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About wolfpaw

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    Western France (16)
  1. Thank you. I suspect my problems with the telescope aren't really relevant to the forum any longer so I'll say goodbye, but you've always been helpful and friendly and it has been appreciated! Clear skies and best wishes Rich
  2. So you don't think people considering buying an f/4.7 telescope have the right to know that they can possibly expect to see over 50% coma through the eyepiece? I'm sorry you feel that I 'hijack' every thread about it but if I don't mention this issue with coma then no-one else will. As I've said, if someone had told me that that amount of coma was 'normal' and 'expected' then I wouldn't have bought the telescope in the first place. I did a lot of research before getting my Skywatcher 250PX and the only phrase that kept popping up re. coma was 'edge of the field'. It was not 'half of the field'. I just feel that people have a right to know what they're getting and the potential issues that can arise.
  3. I am too. Theoretically though, I think I'm right in saying that if you stand two Skywatcher 250s next to each other and used the same eyepiece then they should show the same amount of coma. Maybe the optics on your OOUK scope are just vastly superior to the stuff churned out by Synta.
  4. Well the way I see it there are three options: 1) The telescope is faulty. 2) The telescope hasn't been collimated properly despite showing a good image in the centre of the FoV. 3) The telescope's 50%+ coma is entirely normal in an f/4.7 telescope and I'm just one of handful of people who can see it. I've spent hours and hours collimating my telescope, trying to get it a little better each time, so I find it hard to believe that the collimation is now so far out that it's producing such extensive aberration. And I don't believe 50%+ apparent coma is remotely normal (or the 80% coma I was getting with the MV16mm). So that leaves me with the 'faulty telescope' option which will be almost impossible to prove to the retailer without taking it on an eight-hour round trip, especially as they've already stated that the only 'fault' with it can be miscollimation. The bottom line is that I'm lumbered with a 500 euro telescope that I don't use and, until I can get it back to the UK to flog on eBay, I'm stuck with it.
  5. I appreciate that you think a coma corrector would improve the telescope but I'm not prepared to try and patch up a potentially faulty telescope with a further outlay on money. No-one would. If the telescope's faulty (as has been suggested by a number of people with a similar set-up) then why would a coma corrector work as it would in a non-faulty telescope? I'd be more prepared to try a CC if I hadn't received so many contradictory opinions on here. I must disagree with this. There is very little discussion on the internet of people using an f/4.7 and seeing 50%+ coma (let alone the 80%+ coma I was getting with the MV16mm on Jupiter's moons). Nearly every single discussion about coma in an f/4.7 telescope refers to it as appearing at the edge of the field of view, not over halfway towards the centre. And most of it also relates to astrophotography. I have no spent weeks and weeks reading about people's experiences of coma online and almost none of them matches my own. As for miscollimation being the cause, would that still hold true if the centre of the field of view was perfectly sharp? I thought miscollimation resulted in poor images across the field of view, not good in the centre and then quickly getting worse. Again, here's the view through the focuser: http://oi61.tinypic.com/szd5sl.jpg When I posted this pic up in another thread the general response was that the collimation looked fine.
  6. If the apparent sweet spot was really so small then everyone with a fast Newtonian for visual use would use a coma corrector. AFAIK, they are primarily used by people doing astrophotography. This is my big issue with the claim that the coma I'm seeing in my telescope is remotely normal. I didn't measure it exactly when I took the mirror out. I tried but couldn't find a way of stretching the measure across the face of the mirror without the tape touching the mirror's surface. It seemed reasonably centralised but I couldn't say for sure.
  7. So, again, how would miscollimation increase the amount of coma seen if the centre of the field of view is clear and sharp? Also, as has been mentioned by others, if 50%+ apparent coma was normal in an f/4.7 telescope then why is there so little discussion of it on the internet and why is coma only referred to as affecting the "edge of the field" rather than "half or more of the field"?
  8. I don't personally agree with the miscollimation theory. What would, specifically, have to be wrong to make "coma much more visible" if the centre of the field of view is already sharp?
  9. Coma might be a feature of the optical design but I believe the jury is still out on the extent to which it is visible.
  10. Well, with respect, the general consensus wasn't that there was something wrong with the telescope. The general consensus was that it was coma. When I asked if 50% coma was expected in an f/4.7 telescope I received the reply: "pretty much, unless you use a coma corrector". You can read the exchange here: http://stargazerslounge.com/topic/207286-primary-mirror-defect/?p=2245869 I was told on another forum that 80% was expected as well. Either way, it seems strange to read that there's "something wrong" with the telescope in one paragraph and then be advised to fork out for a coma corrector in the following paragraph. Either there's something wrong with the telescope or there isn't. I've been told that there is and that there isn't (sometimes by the same person). As you said, if coma was as bad generally as it is in my scope then the forums would be full of people complaining, and yet they're not. But apparently 50%+ coma is perfectly normal. As for the EU legislation, it's a bit of a joke IMO. I've contacted the company from whom I ordered the telescope here in France. After a rather protracted exchange of emails they now believe this 'fault' with the telescope can only be a product of a collimation error. They want me to make an eight-hour round trip to Toulouse so they can check it out. There is zero chance of that happening. Caveat emptor indeed, especially when buying a Skywatcher Skyliner 250PX.
  11. This might be true of large apparent objects but not with smaller ones. For example, the last time I used my f/4.7 telescope was to look for the Eskimo planetary nebula. I was expecting it to be very small, almost like an out-of-focus star and so I put in the 24mm 68 degree eyepiece and tried finding it. Unfortunately, with 50% of the field of view obscured by coma, it was impossible to work out what was a faint planetary and what was just a coma-affected star. The only option was to being every part of the field of view into the central 50% of the field of view. Scanning the entire field of view simply wasn't possible because of the aberration. I gave up after a while and haven't used the telescope since.
  12. This does appear to be true, although I'm surprised so many people are happy with half of the field of view consisting of out-of-focus stars.
  13. Hmm...I would disagree with that, tbh. If coma is visible across 50% to 80% of the field of view when using a fast Newtonian and wide angle, low power eyepieces for visual observation then a coma corrector becomes something of a necessity. 'Edge of field' has nothing to do with it in my experience. As I get significant and noticeable coma in my f/4.7 I suspect neptune1 will experience even more aberration in an f/4. In fact it must be a mathematical certainty that f/4 will show a lot more coma than f/4.7.
  14. i) Yes, a Skyliner 250PX means greater expenditure in the future, whether it's a coma corrector or more expensive eyepieces. You can get stuff secondhand, which makes it cheaper of course, but the bottom line is that the desirable additions for the 250PX will cost more than in a slower telescope. ii) I've never used a Heritage 100P but I can't imagine it would be good for galaxies. I struggle to see any detail in galaxies with the 250PX! A dark dust lane with averted vision is probably the best I can hope for. What about one of the smaller Skyliner dobs like the 150P? (again, I've not used one so can't really comment).
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