billyharris72

Thanks all for the advice. Bobro & Daz (good avatar) - I will go back and check the exposure on the flats, though I think the apparent high gradient (from total black to total white) is a result of my poor choice of visualization settings on the fits file. I've attached one with less extreme black / white points and the x axis ADU profile (saturation at 64k) below. Total light fall off from centre to edge is about 25% or so, with the centre slightly less than half saturation (is this correct or should I be exposing more / less)? Wasn't aware of the need to flats for each filter - bit of a pain but hopefully it will make a difference. Art - agree 100% that I need to recheck my collimation. Usually do it every session but was reluctant to remove the camera as I didn't want to have to refocus etc. I've dug a bit further and the whole image is a complete mess to be honest. There is significant over correction, so I tried stacking without flats and got this. As you can see the colours are totally uneven, with some parts of the image green and some red. No idea how this could come about, as I was shooting R,G,B in a repeating sequence using an electronic filter wheel. Anyone seen this before? Cheers, Billy.

Hi all: Here's an LRGB effort from a couple of days back, stacked in AstroArt and adjusted to bring out the issue I'm talking about. It seems the edges (especially bottom right) are brighter and redder. Is this likely to be a problem with my flat frames? I was thinking maybe it has over corrected somehow, but is this a thing that can happen? Flats were taken with the same gain and offset as the subs (on an ASI1600mm), so wondering how over correction could even happen. Another option I thought of was maybe the flats themselves have an issue. I've attached one - I don't think it looks too bad, but could there be an issue with the filtering? I guess the question is does light wavelength have a significant effect on vignetting. The flats were all taken with a luminance filter, using a flat panel made from a LED tracing pad - if the light was biased towards the blue end of the spectrum could this have the effect of producing a flat that would over correct for red? Just a thought - really have no idea. Thoughts much appreciated - has anyone else encountered this, and if so how did you solve it? Billy. EDIT before I've even posted. Just looked at the flat again. The scope looked reasonably well collimated in my Cheshire (checked the day before, so could be out but should not have shifted much), but is that the secondary spot I see near the bottom of the frame?

I think the collimation thing can really be broken into two bits. 1) Collimation of the seconday mirror. Can be mechanically fiddly; less important than the primary; rarely needs changed; is usually okay straight from the factory. Fine if it's approximate, except for fast systems. If in doubt, I'd just leave this alone unless there is something major. At the very least, it can wait until you feel more confident - at which point I would use Astrobaby's guide and do it by eye (I find it gets better results than a laser, though it takes slightly longer). 2) Collimation of the primary. Genuinely important and worth doing regularly (I do it every session). But really quick and easy once you get the hang of it (just move the knobs until the centre spot of the primary and the centre hole of the collimating eyepiece line up). That really is it. Starting out I found the Rigel collimating eyepiece easier to use (it's only about £5) and good enough, but the Cheshire, with its longer site tube component, is more accurate. Billy.

This (or Skywatcher Heritage 130p if still available and if a 200mm Dob really isn't and option) would be a great choice. You won't get a frac that will outperform it for under about £1k. Billy.

Another example (if it were needed) of how supportive this site is. Problem solved, and I've got a good idea for a longer term project. Thanks Neil! Billy.

Here's another way to look at it. The secondary mirror is optically neutral - all it is there for is to divert the image into the focuser. The truth is that minor misalignment of the secondary won't have a meaningful effect unless you are dealing with fast (considerably faster than f5) optics. If the secondary is not pointing down the focuser (so it appears more or less circular) then the sweet spot of the image could be slightly misaligned with the centre of the eyepiece. Not good but not the end of the world. Get it as circular as you can (if you're in doubt, it's fine). If you are not able to see all the mirror clips you might lose a small portion of the light from your primary - it is worth spending time to get this right, but you do have a bit of leeway. Basically, get it as good as you reasonably can (it is fiddly so make a cuppa, get comfortable and block out a couple of hours) and then leave it alone. The bit to concentrate on is the primary. That is important, and worth doing every time, but much less fiddly. Billy.

That was sort of my suspicion. You're right Peter, it is actually Aluminium, and even if it could be welded the surface area of the weld is really small, so I doubt it would be particularly strong. Astroboot sounds like a good idea - if I could just pick up a spare leg from somewhere that would make life much easier. I think I'll ponder my options on this one. Cheers, Billy.

