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sixela

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Everything posted by sixela

  1. The barlow magnifies more in that position and the effective f/ratio of the system (scope+barlow) is increased dramatically, and that usually lowers eyepiece astigmatism significantly. Also, you're looking at a much more narrow field in which the LX90's field curvature is much less bothersome (to reduce the softness without the barlow, try to focus stars that are somewhat closer to the edge, and you might be surprised at how sharp the centre still is: your eye can accommodate for a different focus much easier in one direction than in the other).
  2. There's no need for them. That's something else than saying there's no benefit in them (especially if you have a fairly sizeable collection of simple ortho, Pentax XO and TMB Supermono eyepieces).
  3. Watch out: the Paracorr actually requires a bit of focuser in-travel, so count on using roughly 15mm less out-travel (it varies, because if the visual Paracorr tunable top is set correctly the Paracorr, unlike your eyepieces, always ends up in the same spot.) The Paracorr itself is pretty long. Can you install a 2" parfocalisation ring on its barrel so that you can just reach focus with the original focuser racked out? Try to see if you can make the partially inserted Paracorr work without extension tube (of course, ensure there's enough barrel left inserted for it to be stable!) You might also be able to win a centimetre or so by moving the primary forward on its collimation bolts (assuming the springs are firm enough not to render collimation less stable).
  4. a) things never look like they do on pictures, because you eye isn't sensitive enough. For them to look good (but different), dark adaptation, averted vision, experience and first and foremost a dark moonless sky (with little light pollution) are very important.
  5. If you are the DIY type, it's also not impossible to adapt a knob with Vernier caliper yourself to one of the two existing knobs. I did that with a few bits'n'pieces and a knob from Mentor Components. Here's one from someone who installed one after I did (I had two more components because the shaft diameters didn't match):
  6. The bearings won't work that way. The solution is to get an equatorial platform instead you can just about see my Tom O. platform under my scope in my avatar), but these are usually not very precise and can't be guided (unless you take one of Tom Osipowsky's dual axis platforms with ST4 guiding port, but they cost a bundle).
  7. It's the same thing. The effective focal length relates the entry angle for a light bundle and where it maps on the focal plane, so it directly (together with the focal length of an eyepiece) determines magnification. Yes. The catch is that 2" wide field eyepieces are usually especially recommended for the lowest power eyepieces, and a barlow by definition won't save you there (you can't replace a 31mm ES 82° eyepiece with a 4" barlow and a 4" 62mm 82° eyepiece). As great_bear also correctly points out, a barlow also won't correct problems present at the original focal plane of a scope indirectly related to f/ratio on a given design (a Newtonian f/5 scope with a barlow still has the coma of an f/5 Newtonian, an achromatic refractor short tube with a barlow still has the chromatic aberration of a short tube, etc.).
  8. It's the same thing. The effective focal length relates the entry angle for a light bundle and where it maps on the focal plane, so it directly (together with the focal length of an eyepiece) determines magnification. Yes. The catch is that 2" wide field eyepieces are usually especially recommended for the lowest power eyepieces, and a barlow by definition won't save you there (you can't replace a 31mm ES 82° eyepiece with a 4" barlow and a 4" 62mm 82° eyepiece).
  9. For something radically different: Sumerian Optics [Watch out, the price at the bottom of that page do not include optics and a focuser, see Sumerian Optics for full price list] Mind you, it's certainly not grab and go like a Starblast or small refractor on Alt/Az mount is, but it's eminently portable.
  10. To be honest, if you begin and you want to buy one filter I'd consider a narrow UHC filter (Lumicon, Omega Optical NPB, Orion Ultrablock) first. It isn't usable on all objects but it helps more dramatically on emission nebulae and you'll keep on using it even at dark skies. If you want something for more types of objects, I'd be tempted to pick a Lumicon Deepsky, a Baader UHC-S or an Omega Optical GCE; the Neodymium Baader filter is rather poor at suppressing light pollution and keeping the object intact (I still have one, but that's more as a planetary and moon filter).
  11. You do need some spare focuser in-travel, though (but less than the extension is long, fortunately).
  12. There's no reason to think Powermates are "better" than the plain TV barlows unless the specific features of the telecentric telextender are relevant (small insertion depth, relative independence of magnification factor and Powermate/eyepiece spacing, unchanged eye relief even for longer focal length eyepieces, lack of vignetting in some rare eyepieces, etc.). In fact, for planetary observation, I tend to prefer the 2x TV barlow by quite a margin (and I put my money where my mouth is and sold my 2.5x Powermate).
