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Everything posted by digital_davem

  1. Actually the sensor size does have an impact on resolution. Let's pretend we have a scope with infinite resolution i.e. every square mm of its optical circle could support any arbitrary level of magnification. Then what defines the resolution of the final image is the number of pixels per sq mm. If you have for example a choice between a full frame and a 4/3 sensor of the same pixel count, the full frame would cover a 4x wider area of view but the imager resolution would be half of the 4/3 because the pixels are spread over a larger area. The upside is improved noise and dynamic range characteristics. Another idea jumps out at me, too. Even if the coverage of a telescope is relatively small, small format cameras could be used with a shift adaptor to allow small mosaics to simulate the effect of a full frame. For example, I have a shift adaptor for my m4/3 camera. This is intended to be used with full frame lenses which have a larger covering circle than native m4/3 lenses. The idea is that you shift left take a pic, centre take a pic, shift right take a pic. Then the you can flat stitch the 3 frames to produce a mosaic image with 3x the area and pixel count. I keep seeing comments to the effect that photography is somehow different from astronomy but the optical principles are the same. Maybe some of the terminology and rules of thumb can confuse but there is no actual difference.
  2. Hi Rik Thanks again for all your help. One point I'd like to make: of course you are correct in saying that the crop factor doesn't change the physical focal length. However, it does make a difference when comparing sensors of different sizes but the same pixel count: the pixel density is higher. For example, if I compare my m4/3 16MP to a full frame 16MP, my m4/3 camera is concentrating all its pixels in an area 1/4 of that of the full frame. Which means I can blow up the frame 2x bigger compared to the full frame without loss of resolution (assuming the scope has sufficient resolution to support the magnification). The downside is smaller pixels, more noise and less dynamic range. I had a quick go at the moon with my camera lens (300mm f4 + 2x teleconverter + m4/3 panny g6 = 1200mm equivalent compared to full frame with same lens). I downloaded and installed some free stacking software (PIPP and Autostakkart). Don't really know what I'm doing with it but here's my first attempt: I shot about 60 frames and selected the best 15 (100% - 80% quality). The pic makes a decent 7x7 inch print as long as you don't try and look too closely ;-) I'm not sure what is the biggest quality constraint: the mount, the lens, shutter speed, camera resolution, seeing, magnification, number of frames, processing but sharpness is a long way from great, even with loads of processing...
  3. I've taken a chance on ordering the simplest t2 ring-nosepieces adaptor. We'll see if it works. I'll report back in due course. Cheers Dave
  4. Thanks! What is the likely focusing problem with this arrangement? My scope is a refractor with a fairly long draw tube. Is the problem that the draw tube reach might be too long or too short? Regarding potential targets, the moon is the obvious one but what about saturn and jupiter? Admittedly, the moon is the only thing I've actually looked at with my scope so far but let's put that inexperience to one side! Using my m4/3 camera with its 2x "crop factor", I'll have 50x magnification effectively. With a 2x teleconverter on the camera that could be 100x. I guess the problem is not so much magnification but brightness? I did a little experiment the other day, imaging the Pleides. Using my 300mm telephoto with a 2x teleconverter and the m4/3 camera (effective focal length 1200mm, similar to the native FL of my scope) I did manage to get recognisable shots of the cluster. The problem was that exposures greater than about 1/10th of a second showed either star trails or simply tripod wobble (elongated stars). Admittedly even my best shot wasn't very interesting (seven stars and a black sky!) but it hinted that something is possible even with short exposures.