Hi all: Not a DIY or metalwork buff so thought I would ask on here before I waste my money trying to get it repaired. I just took delivery of a Skywatcher HEQ5 pillar mount (the kind that has its own legs, not the extension). To cut straight to the chase, I over tightened one of the bolts and broke the flange of one of the legs of the mount (see attached for the joint as it should be and the broken one) Is this something that would be relatively straightforward to repair, and if so would the repair be rigid and robust? I'd rather get it repaired if I can, but don't want to waste time and money faffing if it's a write off. If welding the flange back on won't be strong enough, how about welding the thing together (i.e. the legs to the pillar)? Would that be stronger? Thanks for any advice you have. Billy.

Personally, I'm not convinced of the value of an optical finder on an ST80, though others clearly do find it useful. A simple red dot finder, if you can get the hand of it, is of use, but I'd agree that a wide field eyepiece is probably the best option. I actualy aquired an erect image diagonal for the purpose (cheap, off astroboot) but to be honest I never really use it these days. Billy.

What about contact lenses? Don't really like them myself (never seem to be as good as my specs) but worth a try?

Hi Demonperformer, I'll be as interested in other's answers as you are, but can think of a few things, which I'll just list. 1) 9 x faster via binning is nothing to scoff at. It'll translate to beter tracking and a better S/N ratio, and will be faster, so lots of gains there. 2) Seeing is not always limited to 2" - it's not hard and fast and it is possible to go better than this, though you may need to throw away some subs. A 71mm scope has a Dawes limit of about 1.6" (from a rough calculation) - if seeing allows imaging better than this then an 8" will have obvious resolution advantages. Also, on planetary imaging (using lucky imaging) you can get down below 0.2" on a good night - may or may not be relevant to you. 3) [I think - from Chris Woodhouse's Astrophotography Manual] The different sources of error in an imaging train are not independent of one another, but stack. Same for visual with scopes and eyepieces - the idea (often expressed) that each item just is a "bottleneck" and the scope will perform as per its weakest part is wrong - every part introduces error into the system (though it makes sense to go after the biggest error when looking to improve). In a simplified imaging train you might have: a. error from the resolution limit of the scope; b. from seeing; c. from your camera resolution; d. shot noise; e. noise from the camera itself (bias, thermal). The total error in the system grows as the square root of the sum of squared errors. Anything you can do to bring any of these down helps, though some involve trade offs (binning sacrifices resolution but that's very little loss if you are oversampling compared to the gains). If point 3 above is correct (it will be interesting to hear what more experienced imagers think on this), then a hypothetical, simplfied system with a 1.6" resolution scope and 2" resolution pixels would be getting a "real life" resolution of about 2.5", while moving to an 8" (0.57" resolution) would get you closer to 2". Billy.

I'd second Louis D. Those lenses will be capable of some excellent photos and will be a good place to start. You could go with a Star Adventurer, or heavier with an HEQ5 or up (mounting the lens won't be a problem). The larger options are pricier (and bulkier) but are good for visual use as well and also allow you to mount a larger scope for imaging should you decide to acquire one. It really depends how you see your interest developing. If it's likely to be mostly photographic and you think you might one day want to use a bigger imaging scope then an equatorial mount makes sense. On the other hand, a Star Adventurer will handle all your current gear (which is very capable) and a Dobsonian makes a very cost effective visual scope. Billy.

Thanks all - that was a quick answer. Sky Safari looks impressive, but may well check out Alladin first (the price is excellent!). Billy

Hi all: I just grabbed this quickly during a gap in the clouds. I wanted an image of the Double Quasar (had been reading about it in March's Astronomy Now). I uploaded the image to Astrometry.net to see what else I had (in addition to the obvious) and was a little surprised that one obvious object did not get labelled (the one I've put just RA and Dec on). Neither is it listed in Stellarium or in my AstroArt star atlas. It's not an artifact; the image in AS also shows it, so there is something there. No biggie, but it has piqued my curiosity. Anyone know what it is? Billy.

For general viewing I think there is, in the sense that we evolved to view with two eyes and we do seem to be able to see more when we have that relaxed, immersive view. This can also be gained with binoviewers, but binoculars have another plus going for them. One of the advantages of true binocular viewing (doesn't hold for binoviewers unfortunately) is that the exit pupil is smaller for a given combination of magnification and overall light grasp. For example, a 60mm binocular has about the same light grasp as a 90mm refractor; at 10x delivers a 6mm exit pupil vs 9 for the frac. So for low power viewing (relative to the objective size) I think bins do have the edge (though much less flexible than the scope). Billy