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davidc135

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About davidc135

  • Rank
    Nebula

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  • Interests
    Astronomy and ATM, science and natural history, trad archery
  • Location
    Hay on Wye
  1. Very interesting. Can you give the diameter and length of the assembly? The corrector (purple at front) looks flat. I'm assuming there is a secondary mirror behind the metal plate in the corrector. Does it seem flat or curved? The camera looks quite tall which suggests that the focal surface is internal. Following on from vlaiv's comments you could attach some paper flat to the end of a tube just narrow enough to be pushed into the back of the camera. A distant daytime view might do. The image will be bright. If the image is internal, that is. There are some components inside the tube. David
  2. Skymax 150 star test

    Hi Dan It could be that the zone is no more than the expression of the HSA intrinsic as you say in the design. All errors together are probably less than 1/4 wave although it would be fun to quantify if there was the kit. David
  3. Skymax 150 star test

    Good images. They show the difference in brightness of rings inside and outside of focus that you mentioned due (I think) to a high zone near the edge. Or even a slightly turned edge? Mel Bartels quotes John Dobson who recommended working on a mirror's zones that were bright outside of focus. Have you put this into aberrator? Looks like your original assessment of a small amount of LSA and HSA was right. There seems to be a heck of a lot to star testing! Has Suiter's book been helpful? I must get a copy. David
  4. Skymax 150 star test

    Looks to have fine optics. 1/8 wave overcorrected or better from that graph. Congrats. Seems there was around 1/8 wave induced SA(as well) in the first test. David
  5. What about an Evostar 72ED Ota for £265.00. Wide fields and semi apo image quality. Convenient for someone who may find a bigger scope gets in the way of the out there under the stars experience. Or a Meade ETX80 pretty well all in with goto for £299.00. But false colour? David
  6. I think it's good news, Luke. If it's collimation that can be fixed and now the chances are better that my pessimism was wrong! Looking forward to more news. David
  7. Skymax 150 star test

    There's a recent thread on CN in the cats and casses section: 'star test central obstruction size' including a graph for judging sa that may be useful. Says 10 waves defocus recommended for obstructed optics. Your lens likely to be a good one. David
  8. Good luck with everything. David
  9. Yes, mea culpa! But I've read them all since and formed my opinion. It seemed to me that your lengthy and rigorous assessment of the Mak was damning but that it would be unfair for all Skymax 150s to be so judged, at least as lunar scopes. Well, right or wrong, time will tell. ASSA, on a more positive note, insulating the scope was mentioned earlier. There's a thread on the cats and casses subforum of Cloudy Nights where it is claimed, with evidence, that cooling and tube current issues can be quickly, virtually eliminated in scts and Maks. Very impressive stuff. David
  10. Yes, that is what I meant. I'm sorry to sound negative and best wishes for your observing experience. David
  11. Isn't the disappointing truth likely to be that this is a poor example? Hopefully ASSA will find out differently and the Mak will come into its own. David
  12. Skymax 150 star test

    Looking forward to the results. I've been reading a few other threads on star testing and some of them mention 10 waves or more of defocus being more useful. David
  13. Skymax 150 star test

    Hi Dan I may have misunderstood you but the scope's focuser should only be used once in the star test; to establish position of focus. At this position the ep is kept proud of the diagonal or drawtube by one of the shims. When this is removed the ep can be pushed home and is now in front of focus by a known amount. Similarly, using both shims for beyond focus. A £1 coin is around 3mm or a bit less. I don't have one to hand but will measure it asap. In the aberrator example above there are figures for waves out of focus as well as actual distance defocus. In this case .79 waves corresponds to .5mm defocus or .158mm per 1/4 wave. As 4 waves is often recommended, that comes out at 2.5mm. The £1 coins would be a bit more but I don't think it matters so long as they are the same both sides. However you'll need the right figure to feed into the program. This .158mm number is true for an F/12 setup and varies with the square of the F ratio. It may well turn out that your method of filling the screen is fine and establishes a ratio of shadows but the above is going by the book. The ratio looks to be around 67% which, unless it's explained by the source distance, isn't great. Probably is the source distance. A ball bearing in the sun or with a torch is also handy but I'd initially do a high power visual test using the Pole star making sure the scope has cooled down. David
  14. Skymax 150 star test

    This simulator looks fun. As Peter Drew says the artificial star may be too close and that could be causing the significant overcorrection shown although I'm certainly no expert. Did you make sure that the intra and extra focal positions were the same distance either side of focus? For a visual assessment I'd first use the focuser to establish focus with one 2.5mm shim between ep and holder. Removing that shim gives the intra position and having 2x 2.5mm shims gives the extra., these two being both 4 waves from focus (I think). Measure relative sizes of obstruction shadow. Then increase the 'star' distance and repeat to check if proximity is the culprit. There is a table which correlates ratio of the shadow diameters to spherical aberration which I've forgotten but .1 wave looks optimistic. How does the Airy disc look? David
  15. I've come to your interesting account at the last minute and so have probably missed the boat. I know you said earlier that testing isn't your cup of tea but might it not be worthwhile doing a check on Polaris with the 150 Mak? I haven't read all the posts here and if this has been covered already let me apologise. It just seems likely that there may be a problem with your example and, if so, a quick look at a star; at focus, inside and outside focus would reveal any obvious, major faults. The Maks obstruction makes this easier. One basic test is to compare the secondary shadow an equal distance inside and outside focus as well as the Airy disc and rings at focus. But again, perhaps you have done this. Surely, it should snap to focus. It should stand high powers and be nearly a 180 Mak and not be so easily bested by a (very good) small refractor. David P.S Just noticed spaceman-spiff's thread on an artificial star test
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