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About rl

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    Star Forming

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    pershore, worcestershire, uk
  1. John...You have taken the exact phrase out of my brain that I was going to post!
  2. Sadly a lot of these older measurements have to be taken with a large pinch of salt. In many cases the measurement technique would not have been capable of measuring consistently to 1/10 wave. And you have to specify the wavelength of the test...and the mirror has to be realistically supported to avoid sag of 1/10 wave...A Foucault check with the mirror propped up on a few books in a room with air currents just won't cut it. I've got an 14" AE optics mirror by Jim Hysom made in 1977 with the documentation quoting tested to 1/10 wave. I've had it checked on a Zygo and it comes out at about 1/4 wave at 632nm. It's a very good mirror in general use because the atmosphere is the limiting factor, but 1/10 wave it ain't. Unqualified numbers like this without explanations are a salesman's dream. The Strehl number tells you more about the mirror quality but for some reason the wavelength error has stuck as the criterion. Beware of knaves setting traps for fools..ot at least the uninitiated.
  3. I doubt if an Off-Axis-Guider is the solution; you will need an extra guide camera plus a PC to control it. And there will be spacing issues to worry about. The dedicated ZS61 flattener comes with the correct spacing as standard which tends to rule out an OAG unless it goes in front; this just changes one spacing issue for possibly an in-focus travel issue. Is the real problem poor polar alignment or gear cutting errors? Having owned this rig myself I'd strongly suspect the former, especially at the rather short focal length in use. The flattener will reduce the focal length by 25% which makes trailing a bit better cosmetically. Are you using the polarscope for PA alignment? The balance needs to be slightly heavy on one side just to take out any backlash in the gears. Are you using a PC to work the camera or just using it native? I'd look at using the camera and main scope to set polar alignment with one of the plate-solving methods...some camera control programs come with this feature as standard. It's very effective. Sharp Cap Pro? It would keep both weight and budget down. I found I could go for about 2 mins with most of the subs useable just by moderately careful alignment on the polarscope.
  4. That should be fine. You might well find less power is enough but I know how bad dew can be in the tropics... The resistors will probably be ok but might be a little bit close to their maximum wattage. Half-watt parts are available and not much bigger physically.
  5. Possible extra issues starting AP with a big newt. Collimation. Not just the usual visual collimation. The focal plane should be perfectly square to the camera sensor and not tilted. The problem gets more difficult with faster scopes because the focus blurs more quickly as you go away from the focal plane. . Some focusers allow you to set the "squareness". You will have to guide. OAG usually necessary to handle differential flexure, but typically harder to adjust and limited choice of guide stars compared to a guide scope. Mirror flop during meridian flips. Your OAG will help a lot in this regard taking out slow changes but it won't cope with sudden movement of the main mirror Focus tends to change more with temperature. Not specifically a Newt issue but small refractors seem to do better in this regard. Pricier Newts come with carbon fibre tubes for this reason. Big tubes are more susceptible to wind. Not as bad as a 6" f/10 refractor though... The longer focal length makes finding targets more difficult if the mount alignment isn't on the money. The big sensor in a D300 will help here. None of it is particularly difficult to get right once you know about the potential issue..it's just more to go wrong when you're coping with learning all the other stuff. Best of luck..let us know how you get on...there is always plenty of help on this site.
  6. I've got the Skywatcher f/4 aplanatic coma corrector which I think is the same as yours from the description. Baader do a useful variable spacer #2956929 which covers 20-29mm and works fine on my OSC camera...assuming your camera focal plane is the standard 18mm deep, and you're not having to squeeze in a filter wheel as well. The OAG is 11mm thick so the spacewr is set to 55-18-11 = 26mm. I think they do other ones as well. Just tweak for best results.
