billyharris72

I'm not sure on this poem - it could be taken as part of the old "two cultures" dichotomy (okay, I admit, it definitely is), but it's not that bad. I didn't take its main thrust as astronomer bashing but as pointing to difference between the (interesting, valuable, fascinating) world of astronomy (done mostly in offices, lecture rooms and these days computer labs) and the experience of just looking up (from a body, on a moist evening, with human eyes) at the night sky. I'm not convinced that human endeavour since Copernicus has added to the fundamentals of that experience at all. Ignorance might not be bliss, but neither is knowledge; bliss is just looking up. I like both, for what it's worth, but I can understand from my time in academic and other analytical pursuits the idea of being tired and sick, even with subjects that fascinate me. Billy.

Glad you're enjoying the scope - I think at that size there isn't a better scope on the market, at any price (happy to be corrected if anyone has one). The price being riduculously good is an added bonus of course. You're right about aperture, and this is why, if you do want a significant upgrade, you will need to bite the bullet on size. There just isn't a better option that isn't significantly bulkier. To see a real improvement over what you have I'd skip the 150 and go straight to 200mm in a Newtonian (where a Dob is the easiest option). They're a bit of a faff, but surprisingly portable when you get used to it (at least for back garden use or car travel - I wouldn't go hiking with one!). A Schmidt or Maksutov cassegrain could be a less bulky alternative in terms of the tube, but you'll lose both field of view and (a little) in terms of optical performance (again, skip the 150mm - the 130 Hertiage is optically the equal of Skywatcher's 150mm Mak even on planets, and is better on everything else). Also, by the time you mount it it's not really less bulky. I'd say the same for a refractor - you'd need at least a 120mm ED, to equal or surpass the Heritage, both bulky and expensive. If it were me, I'd consider a bigger Dob or sticking with the (very capable) scope you already have for now. In terms of a present that is astro focused, what about a good set of binoculars? Billy.

If you're looking for a course of action you could do worse than check out if there are any astronomy clubs in your area. The chance to talk with others, learn from their experience and try out different types of kit can be invaluable. Billy.

I agree. Despite occasional gripes it's my firm belief that the Moon is, on balance, a good thing. Like the pic! Billy.

2mm is a fair bit, but I wouldn't worry about it unless it is actually causing problems in use. If you want to try tightening it up the best resource is probably. http://www.astro-baby.com/heq5-rebuild/heq5-we1.htm Hope this helps. Billy.

Hi Rob. There will always be a little play because of the gearing - the gears can't mesh too tightly without sticking, which will mean there will be a little backlash in the system. It should be minimal though. How much free play are we talking about here? If it's excessive it might worth adjusting the gears. Billy.

Funnily enough, I've been thinking along similar lines (Bank Holidays, eh?) and taken a slightly different approach based on using the hyperfocal distance theorem, which, with a suitably tight circle of confusison, ought to give a reasonable indication of the point beyond which more accurate focusing has no payoff. I've assumed a (totally arbitrary but reasonably sensitive) instrument of F=1000mm, N =4 (where N is the f ratio). Shooting on an ASI1600, with a x10 crop and printing the result at 16 x 20 inches (so about as hard as it gets outside of exhibition work). Gauss suggested that for an 8 x 10 print viewed at standard reading distance (much more able to show blur than a non-4K monitor) the appropriate circle of confusion would be the sensor (ok, film) diagonal / 1,750. That's the point at whicha human of normal visual acuity would be unable to detect a difference at normal reading distance. This is widely accepted as unrealistically precise (1500 is common for critical work in digital media, but lets stick with 1750). That gives a very challenging CoC in mm of 21.9/10/3,500 = 0.0006. Plugging those (unpalatably challenging numbers into the hyprerfocal formula H=(f^2/(Nc))+f gives a result of 399.5km. In all seriousness, with a 250mm x 1000mm scope, 400km is infinity from an optical perspective. In real terms, we could get away with much less most of the time. Billy.

