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johngwheeler

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About johngwheeler

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  1. Excellent answer Bill, and quite logical when one thinks about it! I think the total distance would need to be from the focal plane to the rear of the draw tube plus the length of the draw tube.. This total could easily exceed 140mm I think. My next question would be how a focal reducer affects this, but this should just be a question of the entrance and exit apertures being large enough for its image circle.
  2. I've been struggling to find an answer to whether a 2.5" focuser is likely to cause vignetting on a full-frame (36x24mm) camera sensor. This would be for an f/7 130mm scope, probably used in conjunction with a 3" 0.79x focal reducer (stated to have an illuminated image circle of 45mm). Whilst the focal reducer should fully illuminate the sensor (having an M68 connector on the scope side and M48 on the camera side), it is not clear whether any "mechanical" vignetting might occur with a 2.5" focuser drawtube. I imagine this would depend on the backfocus of the scope - i.e. how far the focal plane is behind the end of the focuser. If it's a long way back, I can visualize the light cone potentially being restricted when it enters the drawtube inside the OTA. Hopefully, the scope isn't designed this way, but it's hard to tell. If there is no mechanical vignetting, it does raise the question of what the benefits of larger focusers are. Greater load capacity certainly; improved stability? maybe. But does it actually give a wider unvignetted image circle? I'd like to apply some science to this question, rather than buying a bigger focuser "just in case"! Thanks for any answers! John.
  3. I've been looking into getting a new (or used) mount to upgrade performance from my Celestron AVX, and am wondering about the "real world" guiding accuracy achievable with various mounts. I have my eye on a used Takahashi EM-200 Temma 2, which although somewhat old in design, is beautifully build and is advertised with a PE of +/- 5", and I've seen very good guided performance. If the EM-200 doesn't work out, I am considering either finding another premium mount (used, which will take some time to find), or just getting an AZ-EQ6 (or NEQ6 / EQ6-R) in the meantime. There seems to be a lot of variation in people's tracking / guiding accuracy with the EQ6 class mounts. What sort of raw unguided, PEC corrected, and autoguided accuracy could I expect? I will be imaging with up to a 1000mm f/5 Newtonian, with a possibility of a C8 or C9.25 at some point. Thanks for any help you can give! John
  4. I'm looking at a used Takahashi EM-200 Temma 2 mount (not the 2M) and noticed a problem with the polar scope illuminated reticle. Although there is a red light visible inside the mount casing (when looking through the front of the polar scope aperture), nothing was visible through the p-scope itself, and the reticle was invisible. I downloaded a couple of manuals (for the Temma 2 jnr & 2M models) and nowhere does it show a diagram of any switch for the illuminated reticle. It does describe an adjustment for illumination level, which I take to be the small set-screw next to the power indication light. Adjusting this screw does seem to adjust the brightness of the red light visible inside the mount. Does this mean the reticle is permanently illuminated when the mount is powered on? What may have happened is for the light source (LED?) to have moved from its usual position, so that it no longer illuminates the reticle. This will require opening up the mount to see what is going on. Does anyone have any detailed schematics of the EM-200, or have any experience in opening one up? Given the polar scope is a key feature of this mount, I'd really like to have this fixed before buying the mount! Thanks for any help or suggestions you can offer, John
  5. Thanks! My principle criterion is to improve consistency and accuracy compared to the AVX, so I'm looking for low PE with a consistent pattern that lends itself to correction via autoguiding. A secondary requirement is to allow me to use a heavier OTA - I currently use a Tak FC100F, which only weighs about 6kg with camera. I'd like something that can easily carry an 8" Newt, 9-10" Cat or 130-140mm f/7 refractor.M Portability is "nice to have" but not a pre-requisite; the mount will be used 99% of the time in my back garden and will stay set-up under a cover. Of the two choice, I am now leaning more the EM200 because of its simplicity and (hopefully) more consistent tracking accuracy compared to an EQ8.