  5. Hi folks Complete newbie astronomer but experienced photographer... I recently acquired a classic 1970s long tube Prinz 660 3" refractor (1250mm/f16). It came as part of a boxed set with 0.956" accessories but no tripod and an incomplete EQ mount (just parts). I've been refurbishing it for a few weeks and it now has a 1.25" visual back, diagonal, red dot finder and a couple of modern low cost plossls and is fitted with tube rings and an arca-swiss plate and mounted on a manfrotto 055c tripod and pan/tilt head. The tripod is just adequate for visual work if you are careful. Eventually, I intend to get a dedicated astronomical mount (maybe a skytee or an EQ5). For now, I'll make do with the camera tripod (which I'm going to try and strengthen). I've just finished getting the thing set up for proper use and tonight I had my first real go with it (an hour moon viewing). Everything worked as expected thankfully. I even tried getting some snaps and a bit of video with my camera held up to the eyepiece (no lens attached). It kinda worked but a bit iffy! I know that there are various adaptors available (eg from ebay) that allow a camera to be mounted directly on the focuser without an eyepiece. Presumably in this configuration, the scope acts like a 1250mm/f16 telephoto lens. I appreciate with my mount and tripod, nothing very ambitious is possible. There appear to be a whole variety of different adaptors on offer that look quite different. Some are very short 1.25" to T-mount adaptors, some are much longer, some have adjustable length tubes. One even combines a camera connector with an EP diagonal so you can switch between visual and imaging by turning a knob and one even has a built in eyepiece! I have 4 suitable cameras available (all 16MP): a Sony NEX 5n, a Fuji XA-1, a Fuji XE-1 (all APS-C) and a Panasonic Lumix G6 (m4/3). I also have quite a selection of cheap chinese lens adaptors that allow various brands of lens mount to work on these cameras. Any thoughts on what is the most suitable (and cheapest!) way to mount the cameras? Thanks Dave Links to various ebay scope-camera adaptors: http://www.ebay.co.uk/itm/Telescope-1-25-eyepiece-connection-for-Nikon-DSLR-Camera-Photography-Flip-Mirror-/261344855554?hash=item3cd95e0602:g:jD8AAOxyTjNSnEPb http://www.ebay.co.uk/itm/1-25-Variable-Projection-Camera-Adapter-Telescope-for-Canon-DSLR-Camera-New-/261344896846?hash=item3cd95ea74e:g:RAQAAOxy3zNSnFm1 http://www.ebay.co.uk/itm/1-25-Telescope-Mount-T-T2-Lens-Adapter-for-Sony-DSLR-Camera-A58-A65-A77-DC617-/311490063116?hash=item4886412b0c:g:iNMAAOSwnipWUMpH http://www.ebay.co.uk/itm/1-25-26mm-Telescope-Eyepiece-Adapter-for-Canon-EOS-EF-EF-s-mount-DSLR-cameras-/321179886039?hash=item4ac7d021d7:g:-8YAAMXQxU5R~-Ev
  6. Forgot to mention the best bit - the lens cap made from the tin lid of a finnish licorice container
  7. From what I can find hunting for info on the internet, the 660 originally shipped with a 6x30 straight through. I suppose there is a slight chance that one of those might be turned up somewhere.
  8. Thanks very much for everyone's suggestions and tips. Got there in the end. I now have a working classic 3" long-focal length f/16 achromat refractor converted to the modern 1.25" barrel standard, tube rings, mounting bar, two new eye pieces, a diagonal and a modern finder for £166 in total. And as a bonus I still have all the original 0.965" bits plus some parts of the original EQ mount and a lovely wooden coffin. The only money I wasted was £20 on the unnecessary 0.956 to 1.25 adapter which the new visual back made redundant. Not too bad, I think. I wonder how much a new one of these would cost if they were still made today. The main remaining problem being the camera tripod and the pan and tilt head. It's strong gear for photography but marginal for this long scope. I guess maybe a Skytee or at least an AZ4-2 is on the wish list. Time to save some pennies... Thanks again Dave ps And learn how to use it, of course!
