Jump to content



  • Posts

  • Joined

  • Last visited


39 Excellent

Profile Information

  • Location

Recent Profile Visitors

The recent visitors block is disabled and is not being shown to other users.

  1. Thanks Oddsocks, I did wonder if that was the case. I'll remove the spacers used to give the 133mm as I don't use the reducer so backfocus shouldn't be a factor and get the 2 distances as close as I can to each other. Robin
  2. I have an Edge HD8 with the Celestron OAG and a QHY5 IIL guide camera. I have set the backfocus distance to as near as possible the recommended 5.25" (133mm) and have focus on the main camera. The problem is that try as I might I cannot achieve focus on the guide camera. Whether I set the camera to as far in as possible or as far out as possible and all points in between when pointed at something bright like the moon all I can get is a bright blur. Does anyone have a similar combination and if so, how far in / out do you have the guide camera set to achieve focus? TIA Robin
  3. I've just had to wipe drool from my keyboard, that is a thing of beauty. I want!
  4. We can argue semantics about the term resolution until we’re blue in the face, it doesn’t change a basic fact. An image is information, in a 2*2 square I can display 4 discrete bits of information, levels or colours. In a 1*1 square I can only display 1. When you reduce the scale of an image by 2 you reduce the amount of information in that image by a factor of 4. That cannot be argued.
  5. Wrong, imagine an object that covers a 2*2 square of pixels i.e. 4 pixels, reduce by 50% and your object covers a 1*1 square of pixels i.e. 1 pixel. You have reduced by 50% but dropped your resolution from 4 pixels to one. It is 4* faster because you have 1/4 of the resolution.
  6. I give up! By reducing you are optically binning your pixels. What is the difference between reducing 50% and binning 2*2? (apart from the obvious that there will be a bunch of sky you weren't interested in in the first example). By reducing 50% you are concentrating the light that would have been spread across 4 pixels into one. Result = 1 brighter pixel but less detail. I would rather do that in post processing than I would by adding more glass into the optical train. @Astro Mick, F-Ratio is a number which gives the relationship between aperture and magnification, nothing more. As your aperture is fixed by your telescope you can play with the magnification by reducing or barlowing until you find the right balance between the size of your target, it's magnitude (brightness) and the length of time you can comfortably spend on each sub. There is no magic formula for that, only trial and error. Concentrating on F-Ratio is a fools errand. So in the end the answer to your initial question is try it and see what works for you. What works for me is to start by deciding how I want to frame my image and adjusting the focal length to that, then adjusting the length of sub by the magnitude of the target. If that means 30 minute subs where I have to chuck some because of guiding glitches, planes, clouds etc. then so be it - the stars will be there tomorrow
  7. Simply put, the only thing that affects the amount of photons (light) that your telescope gathers is aperture. How that gathered light is distributed over your camera sensor is a function of focal length. F-Ratio is the ratio between the two of them, reducers or barlows affect the focal length they cannot affect the overall amount of light. The only reason for using a reducer is if you want to capture a larger area of the sky to properly frame your image, using one to make an object that could be properly framed at prime focus smaller so that it appears brighter is a waste of information that could be better used in post processing. Even simpler, if you don't have to use a reducer to fit your target on the sensor then don't use one.
  8. Or put another way your 0.8x reducer is actually reducing your image by 0.8 x 0.8 = 0.64 so your 20% less area is false. It's actually reduced by 34% which is why it's 34% shorter exposure required
  9. So where have these extra photons magically appeared from?
  10. Assuming that the object you wish to photograph fits on your ccd chip at prime focus then using a reducer is pointless. Your object will be brighter but will also be smaller and have less detail. You are effectively binning your pixels optically, you can do that with software later if you wish.
  11. I've just bought a Celestron C8 Edge HD for exactly the same reason. It arrived yesterday but looking at the forecast for the next week or so I don't think It's going to get used in anger for a while
  12. Close enough! It is / will be a micro focuser (think FLI). The center of the barrel is threaded M50*07 there is a 2:1 ratio between the pulleys so with a 200 step motor it gives 1.75 microns/step. The central pulley is captive between two plates (not in the drawing yet) and the barrel is free to move +/- 5mm giving 10mm total travel. The bearings prevent the barrel from rotating and also prevent sag. It will also be anti-backlash sprung. When I have enough bits made I'll make a thread on it.
  13. @Chris - No but good guess and you're in the right ballpark @SnakeyJ - 72 There is quite a lot more to put into the model and an awful lot more machining before it's finished. A Clue, a few parts assembled, finished with in the lathe but still have to go through the mill to have flats machined....
  14. The more I play with it the more impressed I am. It's actually a slightly cut down version of SpaceClaim which is a professional CAD program aimed at mech engineering. RS also do a program aimed at circuit design which is supposed to be better than Eagle which I currently use but is a bit dated now. I haven't gotten around to downloading and trying yet but will do so soon. The model is growing, answers on a postcard...
  15. As I do, RS have made available a free (registry required) mechanical design package which as far as I'm concerned knocks the socks off SketchUp. Want screw threads? No problem! In addition it will load and save to SketchUp files so you won't have to ditch all you old models. This is a couple of pieces from something I'm building at the moment (Yes, it's astronomy related, I'll reveal all when I've finished building it ). You can get it from http://www.designspark.com/ Robin
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.