  6. I'm looking at two used mounts with a view to upgrading my Celestron AVX mount for imaging. One is a Tak EM200 Temma2 of unknown vintage (but great condition) and the other a SkyWatcher EQ8 (Orion HDX110) about a year old. These are obviously two quite different mounts - the EQ8 is a heavy-weight semi-portable mount and the EM200 is an lighter more elegant (but older) design, and hopefully very well engineered. Is anyone using an EM200 for imaging, and are there any "gotchas" I should know about? Is interfacing it with a computer straightforward and does it support ASCOM or similar? What is tracking accuracy like? I understand it has no PEC - but maybe it's so good it doesn't need it! I don't need to the capacity of the EQ8 (a whopping 50kg/110lbs) for my 8" Newt, but I like the idea of something considerably more accurate than my AVX. Given the prices of both mounts are similar (used), which represents the better deal? My thoughts on the choice between EQ8 & EM200 are these: 1) The EM200 probably has the capacity I need - as long as I can mount a 10-11" SCT / 8" Newt / 5.5" Apo on it without overloading, it will be enough for any future scope I envisage. Opinion on an 11" SCT (e.g. C11) is divided - some say it will handle it for imaging, others say it's too much. 2) The EM200 seems to be an old design, and it may be harder to interface with modern equipment - although I understand it has ASCOM drivers. 3) The EQ8 would allow very large scopes to be mounted, so gives a few more options for the future. 4) The EQ8 is not really portable - maybe "transportable" is a better description. It would live in my garden semi-permanently. The EM200 is still field portable, which might encourage me to go dark sites more often. In any case I would keep the AVX as a light-weight imaging option & for visual use. 5) Bits of the Takahashi design may make imaging a little bit trickier (some manual selections required for tracking in the southern hemisphere), and no GoTo features without a computer (or at least SkyFi + tablet) 6) The mechanical accuracy of the EQ8 may be as good as or better than the EM200 (quoted +/-3" PE for the EQ8 vs +/-5" for the EM200, which has no PEC capability. I'm not sure what the reliability of these figures is in the real world. So it's quite hard to decide between them based on accuracy and price! Which would you choose? John.
  7. You can also compensate somewhat for the smaller image scale of the telescope by choosing a camera with smaller pixels. This increases the available resolution of the system - although it is often limited by atmospheric seeing. In effect, smaller pixels allow you the "blow up" the image to a larger size without losing detail, much like any photographic enlargement. As a rule of thumb, large DSOs can look pretty good at about 2"/pixel (or even larger), and small ones (or planets) often benefit from 0.25-0.5" / pixel. You may have heard of "under-sampling" and "over-sampling" - this refers to how much you deviate from an "ideal" resolution for the object. As with everything, it's all a juggling act - there is no point in having more resolution than your scope or the seeing will support, but if you have less, you are losing some information. For 2-dimensional images it's often stated that an image scale about one-third of the limit of seeing or your scope's resolution is ideal. So if you have a 4" scope (c. 1.1" resolution) and 3" seeing, then an image scale of 1"/pixel would be the lowest worth considering. A bigger scope may allow you to have a finer image scale, particuarly for planetary imaging where "lucking imaging" can help counter the limits of seeing. To answer you original question, yes, a shorter focal length (irrespective of aperture) will reduce the effect of angular tracking errors (or periodic error) of your mount. You will still need to use an auto-guider unless you have (a) very short focal length or (b) short exposures (< 30-60s) or (c) a very expensive (c. $10,000) mount. The added advantage is that by reducing for focal length you also reduce your focal ratio, making the whole system faster so you can choose to reduce the duration of your subs whilst getting the same signal. John
  8. I have a 200P (f/5) and the only difference between this an the PDS (apart from the 2-speed focuser) is the position of the primary mirror (AFAIK), which is supposed to make it easier to reach focus. I have no problem focussing my Canon EOS 100D using a t-ring to 2" nose-piece that goes straight into the focuser. I can reach focus with the focuser wound out about 10mm from its innermost position. I think many people would say an HEQ5 is fairly marginal for imaging with an 8" f/5, and would recommend an EQ6. It should be fine for visual and getting started in imaging, without necessarily expecting top-quality results. I'm doing exactly the same with my 200P and a Celestron AVX mount.