  9. Forget EVERYTHING I have angst over! I just don't have an innate Mr Fixit mindset. I have by pure chance found a solution. In retrospect it is perfectly obvious but I just don't have the right kind of imagination or resourcefulness. Still, dumb luck can come to you aid sometimes. I have a bag of junk - old camera bits I've collected but never had a use for, that I have been encouraged to throw away as we don't have space for it. Being congenitally lazy, I have collected the bag of junk but just left it lying around waiting to be taken to the dump. In that bag is an old zoom slide copier gadget. It uses a rotatable T mount that has two thumb set screws to lock in place. And as it happens, those set screws are exactly the right thread, the right length and the thumbscrew is a nice big head on the end that is exactly the right size for the countersunk hole in the Skywatcher plate... I needed to use needle nose pliers to tighten it in place as there is no slot for a screwdriver. It works perfectly. The finder is now installed and aligned. Yay! Thankfully, no drilling required and I can keep the original plate and screw safe for posterity. I doubt that single screw would be strong enough for a heavy optical finder but the Skypointer pro reticle finder only weighs about 100g - fragile but light.
  10. I have obtained a lightweight Celestron Starfinder Pro red dot bullseye finder. It is disappointing cheap and plasticky which seems out of place on such a scope but if it does the job... It shipped with for different (plastic) mounting plates, none of which seem to be remotely suitable. I have also got a skywatcher plate which is reasuringly solid metal. It has two mounting holes and two bolts. Unfortunately, the original mounting plate is secured only by a single tiny (3mm long) almost grub screw, way smaller than the SW plate bolts. So if I'm to fix the SW plate to the original hole, I'll to find a tiny skinny screw 4x longer than the original and fit it with a big washer or something so it doesn't fall straight through the hole in the SW plate. It all seems a bit impractical. I'm now considering other options - like sticking the SW plate on with hot melt glue or a sticky pad maybe. Alternative, I wonder if it might be easier to attach a plate to the focuser unit instead. Does anyone know whether the stock mounting shoe used to be a common design or is it unique to this telescope?
  11. That makes sense. I was thinking about this before I went to sleep. The drawtube in my focuser has a 36.4mm diameter. So no matter how big the patch of light produced by the objective, the largest diameter circle the eyepiece/camera/unaided eye could see is 36.4mm. If the eyepiece has a bigger apparent field of view, all it would see at the edges is the inside of the drawtube. So, in practice even the fanciest EP has an absolute limit defined by the diameter of the drawtube (or less if there are baffles in the drawtube. That is why as you note, bigger barreled EP in bigger drawtubes can potentially have a wider apparent field.
  12. So there are 3 ways you can achieve a wide angle view with a bigger true field of view: - don't change the eyepiece but change the telescope to a shorter tube - keep the telescope but use an EP with a longer focal length - keep the telescope and don't change the EP focal length but swap the EP for a different design that has a wider apparent FOV Given my current theory of how this probably works, the third option of using a fancy EP comes at a (technical cost): the extra field of view is making use of the outer edge of the objective's image circle which is of poorer quality than the central region. This is analogous to what happens if you put a lens designed for APS-C sensors on a camera with a full frame sensor: you can get a wide angle view but chances are the sweet spot of the image circle is too small for the big sensor and the edges of the frame will be a lot fuzzier than the centre of the frame. And if you go too far, the lens circle won't even cover the full frame centre and you will end up with a circular image in the middle of blackness.
