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About rl

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    pershore, worcestershire, uk
  1. rl

    It does what it says on the tin.

    I bought mine in a fit of pique after having messed up my polar alignment for the umpteenth time because the spirit level in my AZEQ6 was playing up. I expected a lot of buyer's regret but am happy to report being very satisfied. It is undoubtedly a massive wedge to fork out for a posh webcam and some software but it does a brilliant job. The value is all in knowing you can trust the polar alignment and the improved guiding. There are cheaper ways of doing it with platesolving programs and finders but the overall package is hard to beat. RL
  2. From personal experience the ZS61 + flattener + Canon DSLR combo gives good results on the star adventurer mount; it can be made to balance correctly without having the counterweight all the way to the end; I add an extra piece of metal to allow the mounting point to be more towards the camera or it will be a bit bottom-heavy in dec but it's not the end of the world without this refinement. The ZS61 is a very well-made scope with exellent optics but a bit expensive for a 60mm. I believe there are now some ZS61 clones out there which might be a bit cheaper. As already stated, you need the flattener to make use of an APSC-size sensor but also the focal length is reduced (to 245mm from memory?) which helps mask the guiding errors. A very worthwhile addition is a red-dot finder which clips on to the hotshoe on top of the camera. I didn't realise these thinks existed until I got feedback on the review; since the ZS61 has no finder option it's worth it's weight in gold for centering targets quickly. Regards to all, RL
  3. rl

    NGC 891

    Nice work, it's a beautiful galaxy. I always find this one difficult visually in a 12" so I'm not surprised it took so much data. RL
  4. rl

    Astrophotography in Southern Africa

    Some of my most memorable astro nights have been at SAAO at Sutherland in the Karoo, South Africa, which is several thousand feet up. With good dark adaption, you could virtually read a newspaper by the starlight and Venus would throw a visible shadow on white paper. Many Messier objects normally considered binocular targets were naked-eye..there was so much om view it was quite disorientating..it was like learning the sky all over again. Having everything upside-down didn't help. I took a lot of wide field shots with a home-made clockwork drive and a 135mm lens on Ektachrome 100..if I can get them digitized I will put them up here. RL
  5. rl

    Coma Corrector for Visual Use

    I've got the Explore Scientific HR coma corrector in my 8" f/4.5 scope and it works really well. It gives a flat image plane as well as coma correction which means that the overall view is very good indeed with eyepieces also having a matching focal plane, which includes all the more expensive Televue offerings. The view of the moon at *200 with everything in perfect focus all at once is seriously impressive. Downsides; the screw thread adjustment can be a bit of a pain to set in a hurry. The Paracorr tuneable top idea works better in practise. But to be honest it is not that big a deal since there is a compromise setting that works well enough with most of my eyepieces, but it might bug the perfectionists. Also, I find the view through the HR a bit "warm" in the same way as some criticise Nagler eyepieces. But on balance it's a really good bit of kit. If buying secondhand make sure you get the two camera adapters with it. Regards. RL
  6. No-one's said it yet so I'm going to.......the mount is probably the most important part of the deal when starting AP. I'd get at least the HEQ5. By the time you'be added a camera, guidescope, filter wheel, guide camera, focal reducer, flip mirror, dew heaters and any other toys that take your fancy it's very easy to get up to silly weights even with a small scope. You may have to add weight unintentionally in the form of a dovetail/ bracket of some description just to move the scope forward enough to balance with all the accessories attached. A smaller mount can work if there is no wind but you need something that is rock-solid enough to work whenever the sky is clear. The HEQ5 is not without its faults but most of them are well documented by now and have fixes. If you saved a lot of cash buying the Altair scope you might like to invest some of the change in the newer AZ-EQ5 which solves a lot of the original HEQ5 issues The truth is that for most of us lesser mortals the learning curve is steep and painful before getting good results reliably, and guiding issues (along with processing) tend to be a biggie. I'd take advantage of other people's pain and go with the tried and tested route.
  7. rl

    Different Eyes = Different Focus

    If you're short-sighted you will have to rack in the focuser a bit....I believe you also get a little more magnification than the usual simple formula predicts (Fobj/Feyepiece) which assumes the eye is focussed at infinity.
  8. rl