  7. The learning curve for astrophotography is steep, painful and expensive! Personally I'd get the 200PDS and keep the load lighter on the mount,and put the difference in price towards the extra bits you will need. You will also need to buy an off-axis guider setup and guidecam, and a coma corrector. The spacings are a bit critical so do some research before randomly choosing the bits hoping they will all bolt together and just work. Many good shots have been taken with the 300D...mainly 20 years ago when it was state-of-the-art. You might want to find a SLR with live view facility which makes focusing much less of a pain. I think the 450D is the earliest with this option. Don't forget the usual way in to AP is with a short focus 80mm refractor for lots of good reasons. You can just dive in with a big Newt but you will hit lots of separate issues to be solved all at once. But it can be done...
  8. Just had to look that one up. That's a big boy's big budget coma corrector. It's almost insulting it to stick with 62 degrees...Is it worth looking at a Delos or Ethos?
  9. Can't really go wrong with either. Pentax has wider AFOV, is less bad value for money. Delite has possibly better edge correction in fast scopes and flat field plane which may be a plus depending on what coma corrector you're using, if any, at this high magnification. I like the Delites very much, especially the lockable eye guard, but in this case I'd take all the AFOV I could get just because it's a dob at high magnification. Although I guess it depends how much of that field is still good at F/4......John is probably the oracle on this one.
  10. I've found the statement to be essentially true. Polar alignment is good to better than an arcminute if done with care. On my AZ-EQ6 I simply align on whatever star is easily available near the celestial equator and after that virtually all objects in the same E/W hemisphere are on my camera somewhere even if it's not exactly in the middle....I'm using a QHY183 at 900mm focal length which is approximately a degree across. The only time things sometimes go wrong is after a meridian flip. Stuff moves in a Newtonian. I bought mine in a fit of frustration when I could not get the mount to align properly, and fully expected a bunch of buyers' regret the following morning. It's certainly very expensive for what is basically a webcam and a bit of code, but I've found the results far more reliable than using the handset and polarscope. The software has the nice characteristic of doing one thing and one thing only, and doing it well, which makes it comparatively bulletproof. The few crashes I've had have been down to too many other things on the same USB port. Mine won't be on ABS any time soon....but anyone reading this thinking about buying one might do well to investigate the finder/ guidecam/ sharpcap option before splashing the cash. Having said that. it is nice to have it set up and ready to go without reassembling the finder and realigning all the time. First world problems....
  11. It's worth keeping this in context; the star in question is Deneb which is 1st magnitude and very blue..the B-V colour index is +0.1 all of which means a lot of blue light. It's a tough test for any doublet system when you're trying to bring out faint nebulosity.
  12. Cooling the camera allows longer exposures because the dark counts drop by half for every 7-10C cooling. But the maximum capacity of the pixels remains unchanged.....if a pixel saturates out at 20,000 captured photons at 20C it will still saturate at 20,000 photons even at -20C. Once a pixel is full that's it....you're only option is to allow in less light.
  13. I guess this is the reason people invented triplets...the colour correction is better, especially for photography. I'd try halving the sub time just to see if it's some sort of saturation issue on the camera. A luminance filter that cuts off sharply in the violet might help.
  14. That's a nice shot seen in context. Assuming the Bahtinov mask is matched to the scope then the focussing should be spot-on..in which case then yes, you're seeing residual CA, which is a little bit strange because it's not the fastest 80mm doublet out there. The slower ones usually have CA better controlled. Is yours the original Starwave 80ED? I note they now advertise a Starwave 80ED-R which claims "better colour correction in the blue regions of the spectrum" which probably tells you something. https://www.altairastro.com/starwave-80ed-r-ed-doublet-refractor-telescope-466-p.asp
  15. Difficult to say from a screenshot, but that looks like over-exposure to me. You might have some focussing isssues as well. Is the image a crop or the whole frame? What camera and exposure time were you using? What is the target? Were you using any filters? An ED scope with FPL53 is unlikely to have objectionable chromatic aberration unless there is something seriously wrong with it. Some doublets do show minor CA occasionally referred to as blue bloat..but that looks a bit excessive!
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