  13. Can these forums be viewed as a threaded tree structure? The flat view makes it very difficult to carry out three simultaneous converstions and keep track. I like your analogy as an aid to visualising what you will actually see. But it does not seem to explain my central condumdrum of why telescopes can do the impossible! With my camera I can use a telephoto to get a close up view or I can use a wide angle to get a lot in. But I cannot swap out my telephoto to another telephoto of the same focal length that will also show me the wide view at the same time. The two are mutually incompatible. Yet in eyepiece land I appear to be able to buy a high magnification eyepiece that has a wider view than a low magnification eyepiece of a different design. That's pretty counter-intuitive, you must admit. Again I come back to my idea that the objective must project a large circle most of which is wasted by cheap eyepieces. Only the expensive multi-element designs can "see" the outer regions of the objective's image circle. To create another analogy: imagine you are watching TV, say the evening news. The TV cameraman zooms out to show the studio and sometimes zooms in to the face of the news reader. This is analogous to swapping from say a 25mm eyepiece to a 4mm eyepiece: everything on the screen is magnified. Now imagine that you are viewing the TV not with you eyes but through the zoom lens of your camera. You can zoom in so the TV exactly fills the viewfinder. Then when the TV cameraman zooms in or out, your viewfinder view follows suit. Now zoom your camera in a bit further so you fram only the centre of the TV. Again, when the TV cameraman zooms into a close up you see that in your viewfinder. But when he zooms out again for the studio shot, your viewfinder chops off the edges and you can't see the wide view. I think how this must work in like this. The objective projects a nice circular image at the focal point. Your eyepiece photographs this circle. If your eyepiece has a wide apparent field of view you see all of the image circle including the fuzzy edges. If your eyepiece has a narrower field of view you see only the central region of the objective image circle. In fact (just a guess) I reckon that orthos and plossls and the like actually have a much wider apparent field of view that you see in a practical eyepiece. However the edges of that apparent field of view have embarrasingly terrible image quality so the manufacturers include a fixed aperture stop in the eyepiece to block off the fuzzy regions, That would explain why the edge of the visble circle is always a knife edge sharp line. Real optical circles just sort of blurrily fade to black at the edges, that sharp circle has to be artificial.
  14. This is an aside but an interesting parenthesis. It's true that a telescope does not have a variable iris but it effectively still has an iris: its fixed aperture lens. Camera lenses used to work the same before the iris aperture was invented. The lenses were telescopes with a fixed aperture defined by the focal length/size of the front lens. Exactly like a telescope. The first "stops" used in photography were literally metal plates with different size holes in them the were inserted into slots in the lens barrel to vignette the incoming light cone. The key thing when making anologies between telescopes and modern camera lenses is to think of the camera lenses at their wide open aperture and forget about the iris. A zoom lens for example, used with the iris wide open will have a wider aperture at the wide end and a narrower aperture at the long end. Typical cheap zooms might be something like a 80mm f4.5 to a 200mm f6.3 - even with the iris fully open. The reason the f number rises in this case is nothing to do with the iris but is caused by the focal length getting longer - just like the difference between a ST80 and a class refractor. And with higher (ie narrower) f numbers comes a greater depth of field (zone of sharp focus either side of the actual focus point). This largely happens because the narrow aperture blocks off the light rays at the edge of the light cone that are diverging, letting in only the ones that are more parallel. This is significant because Chromatic aberration is caused by colours coming to focus at different points. A wide aperture low f number lens has a very thin depth of field and focusing has to be very precise - too precise for all the colours to be in focus at the same point unless exotic glass in used. A narrow aperture high f number lens has a thick depth of field sufficient to include the focus point of all the colours even with a simple objective - hence chromatic aberration is gone. F number is still important in discussions about telescopes for this reason.
  15. But the increase in depth of field that applies to stopping down a camera iris must also apply to the telescope, yes? One of the reasons why my simple f16 achromat gives views the equal of an expensive APO is because the increased depth of field at f/16 makes chromatic aberration go away. The same achromat type objective with an f2.8 focal ratio would give appalling image quality because of all the out of focus colour fringing.
  16. OK, this just make no sense to me. It seems impossible. If I have a zoom lens on my camera, there is only one way to get more stuff in and that is to zoom out to a wide angle view: which makes everything smaller. It is impossible to zoom in a way that keeps the centre of the field the same size while making the edges show more. A lens is either zoomed in close or pulled out wide. Your images appear to show a view which is simultaneously wide angle and zoomed in! Does not compute...does not compute...illogical Kirk, illogical... The only way I know to duplicate the effect your images show is by stitching a pano - basically taking two or more photos and joining the frames together. There is no camera that can do what you are showing here. How does a telescope do the impossible???? ps And this is not what happens when I use my spotter scope with a zoom lens. That behaves exactly like a camera. At low power you get get a wide view and at high power a much narrower view. pps Which gives me a clue. Is it perhaps, that the image circle produced by the objective is quite large in diameter, somewhat like the large image circle produced by a plate camera lens. And the eyepiece doesn't see the whole circle, just the centre of it. I have always assumed that the boundary of the view you see through the eyepiece is the edge of the image circle produced by the objective - but actually that can't be true because the edge of the objective image circle will be quite fuzzy and the eyepiece view is very clean cut. So there must be an artificial edge introduced in the eyepiece - an artificial vignette essentially. Which suggests that these fancy eyepieces make use of more of the objective image circle than the basic ones. That would make sense. It is like replacing an APS-C sensor with a full frame of twice the area. It could see more of the objective image circle without having to zoom out. Which leads to a question: are there eye pieces whose apparent field of view is so wide it exceeds the objective image circle so you can actually see the whole thing with its fuzzy edges blurring into darkness...