  13. A barlow and a simple eyepiece usually has "less glass" than many more complex eyepieces (for wider fields or for long eye relief). In fact, many eyepieces have a built-in lens group at the field end that functions pretty much like a barlow.
  14. Not really, but it doesn't have a tube, so its aspect ratio is never too narrow. If a collimation tool is too narrow, then it's possible that even if you rack the focuser in completely you can't see the whole of the primary reflection (and if you rack the focuser out, the secondary may clip the reflection of the primary before you can actually see all of it).
  15. No, because we don't know if the camera was centred, and if it wasn't, then the effects of that parallax dominates all the "errors" we see. And we also don't know what collimation tools you have, you haven't said whether there is a center spot on the primary (despite my question), etc. so we can't tell you how to fix it either. As I said, the one error we can see (a rotational error of the secondary) is the least relevant one, so I'd rather not make you chase red herrings if there are big fat tunas around. If you want to know what it has to look like, look up "Jason D"'s avatar. Or this thread: http://stargazerslounge.com/equipment-help/121989-centring-secondary.html
  16. If I were you, though, I'd make sure that you are indeed resolution limited with your current scope (i.e. that you're not limited by the thermal behaviour of the scope, your ability to focus, the seeing, your processing skills, how you configure the webcam etc.). If your photos really are fuzzy even when taking pictures at f/20-f/25 and a webcam sensor, then your scope is not the issue. A bigger scope is something you get to get larger image scale, not less fuzzy pictures. If you aren't imaging at the current limits of your scope, then taking a bigger scope is just going to make it more frustrating.
  17. They'll become rarer at first; there's a big difference between the moon size at perigee and apogee.
  18. If it has an M54 thread inside the focuser like many Synta scopes: Adaptor M54 to T2 - low profile or anything similar (I'm just quoting the TS website because I can locate things easily there). Otherwise, if it's got a good 2" eyepiece adapter, you can always by a T2 nosepiece (the Baader ones are very good): Baader Adapter auf T2 (M42x0,74) für 2' Steckhülse - Fokaladapter On the focusers that have an ugly long thing sticking out of the draw tube for 2" eyepieces, if there's an M54 thread inside and you want to get rid of that ugly thing, there's this: TS Adapter auf 2' mit Ringklemmung für Skywatcher Newtons
  19. Especially if you're doing planetary imaging, you really need to collimate them both before every session. The GoTo version of the Flexitube Dob is not too bad - the Auto version has a mount that's a lot more fickle. Apart from that, the Dob mount is actually a lot more stable than a 200mm Newt on an EQ5 (that scope really wants at least an HEQ5 even for planetary imaging), but of course unsuable (like all Alt-Az mounts) for DSO photography. Those two are very, very different in almost all respects. Visually, I'd rather have the Dob but that's very personal; if you have an EQ mount you can reuse it for a small refractor and DSO photography (although if you want that scope for DSO photography you'd better get an EQ6, but that's not a very portable mount.)
  20. It's impossible to tell whether your scope is collimated unless you take a picture through a centred pupil. what is apparent is a tilt/rotate error: the secondary's major axis is not towards the focuser and that's compensated with tilt. But that's an error you can usually safely ignore (all it does is make the secondary, and thus the fully illuminated field, slightly elliptical as seen from the focuser, but as long as you can see the entire secondary that's really not that important). Apart from that, it's either really badly miscollimated or the camera's not in the middle of the focuser, with the latter (fortunately) much more likely. From a tool with a centred pupil, the cross hairs and the reflection of the cross hairs should overlap. They don't here, so either the camera is way too low of there's massive miscollimation. By the way, there's no centre spot that I can see on the primary. Since you have a Cheshire, I'd certainly consider putting one on the mirror if there isn't one. Ignore the spider vanes - due to a perspective effect, the optical axis doesn't usually hit the geometrical centre of the primary and depending on how that offset is implemented (secondary holder centred or offset, secondary glued offset or centred) the spider vanes can get offset with respect to the rest. The silhouette of the secondary's reflection is *always* offset away from where you see the focuser intrude and towards the back of the scope. In your case it's offset a bit more anticlockwise, which is what hints there's a tilt/rotate error (the secondary's far side needs to move down a bit).
  21. If you use the mount for visual observation you may not want to do the polar alignment as someone with an obsessive compulsive disorder, and then you'll appreciate the ability to nudge the object back to the middle from time to time.