  9. I've been thinking about getting a larger reflector and had been considering a 10" Dobsonian. However, and opportunity to get a used 10" Newtonian on an EQ6 Goto mount has come up, and I'm looking at this. The OTA seems to be pretty much identical, so the choice is really down to the mount. I'm used to a GEM (Celestron AVX) with my 6" Maksutov, but I've never used one with a large Newtonian. I'm aware it will be a bit awkward at times (although the OTA has "slip rings" to allow tube rotation). The EQ6 will also be a lot heavier than a Dobsonian base, and will certainly require 2 trips for the mount alone (tripod/head + counterweights). The main advantage of the GEM is the GoTo and tracking capability, which not only makes observation easy (especially in my light-polluted home location), and also opens the door to astrophotography. I can also use the GEM for other 'scopes. The cost of the OTA + EQ6 (used) would be a couple of hundred dollars less than the cost of a GoTo Dob such as the Orion XT10g. The used scope also includes 4 EPs, a laser collimator and some camera adapters. So my choices are: 1) New 10" manual Dob 2) A New GoTo 10" Dob for about twice the price of the manual Dob 3) The used Newt+EQ6 Goto GEM for a bit less than the GoTo Dob. What would make most sense for mostly visual use, and mostly at home? I'm tending towards #3 because of the more "serious" GoTo mount that can used for other scopes, but am cautious about the size & weight of the whole setup.
  10. It sounds like you know what you are doing with your optical tests, but could the problem simply be a poor example of the telescope itself? I bought an ES ED80 CF Triplet a couple of weeks ago (which I think is what you have, although without the field flattener), and wasn't very impressed visually. It seemed to have an unacceptable amount of field curvature and poor edge contrast. The mechanical precision of the focuser rotation was also very poor, allowing easy miscollimation of the focuser to the optical axis. I returned it to the dealer. I know many people say good things about Explore Scientific, but my experience left me unimpressed with both optical and mechanical quality. Maybe I was just unlucky! I hope you have better fortune in fixing your issue.
  11. Hi, I've just bought an Explore Scientific ED80 Triplet Apo and have a problem with it. I understand that this type of scope is often used for imaging, and that extension tubes are often used to achieve focus using a camera (where a diagonal isn't used). In my case, I want to use it initially for visual observation, and have seen several videos, reviews etc. by people using it for this. The scope was supplied with a 1.25" diagonal (instead of a 2" one for the 2" focuser), and I've found that I can't reach focus with any eyepieces. With a 2" diagonal it will just about reach focus if I pull the eyepiece out of the holder 1-2cm - clearly not a satisfactory solution. I estimate that the back-focus distance is about 100mm behind the maximum extension of the focuser tube. The focuser travel seems short - only about 35mm according to the scale on the tube. Is what I am seeing normal? I am disappointed that I was sold a scope incapable of being focussed! (I understand that 2 focus extension tubes were once included, but the dealer told me he hadn't received them). Can anyone advise me whether what I'm seeing sounds reasonable, or whether the scope could have a defect in it focusing mechanism? Thanks for any help you can give! John.
  12. I have a mid-range 80mm APO spotting scope (Celestron Regal M2 80ED) as a "grab and go" set-up for quick views and as a (potential) travel scope. Whilst it's a great terrestrial scope, I've found it a bit limited for astronomical use for the following reasons: 1) 45 degree eyepiece is uncomfortable for high elevation targets 2) Lack of finder or easy method of fitting one to a tapered tube makes locating objects really hard 3) Changing eyepieces requires undoing a large locking nut, which often moves the scope from the target. 4) I have my doubts about the magnification ability of the 45 degree prism compared to a star diagonal (the image gets noticeably dim at 60x). My question is this: would I notice a significant improvement if I got a similarly sized astronomical telescope of equivalent optical quality? (e.g. F/6 80mm APO) The key requirements are optical quality and portability. This needs to be something that will fit in airline hand baggage, and can be carried assembled on a medium-duty tripod in one hand. You can obviously spend a lot on a small APO refractor - the TeleVues and Takahashis are beyond my budget, but I might stretch to the next tier (e.g. William Optics, Stellarvue, SkyWatcher Esprit) if there are *significant* improvements to be had over my current set-up. I'm not really looking at catadioptrics (I already have a 6" Maksutov), unless anyone can recommend something execeptional. Any ideas? Thanks! John.