  17. This is where what seemed to have started to become clearer starts to get muddy again! Ignoring the eyepiece for a moment, what difference does the objective make? Let's compare a 3" 300mm short tube refractor with a 3" 1250mm refractor. They have the same size objective lens so they collect the same total light and offer the same absolute resolution potential. However, one has a focal ratio of f4 and the other f16.4. And I know from my photography that when the f number gets bigger, the image gets darker. What is the explanation for this? The answer must logically be that the long focus refractor objective magnifies more than the 300mm refractor even without an eyepiece. That's why it's darker. If it were possible to view the image created by the objective with a 1x eyepiece, you'd see that that the 1250mm refractor was more zoomed in that the 300mm. I think this means the image created by the objective is not like what you see with naked eye - it will vary from scope to scope depending on the focal length of the objective. To mimic the naked eye, the objective would have to have a focal length of 40-50mm (and I don't believe telescopes this short are that common). Or is there something I'm badly misunderstanding here?
  18. Hi John Thanks for the attempt It seems to me that more mathematical formulae or words I read, the more confusing it gets! However, I just watched a Youtube tutorial in which the words used were so confusing it was better with the sound off (!) but in which the pictures might have provided some illumination. Let me try it on you... - The patch of sky that the telescope sees is the True field of view. A high magnification will show a very small patch of sky with the subject (say Jupiter) quite large. A low magnification will show a larger patch of sky with the subject smaller and lots of surrounding stars visible that were outside the frame with the higher magnification. This is consistent with what you see in a camera viewfinder when you swap from a tele lens to a wide angle. The more wide angle you go, the more stars you can squeeze into the frame but the smaller everthing is. - Apparent field of view. This is the spec that keeps getting quoted in adverts. What confuses me is the descriptions in the adverts always imply that if you use an eyepiece with a wider apparent field of view, it's like fitting a wide-angle lens on your camera - yet it can still magnify x300. This must surely be nonsense. The youtube diagrams imply something quite different: They imply that the eyepiece apparent field of view makes zero difference to the amount of sky that your telescope can see (the number of stars and how much jupiter is magnified). One of those fancy Naglers or an ortho that yield say 50x magnification will see identical patches of sky with exactly the same subject matter inside the frame. What does change instead, is how close the circle you see feels to the eyepiece. In other words some eyepieces make the circle look like it's down the end of a long tunnel, while others make the circle seem really close, in your face with no wasted space outside the circle frame. It's similar to the difference between the viewfinder of an APS-C format SLR camera and a full frame SLR. With the APS-C camera, the viewfinder puts the frame down the end of a long dark tunnel where it looks small and far away while full frame viewfinders are bigger and closer. In fact, often so big and close that you can't always see the edges with your eye in one position but have to move it around the eyepiece to scan the frame. The key thing for me about this camera analogy is that what is in the frame of both viewfinders is exactly the same subject and the pictures you take will look identical. The difference is basically cosmetic - how comfortable the viewfinder is to use and how bright and impressive the view is. The framing doesn't change. Likewise with telescope eyepieces, what is contained within the circle is the same for a Nagler and an ortho, what changes is how much wasted space there is outside the image circle and how "close" the circle itself appears. It's basically like watching your TV from a foot away or across the room. The same field of view is in the TV picture either way. Sorry for the long windedness of this but does it sound like l'm on the right track? p.s. And if I am, with my 1250mm long focus refractor, if I want a wide field view showing the sky from horizon to horizon, getting a Nagler won't help at all. The wider apparent field of view won't make my telescope show a bigger patch of sky, The only way to do that will be to use a longer focal length eyepiece to reduce the magnification and increase the true field of view or use one of those gadgets that reduces the focal length of the objective (thus increases true field of view)....