    Guide scope upgrade

    Taking up Olly's suggestion on the maths... Main scope resolution at 600mm f/l = 1 arcsec per pixel for ASI290 with 2.9 micron pixels Guide scope resolution at 200mm f/l = 3.8 arcsec per pixel for ASI120 with 3.75 micronpixels Autoguiders will generally control to 1/10 pixel without too much bother, say 0.4 pixels so if it's all working ok you should be well in. If not, are there other issues like backlash or wind, polar alignment? What does PHD2 report as the guiding error? I have a very similar setup (ASI120MM 60mm guidescope) and generally get better than 0.5 arcsec RMS on a wind-free night. Bear in mind that the Rayleigh limit for an 80mm in green light is only about 1.5 arcsec. If your RMS error were 1 arcsec it would'nt be a disaster.
  9. rl

    Guide scope upgrade

    It sounds like your skies are a lot better than mine...is light pollution really a problem for guiding? Can't you just up the guide cam exposure time? Try 2 or 5 sec..should still be ok if your polar alignment is up to scratch.
  10. rl

    Guide scope upgrade

    If you've already got a filter then it's worth a try. One practical test is worth a lot of posts here, but I would not expect much benefit. It depends heavily on the nature of your light pollution. A star shows a continuous spectrum rather than a line emission spectrum. The filter is going to cut down the star's light along with the light pollution. If your light pollution is old-fashioned sodium light with a line spectrum there might well be an improvement but most modern streetlights are unsportingly white in their output, which will leave the optical SNR little changed but the reduced photon count will only serve to emphasize the read noise on the camera.
  11. rl

    Guide scope upgrade

    I regularly guide with stars with a SNR less than 20 without too many issues, with a 2 second exposure in PHD. The increased aperture will definately increase your SNR in the ratio of 64/25 (areas) which is good, assuming light pollution is not the limiting factor. In addition , if the ST80 at f/5 is slower than your current 50mm guidescope (f/4?) the light pollution will be reduced improving the SNR. On the other hand, doubling the focal length means the guide setup will see only 1/4 of the sky area with the same guide camera which decreases your chances of having a bright guide star in the field in the first place..... Swings and roundabouts I'm afraid!
  12. rl

    Guide scope upgrade

    It's a tradeoff between extra weight on the mount against the advantages of brighter stars and a longer focal length. If you've already got enough focal length on your existing guidescope compared to your main scope to give accurate tracking, and you've never had problems getting a bright enough guide star, then it's probably not worth it. I think you're probably looking at 200mm and 600mm (without reducer) respectively which should work ok for all normal camera pixel size like the ASI120. After all , you only need 1 star to guide on and that's not usually an issue with a separate guidescope. If the pixel size on your main camera were very small compared to your main camera then you could make a case for greater guiding accuracy but looking at your kit list I don't think this is the case here.. What's your expected total weight? And will it require an extra counterweight because the scope/ guidescope is too far from the axis? The focuser on the ST80 might want some work to make it stable enough. A lot of the 50mm finders dispense with the r+p focuser using the threads on the objective to set focus which is more stable in my experience. I like big guidescopes if they can double up as finders with a flip mirror assuming the focuser has enough in-travel, but that's a different argument! An old ETX60/70/80 OTA is good and they're very light. Not sure if that helps any... RL
  13. rl

    Defraction Limited.

    Enter the legendary Ralf Ottow whose 12.5 inch Newt has a watercooled primary and sensors inside the mirror itself and the tube so that observation is deferred till all sensors are in agreement and the cooling can be disabled. Olly Too many cables!
  14. rl

    Defraction Limited.

    Sorry, I did not answer the real question above. At 550 nm the actual error in nanometers is a slightly larger fraction of a wavelength...hence the accuracy of the mirror in wavelengths is lower. The accuracy of the measurement, not the mirror, is a different thing and depends on the quality of the light source and the mechanics of the jig.
  15. rl

    Defraction Limited.

    Commercial interferometers used for optics testing are often built to a standard in excess of that required for testing telescope mirrors in the optical range....they multipurpose test instruments used to test other things as well. In reality you do not need this level of numerical accuracy for our purposes but that's what the kit does. For general testing of astro mirrors a decent LED source would be good enough. Coloured dye filters will do at a pinch if all you need to know is the difference between a 1/4 and 1/10 mirror. I think my main point is..beware of specmanship. Of course the elephant in the room is atmospheric seeing in the UK which makes most of this moot on most nights...

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