  13. Nice setup! I like the look of the long dovetail on which both the scope and camera are attached. This looks like a really solid setup - far better than hanging a ton of adapters and cameras off the end of the eyepiece! Where did you get the dovetail from?
  14. Thanks for the replies. As luck would have it, I've just remembered that a family member has an older Canon 35mm film camera, and a couple of Canon EF lenses (50mm f1.4 and 28-80mm f4.0-5.6), which appear to work in my DSLR. I haven't seen any system that allows eyepiece project *with* the DSLR lens fitted, so I may try one of the digiscoping brackets, which would also work with my compact camera. I haven't found any option for connecting eyepiece to camera lens via coupling adapters, and this may not be possible due to strength limitations of the threads - hanging the entire DSLR + lens from the filter thread on the lens sounds risky. Putting all of this on the eyepiece thread sounds worse! I fear that as suggested, the 'scope's focal point is deep inside the telescope, so there's no hope of using it as prime-focus lens without some kind additional optics, such as a Barlow. I know that Celestron does make a 1.25" T-adapter that includes a 2X Barlow, so this might be worth experimenting with. Swarkovski make something similar for their modular scopes, but it costs over $550! At that price, I might just as well buy a dedicated small astro refractor, without the focussing problems. As you say, I could be on a slippery slope, but as Macbeth said "I am in blood stepped in so far that should I wade no more, Returning were as tedious as go o’er"
  15. Hi , I have a Celestron Regal 80mm ED spotting scope (f6) and am thinking of mounting it on my Celestron AVX EQ mount to try a bit of photography. What would be the best way to attach a camera to the scope? I have read quite a few posts on various bird-watching forums but still have a few questions. I have a good quality micro 4/3 fixed lens camera, and a Canon DSLR (body only at the moment). I have tried "eyepiece projection" using a T-ring and T-adapter to couple the DSLR body to the spotting scope eyepiece, and used the fixed-lens camera just held up to the eyepiece. Here are my observations from terrestrial photos: 1) DSLR body only through scope eyepiece: the magnified image fills the frame, but is very distorted on the edges of the frame. I would say that only about the centre 50% of the frame is usable. The downside is that the camera without the lens can't use the LCD screen as a viewfinder (so I have to use the optical viewfinder which is much less convenient). It also doesn't seem to do any through-the-lens metering because the camera thinks the aperture is f0.0, so exposures take a lot of guesswork. 2) Fixed lens camera through scope eyepiece: the image is nicely focussed, but seems to suffer a lot of vignetting - the diameter of the image circle is only about 50% the width of the frame. I have tried changing the zoom of both the camera and the scope, but it doesn't make much difference. Maybe I need to hold the camera much further away from the eyepiece? So, my questions: 1) Can I fit the DSLR body to the spotting scope *without* an eyepiece, and use it a telephoto lens, i.e. use the prime-focus method? I don't have the T-adapter for this yet, but my major concern is that the spotting scope won't have enough back-focus range to allow the image to focus on the camera's focal plane. Just holding up the camera body to the scope (without eyepiece) didn't get anywhere near focussed images. Are there any T-adapters that include optical elements to allow this? 2) Will the quality of prime focus be any better than the afocal (eyepiece projection) method? If not, should I just a lens for the DSLR and use the afocal method? 3) Are there any couplings that allow a camera *with its lens* to be screwed to an eyepiece, rather than using a "non-contact" coupling platform that just holds the camera behind the eyepiece? Presumably such a coupling would need to fit to the filter thread of the camera lens - would this be strong enough to support the camera? Thanks for any advice you can give me! John
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