  19. What little understanding of optics I have comes from the photography world. There are many similarities in the concepts and terminology but sufficient differences to utterly baffle me ;-) For example Field of view. In photography: - The lens projects an image circle at the focal point. - Lenses designed for different size sensors produce larger or smaller image circles, The diameter of the image circle needs to be sufficient to cover the format of film or sensor you are using. For example a lens designed for 35mm format will typically produce a 43mm diameter image circle, while a lens designed for 5"x4" sheet film will produce an image circle larger that 5" across. How that is achieved I don't understand. - Within any sensor format, the angle of view and inversly the magnification of the lens depend on the focal length. A long focal length will see a narrower angle of view/greater magnification; a short focal length a wider angle of view/lower magnification. Thus wide angle lenses are shorter focal length than telephoto lenses. - However, there doesn't seem to any absolute angle of view/magnification determined by focal length; you always have to consider the sensor size as well. So, for example, a 300mm lens on 35mm is a longish telephoto but on a 10x8 plate camera it is merely a standard lens. Now to telescopes... - What size image circle does a telescope objective project? If I were to place a sheet of paper at the focal point without an eyepiece installed what would be the diameter of that circle? Is it the same for all telescopes (i.e. is there some kind of standard) or does it vary? If it varies what is the cause of the variation. The effect of the eyepiece - this gets really confusing for me. You read about eyepieces of the same focal length having different fields of view. I don't understand this. How can this be - magnification and field of view are the inverse of one another: if one goes up, the other goes down in lockstep. It is impossible to have a wide-angle telephoto, yet eyepiece specs imply that some eyepieces are simultaneously wide angle and telephoto! A 100 degree angle of view is inconsistent with high magnification, it is by definition the opposite. A high magnification would surely be a 1 or 2 degree angle of view! Very confused and would be very grateful for some insight from you wonderfully knowledgable and helpful people... Cheers Daver
  20. Also, do they take into account the lengths of the scope? A one foot long scope won't wobble as much as a 5 foot scope which is a great big lever arm...
  21. Focal length of the 660 is 1250mm. My 12.5mm gives me (obviously!) 100x. Barlowing it would be 200x compared to 8mm which would give 156x. I have no idea what magnification a 3" classic f16.5 scope will tolerate on the moon. I've seen suggestions on forums that 300x is doable and elsewhere suggestions that 150x is too much. I guess it needs input from someone with hands on experience of this class (preferably model) of scope.
  22. On my 660 the 20mm seems usable but I haven't really be able to see anything through the 6mm and 4mm ortho! Not sure why, just a fuzzy patch of light full of hairs, dust and spiders!. The view from modern 1.25" inexpensive plossls is incomparably better. I have a 25mm and a 12.5mm. I'm wondering whether I should get a high mag EP for moon viewing as well, but if so which one? Something like an 8mm? Or maybe a barlow?
  23. There may be other solutions for tube rings but I have found a workaround. The tripod collar for the Canon 80-200mm F2.8 lens is just a millimetre or so larger than the telecope tube. A thin layer of felt or (as I did, electrical tape!) and it grips just fine. The original canon rings are some absurd price like £120 each but I googled a chinese knock-off for £10 per ring. They are understandably not as well finished as the Canon originals but mine are fine as tube rings. I bought a 200mm arca -swiss plate which has two built in 1/4" tripod bolts for £6 and it works fine on my camera tripod. One thing to note with these collars - they don't open fully, just enough to loosen them a little. To get them on the tube I had to remove the focuser unit, slide them on, then screw the focuser back on.
  24. Hi Roy I'd appreciate it you could copy me in on those instructions - there might be something useful there I've missed. Cheers Dave
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