I'm not sure how this works to be honest. I'm pretty sure angular size has no effect on focus (or we could make the same argument about M33). The thing about having the rays coming in parallel is that they come in parallel across the mirror / lens from every point on the object, so the thing that would matter would be the size of the objective (more properly f ratio). Please correct me if wrong on this though - it's been known! I also reckon on this basis that focus should be identical, while agreeing with Olly that it never seems to work out that way. To what extent that's just an illusory result of seeing (and trying to focus to a tolerance that is less than seeing will allow) I have no idea. Billy.

I was about to post about this must be wrong - but just realised it isn't! Thanks guys; that's been worrying me for a while! One more "problem" that I just learned I don't need to solve :). Billy

I'd avoid ebay if possible - lots of good bargains on there, but also the possibility of getting something that's not great. This place is good for second hand - everyone sort of "knows" everyone else and plays fair, so what you'll get will be honestly as described. I've bought a couple of things on UK AstroBuySell and never had a problem - tend to buy only things with photos and make a point of checking the pictures (if they don't have a shot of the objective then I'd email them and ask for one). Generally if you ask people about the item they'll give you an honest answer. If in doubt, you could try to arrange to collect rather than pay and post. That gives you a chance to check the glass and the focuser and make sure everything is in order. Hope this helps. Billy.

That's completely reasonable- Maks are great specialist high magnification scopes but there are better all rounders. A second hand ED80 (or ED72) will do a pretty good job on just about everything. Best of luck in tracking one down. Between here and Astro Buy sell you shouldn't have to wait too long. Billy.

Given that you have said you are interested in lunar and high magnification terrestrial viewing, I think the ED is the only one on your list worth considering. The Startravel range are very much designed for low magnification, wide field viewing. Really bright targets (like the Moon) and high magnification (above about x60 or so) don't bring out the best in it (I'm being kind here). For your stated goals, if you've decided you really don't like the Mak, you'd probably be best looking at either a long refractor (about f10 or so) or an ED type scope. The latter, while more expensive, will give you both wide fields, low CA and high magnification performance in a portable format. I'm intrigued as to what you didn't like about the Mak. Was it just the relatively narrow field? Billy.

Great post Dave - many thanks. I've been meaning to have another go at this with the ASI1600 but one thing that has been bugging me atmospheric correction. Do you have a view on how wide a field you can get away with in comparative photometry without doing corrections? I've seen some sites that suggest half a degree, but trying to find variables with decent range of comparison stars in a half degree field has proven tricky. My own attempted workings would seem to suggest that, for anything above 45 degrees, a field of two degrees would probably be okay, as the error would be only a few hundredths of a magnitude, but I could be wrong and have not seen this anywhere else. Thoughts appreciated. Billy.

I'm with alacant on this. The cheapest dedicated astro camera I could find out there with a half decent sensor size(still smaller) was the ASI1600, which was about £1200 (maybe a bit cheaper now). That's by no means a bad deal (you get a lot of camera for the money) but still quite a bit more than the DSLR. Add to that the greater flexibility (are you totally sure you'll never use it for anything else) and I still reckon it's the best way to start out. Bear in mind also that you don't need to mod it, though you might want to. Clusters, galaxies and a lot of nebulae don't really need it - depends how much you want to image the fainter Ha emission nebulae. Billy.

I've struggled with the same issues. I bought a collar type dew srtip from 365 Astronomy (wraps around the secondary stalk) but it tended to change the profile of the secondary mirror (which affects star shapes in images). I emailed Altair Astro to see if they have something that would do the trick (the ones on their site are too big) but that petered out. In the end I've gone for the DIY option, with a loop of 4 x 24 ohm resistors that is taped to the back of the secondary mirror. In testing it runs a bit hot, but I'll make a plug to connect it to the dew controller that I use for everything else and vary the voltage appropriately. Othr advantage is that can use thinner wire than commercial dew heaters, which won't unduly increase the profile of the secondary strut that I run the wire across. Not a lot of work and it should do the job nicely. Billy.

Yep - f5.8 so should be right in the sweet spot. I reckon it's usable with a bit more tweaking; it's proving a bit of a painful experience. On an altogether more positive note, I've just tested the Skywatcher flattener at 58mm and it's a massive improvement. Not perfect, but much closer. Billy.

