer is added by threading it on? The 'eyepiece adaptor' I can see is after that - just ignore the mis-typed wrong label.
  4. I have been building an ultra low profile '~Crayford-style' focuser for imaging only this weekend. Leverage was my main concern with a DSLR. So I did some levering as I assembled. Without resorting to some CAD analysis software, I have to say that the basic design is for smooth movement without precision engineering skills, not holding the drawtube perpendicular (it has a spring in it - surely a big clue!). Or for pulling heavy loads against gravity. A three-legged stool doesn't wobble and can be quickly made by the milkmaids - but is less stable. An engineer care to analyse and confirm this? After adding a fifth bearing things improved. An internet search reveals some monster engineering to overcome the inherent lack of stiffness. And even a (different and revealing) 5th bearing design! All the expense and extra bulk of metal and big bearings thrown at the problem is evident in the 'good' products on the market. To push a shaft onto a tube with enough force to grip needs three ball bearings set up behind it, not a (squashy) 'virgin teflon pad'. So it can't flex away, and allow the drawtube to flex. So the adverts for red anodised CNC perfection should really say - 'we've taken the basic design from an amateur for a low-precision focuser and added as much (or little) metal as possible to make it work, made it look really high-tech, finished it in shiny red or green, and even then some people are disappointed' Mine works in the sense that the drawtube moves smoothly over the intended 6mm range, it is probably extremely rigid compared to most commercial units. It cost 45p for some nuts and bolts I didn't have the right size. There isn't a lot of metal in it and I used hand-tools and a pillar drill. My Crayford is not as rigid as the set-up I had used before, where all the parts were machined and bolted together and fine-focus involved tweaking a sliding intereference fit between two concentric tubes and locking them together with screws. The Crayford is the wrong engineering solution for heavy imaging trains. Is it a marketing ploy for one commercial unit to put a top secret 'warranty void if removed' sticker on its' product? - at the same time quoting on its' website the generosity of Wall to donate his wonderful idea! Please can someone else like Wall come up with a new design? Try throwing away all those catalogues of Crayford focusers and get busy in the shed.
  5. No, I have got it wrong! The 45mm 2" adapter referred to in the photos and the spacing distances list by Chris is the GSO 'eyepiece 2" adapter' which is not supplied with the cc from Teleskopexpress? Is that correct? The 2" adapter extender (2" long?)on the focuser drawtube is in place in both photos. The eyepiece shown in the vertical setup thumbnails is a 2"? There seems to be an approx 5mm shoulder sitting on the eyepiece holder WO cc in place and in last pic with cc in place it is also just sitting on the top of the 'eyepiece adaptor'. Can I ignore this as just being part of the eyepiece? Is the GSO 'eyepiece adaptor' itself sliding into the 2" holder at all, or just the filter ring and Hyperion spacer/youghurt pot ring? Had all the things set out ready to machine, now I'm not so sure... Many thanks if you can see what I mean and assist.
  6. I have looked carefully at the photos posted here with before/after cc and eyepiece. The movement of the focuser 'inwards' towards telescope is given as about 10mm - after 45mm 2" spacer is in place. So about 35mm movement of original, non-CC, focus relative to telescope tube/focuser mount, outwards. Which is what I observed when testing on distant trees. The photo is neccesarily at lo-res - but the 2" spacer looks the same diameter as the focuser drawtube...so it is a 2" drawtube adapter being used as a "spacer" with the CC sitting inside it. Is this understanding correct? I have only 20mm left to move the secondary right up to the spider vanes - so will have to move the primary down too. Why not just hang the camera out on extension rings/adapters like everyone else? Because it must be squared on exactly and I have to make another low profile crayford micro-focuser to accept the 2" CC anyway. My secondary is also only just big enough at 50mm minor axis for DSLR sensor. I will re-fit the OAG pickoff mirror/tube into the CC-to-DSLR adaptor I must now make using a lathe. I hope this helps anyone who is in a similar position, wanting to swap in the CC to a newtonian astro-imaging setup that has already been optimised/customised for stability with little room to just re-focus by as much as 35mm.
  7. Received my GSO corrector - sent from Germany in the middle of the night - excellent service from Telekop-Express! I checked it out this afternoon on some trees about 200m away. I thought I was going to be making up an ultra low profile mount to accomodate this CC - a Crayford with half the bearings inside the telescope tube. But the final focus is not inwards - towards the scope - 'by about 10mm' as read on forums. It was measured as moving outward - by about 35mm. Double checked I had the unit the right way around and spacing to eyepiece field stop was 75mm! This isn't mentioned anywhere on reviews. The issue hinted at most often refers to 'infocus' travel not being sufficient on focusers and having to move the primary mirror up. I will say there was poorer contrast and definition with the CC compared to same view with just the eyepiece - but this is quite likely the result of not being adequately collimated with a temporary rig on the side of the tube. Is this particular unit behaving normally? Looking very carefully into the CC with a bright light I can only see a single lens element at one end and a cemented double at the other. The single element may be two which have been so cleverly cemented and afterward edges ground so well that the edges appear as one - but I have never experienced this in a life-time of taking optics apart. Anyone comment on the outward focus shift? It would be a helpful addition to the GSO user guide if this change in final focus were resolved - I'm reluctant to make a focuser until I know what's to be expected and whether this is ok. Many thanks for previous comments.
  8. leonardo2012

    astro

    general astro stuff
  9. I have just ordered a GSO 2" coma corrector from Teleskop-Express and will be interested to see how it performs with my 16cm f3.8/f13.5 scope. Last Autumn I removed the cassegrain secondary and put in a flat to try some photos with a DSLR. Everything I have is home-built, so a quick fixup with an ancient cast-iron stand (1900's?) gave me an image of the Andromeda Galaxy surrounded by lovely 'seagulls'. The GSO should do better both on the image and with the OAG star images. But by how much, will I lose the resolution in the centre of the field that the seeing alone allows? OAG - small mirror right on edge of DSLR sensor field coupled to ex-view 1024x B/w CCTV board. Guiding with PHD. Parallel port control via Transcard in games slot on old laptop running XP. Small stepper and salvaged worm drive, stainless wire (.5mm multi-strand from scrap photocopiers) wrapped around output shaft and then 6" wood pulley. Yes? Sub arc-second 'string' drive! 8 microstep driver chips, Rewired 1970's era TV remote control paddle - 3 slewing speeds etc! 2 hours imaging 5 mins x 20, plus darks, flats. Frost on tube, condensation on laptop, frostbite in feet. And then the processed image shows DSLR not quite square to optical axis. At least the guiding worked - discards were all due to me being too slow/over reacting on noticing slight polar mis-alignment creep in the DEC axis. Took an hour to find target that night (in scope field as opposed to just binos in hand and tilting head back!) - now set up Arduino-based setting circles - salvaged magnetic encoders from photocopiers. I must finish the dome - no more wires to connect every time! And a 240v mains extension trailing from the back door... When I set up with the GSO corrector - I'm going to be looking very closely at the star image quality. It's rare for me to be able to spend money on astro equipment and I want to see a big improvement. Before GSO image in my gallery. Plus photo of string drive. Note this is a suburban location with plenty of orange skyglow! Can someone enlighten me as to who Yves is and what is the 14" ODK telescope mentioned on this thread? Interesting to read comments from so many UK astronomers here. Anyone still make there own scopes?
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