  22. There are tidal bulges and tides on land as well. It's jsut not very visible because there's no horizontal displacement of land and dramatic changes in landscape (and because when you're sitting on the bulge you can't notice you've crept up).
  23. A barlow is one form of telextender. Not all telextenders are called "a barlow". So we'd have to know which "telextender" you mean to say anything.
  24. Etymologically, apochromatic and achromatic say the same thing. For refractors, though, Abbe's original definition was for an "apo" to have three zero crossings on the best focus location vs. colour graph (i.e. for a vertical crossing an "S" or "3" --technically "superachro", not "apo"-- shape, and not a mirrored "C" shape) and for an achro to have two. Nice and clean, but it's not a performance criterion. Of course, since fluorite doublets can have better colour correction than triplets using standard flint/crown glass types even though using Abbe's definition the first were "apo" and the second weren't , people started using "apo" for fluorite doublets as well, and then for doublets with one low dispersion glass element with a dispersion not quite a low as that of (non-glass) fluorite (in the vein "it's as good as some Abbe definition "apos" so I'll call it an "apo" as well). We can all thank William Optics for even having etched "fluorite" on some objectives that actually used FPL-53 glass so that the waters then muddied even further...and don't get me started on "semi-apo": there's not an etymological "middle" between apo- and a- (given they already mean the same thing, really) and certainly no graph that has 2.5 zero crossings somewhere (even if you define a tangent as "half a crossing", if you have that you have three crossings somewhere else), so that's clearly the invention of a marketroid spinning very rapidly. All that prompted Thomas Back to come up with yet another definition for "APO" that included both Abbe's original criterion and some objective criteria for the secondary spectrum and spherochromaticism (i.e. the spherical aberration away from green). That's by far the more useful and stringent definition, but obviously not everyone (and certainly not the people selling FPL-51/53 ED doublets and relatively cheap FPL-51 triplets that have no chance of meeting the criteria) was inclined to stick to it... By the way, FPL-53 and FPL-51 don't mean anything used in isolation as well if they're just thrown at the prospective customer's face. In the "extra low dispersion" picking order it's indeed Fluorite, FPL-53, FPL-51, but a lot depends also on the design and the matching glass types for the other elements. Not to mention that actual performance doesn't only depend on the design but also the quality of the figure, polish and what the tolerances are for alignment and spacing of the elements in the lens cell (if you have a Chinese triplet with a design that could be excellent but the internal collimation and spacing isn't precise or the lens cell pinches the optics when it gets cold, the fact the design could do better is small comfort). I've never seen anyone describe a Newtonian as "apochromatic". The distinction between "apo" and "a" simply makes no sense if you're not talking about refractors.
  25. "No need for anti-dew" doesn't go well with refractors, I'm afraid. If that's what you're thinking then you should give the 130mm Heritage or better the 150mm Starblast a good look. No power tank, no HEQ5 mount, nothing, and it's got a pretty good chance of continuing to work for longer without any anti-dew gizmos. As far as electricity is concerned I'm just using a LiFEPO4 battery pack for the fan (which you *do* want) when I only use my StarBlast, and I'm glad not to have any hassles with too much electrically powered equipment prone to give up on you (even when you have no power at all you can still observe with these beasts). You do need a low table or crate in addition to your observing chair, but I suppose you can store that where you don't have to drag it up and down flights of stairs. Well, I hate to break it to you, but something as complex as a good refractor on an HEQ5 mount isn't going to fix that issue.A Dob will, at the expense of forcing you to track manually. Yeah, you have to collimate it, but if that really bothers you that much you can just adjust the tilt of the primary with a Cheshire or collimation cap and let the rest fall where it may, and that takes twenty seconds. And if you do want to observe sooner, you'd better have a fan with three rubber bands around it and a small battery pack, but you don't have to use it (you can wait for somewhat longer before you start your high power observing). BTW, I've seen people make tabletop mounts for short refractors as well, it doesn't have to be a Newt even though anything with a long tube is a no-no: Now, a "classical" 150mm Dob can actually (barely) be taken up/down stairs in one piece and isn't that heavy, but it's not very stairs-friendly because of the bulk and there are some stairs I'd never want to navigate through with one of these. A 150mm Starblast Dob, on the other hand, is no problem, and you carry it in one hand while carrying the eyepiece/equipment case in the other (unless you are paranoid and want one free hand while navigating the stairs, in which case that's two trips). Refractors also have lenses at the front, and they radiate just as well to the night sky as the corrector on a Mak does. No free lunch there, although the advantage of the refractor is that you require almost no cool down period (unless it's a very large an bulky triplet).
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