I've recently purchased the Skywatcher reducer and I'm getting pretty bad curvature with 55mm backfocus. Thinking that it needs more space myself, so interesting to hear that 57-58mm range seems to work for some people. I can see a lot more fiddling in store before I get this right. I've been trying with the OVL flattener and have not been able to get an acceptable result - it performs oddly, with curvature bad at 53mm, getting worse at 55mm and then getting better at 58mm. Best performance I have got from it was with 38mm of t-extension tubes (so about 64.5mm back focus taking the filter wheel into account). That was getting there but there was still a little curvature, and the focuser does not rack in any further to allow additional spacers. Test shot attached is the best I have managed to date; could use people's views on that - I don't hink it's catastrophic but should I be aiming for better? Billy.

That's a result! Cheers Olly and Skipper Billy!

Could well be that the flattener is causing the tilt. I'd try a few shots without it - field curvature might be horrible but at least it'll narrow down the source of the problem.

Ow! Was that shot on tghe 130 P-DS (I though Newts had a pretty flat field generally)? That far out would have me wondering about focus and collimation (FWHM looks huge). What did the actual sub come out like?

Interesting and useful to know - I did some variation between individual subs despite nothing changing in the setup so makes sense to take a larger sample. Sub length for these was only 5 seconds, and I'd checked the alignement. With the target area being in Cas. I'd be surprised (though not astonished) it was tracking. Worth double checking I suppose, but I'd expect to see the same elongation across the field. I'm a bit stumped but think I need to check the performance with slightly greater backfocus. I'll also try a few shots without the flattener or any spacers - that should at least help me narrow down where the tilt is in the system. Billy.

Hmmm.. Well, I've tried imaging again, having done the following: 1) Replaced the push fit eyepiece holder with a screw fit so the whole imaging train is threaded. 2) Tried varying the spacing (currently up to 59mm and increasing - or will when the weather clears). Results so far are not great. CCD inspector shows the curvature lowest at 58mm and 61.5mm, and higher in the intermediate values (is this normal)? The elongated stars are still there with all setups - with some spacings it seems to be in all corners, in others only the right hand side of the sensor. The whole setup is threaded and I can't detect any movement in the focuser (if I hold the back of the camera and try to move it up and down the whole thing feels rigid - could move it slighly by applying force I'm sure but there is absolutely no play or wobble that I can detect). Pointing the setup at the zenith for test shots makes no difference at all - elongation remains and is about the same. Again, I've attached a couple of images for reference - I think there might be a slight improvement, but the edges (especially on the right) are still very poor. Struggling to know what to try next, so thoughts are much appreciated. Is it worth continuing to adjust the distance, or do the elongated stars mean I'm wasting my time? Billy.

Absolutely. IMO this bit really isn't optional if you want to ensure good collimation. An unbarlowed laser can be useful for rough alignment of the secondary, but that's about it in my view. On its own, I'd rather use a basic collimation cap than a laser. Billy.

Good news - unscrewed the nosepiece on the OVl flattener to reveal an M48 female thread. And with this on FLO ... https://www.firstlightoptics.com/adapters/lynx-astro-m48-adapter-for-sky-watcher-72ed.html Goodbye tilt (hopefully)!

Thanks - pointing at the zenith seems a good idea. I think I've identified the set-screw eyepiece holder as a definite source of a little flex in the system, so that seems an easy one to fix. Focuser movement would be a lot worse, but I tend to agree with Ray - the tube is only racked out maybe 10mm or so and the camera and mini filter wheel is not that heavy (fingers crossed...) Something that concerns me - is it common to have this elongation predominantly in one corner of the image? I would have thought if the camera is tilted it would affact the whole image plane, at the angle of the tilt, but the elongation I'm seeing tend to be concentrated in one corner and looks radial, with the elongation pointing towards the centre of the image. I've attached a test image (see how the elongation is much worse top tight corner) and an output from CCD inspector in case anyone can make some sense of it. Thanks, Billy.