ollypenrice

What do I think of the article? Olly

This has that 'just right' look about it! Olly

IC348 and friends imaged with Mr and Mrs Gnomus and with the participation of Tom O'Donoghue. HaLRGB. The large, faint IC342 with an inset showing M101 at the same scale. A head-banging attempt to find the tidal loops around the Sunflower in the TEC140. When I learn more about inverse masking I'll try to improve this! And finally two 'improvers' where substantial new amounts of data have helped older images. M42 is LRGB. The Rosette is HaOIIILRGB. Olly

I doubt it, personally. You have two crisp stellar images in the screen grab you posted. Murky nights produce soft fuzzy stars, not pairs of crisp doubles, and faint ones tend just to disappear. This is certainly a new one on me. Is the turret of your OAG absolutely rigid? It couldn't have the ability to rock slightly? We had this once with an older model SX OAG and I made a steel strap to stop it. The thing is that whatever is happening in your setup is happening quickly because both the real and the spurious stellar images are sharp. Can we completely rule out hot pixels? Are you subtracting a dark as you guide in PHD2? Olly

...but if it was wide and long the water would flow just as fast as it would in an equally wide short scope. And what does that bring us to? Oh no, it brings us with perfect precision to the F ratio myth! Olly

Awe, I'm only advocating a second hand 106N and agree with Sara that the only reason for going for a new FSQ would be for the big chip it can cover! Tak have also blotted their QC copybook rather badly of late. I, too, like the Esprits and would be happy to have one. Olly

The last time I looked, Gav, the fluorite 106Ns were not fetching £3000. They have the blue rather than the red bands round them. Olly

Second hand Tak FSQ106N. Mine came in at about £2K. The other one, Tom O'Donoghue's, was a bit dearer but was much younger when he bought it. If (unlikely ) Tak offered to swap my Fluorite 106 for their latest Flatfield 106, I would decline. I prefer the old one. It is less sensitive to focus drift while cooling and I don't want to use the reducer. The captain's wheel hasn't made many friends, either. Give me a 106N (or two!) I love this rig because it works: Olly

I'm heartily inclined to agree with your dislike of the company, for sure, and I haven't tried my Meade Plossl in our 14 inch Meade SCT! (I wasn't expecting this scope to be anything like as good as it is, by the way, but in fairness to an unpleasant company it is actually very good!) Olly

I wanted to try a TV Plossl so I bought a used 25mm. It's good, but quite honestly the improvement on an equivalent Meade Plossl is, to my eye, marginal. Where the TVs come into their own is in the more exotic designs. I don't have words of praise enough to describe the 13 Ethos but there are other TV greats as well, notably the 19 Panoptic in a more moderately priced EP. Do I think the best way to spend the price of a TV Plossl is on a TV Plossl? Actually no - and I'm a huge TV fan with 6 in my box. But I'm not a great Plossl fan at the best of times. I once heard them described as 'mushy' which is going far too far, but they don't have the crispness I like in an EP. Of course they're notoriously personal things, EPs. Olly Edit: Your photos of eyepieces on a star chart are very attractive one in their own right. I might copy that one for our kitchen!

As Carole says, yes, that's right. In truth I often do this myself but I'm using cooled CCDs which produce low background noise. The advantage of calibrating the subs individually and then combining them using a sigma clip routine is that the total number of images used to define what's normal (as opposed to what are rogue outlying values) is higher so the rogues are better identified and normalized. If you have only two stacks to stack the only option is to combine using 'average' which will not be as effective against noise. Olly

Good! But you'll be knocked out when you see what flats will do for you. Olly

I think that by 'stacking the stacks' Carole means combining a stack from one night with a stack or stacks from other nights, and you can certainly do this. You'd need to find the best way to weight them in the process. Under no circumstances would I put different sub lengths or settings into one stack. So, assuming consistent skies and the same settings, you'd just weight the stacks based on their total integration time. The best way to do it, though, is to calibrate each individual sub in the entire shoot without combining them and so generate a new collection of individual subs duly calibrated. You then take these calibrated subs aside and stack them as a separate operation, obviously without any calibration files since that has been done to each sub already. This way you get the maximum value from the Sigma Clip algorithm and the best benefit from dither (even if you weren't dithering between subs.) I suspect that this method would be of even greater benefit to DSLR imagers since it would attack the background 'colour mottle' problem most effectively. Now, different sub lengths. Do it only if you know why you are doing it. Of the 100 or so images on my gallery site I've done it precisely twice, once on M42 (which everybody has to image in multiple sub lengths) and once on M31 (where I'm not even sure it did any good at all.) I never worry about white clipping stars. You can pull the colour into the cores in post processing and if you expose so as to avoid clipping stellar cores you won't go deep enough. The key thing is to look at your linear stack. If a galaxy core isn't burned out there then it doesn't have to be burned out in the final image because you already have the data in the stack. Olly

My answer is always the same. Mono is fastest. On NB it is way faster and it opens up the moonlit nights for imaging. Olly

The fault, dear Brutus, lies not within our stars but within our wallets...

The arrival of monochrome CMOS cameras similar in format to DSLRs and at considerably sub-CCD prices inclines me to remind you of a phrase in your original post: Or am I in desperate need of a new camera? I hesitate to answer this too assertively but if I were to do so the answer would begin with a 'y'. lly

The term 'magnification' is only meaningful if we know the dimensions of the thing being magnified by 42x. To know its dimensions we have to know what it is. When we look through a telescope giving 42x magnification we do know what it is we are increasing in size by 42x. It's the image perceived by the naked eye. But there is no such thing as a 'naked camera.' This might be a camera without a lens but in that case it wouldn't give an image at all. So my question' 42x what?' was perfectly serious. Of course we can certainly say that lens x will give 42x the magnification of lens y but this is only of interest to the owner of lens x. This magnification has no universal currency which is why we use the term resolution in arcseconds per pixel or, in professional circles, plate scale. Besides, if I try two cameras in the same scope, one with small pixels and one with large, the plate scale will be unaltered but the resolution will increase, as will the size of the output image. It would be odd, though, to say, 'this chip magnifies more than that chip...' even though, in one sense, it does! Olly

But 42 x what? (That's why the term isn't used in imaging.) Olly

I think this is said because a 50mm lens's picture will present near and distant objects at the same relative sizes as they appear naked eye. In other words there is no 'foreshortening' effect as created by zoom lenses. Since astronomical targets are all at infinity this effect does not exist for us. Olly

Earlier on you used the term 'crop sensor.' This is a term best forgotten when you move into astrophotography and imaging with telescopes. Cameras, and particularly astronomical cameras, come in a wide range of chip sizes. In the image below I've modelled two chip sizes, large and small, in one of our telescopes. The object on the chart is M33 shown in green. The problem with the misleading term 'crop factor' is that it might lead you to believe that you are more 'zoomed in' (and capturing more detail) with the small chip than with the large. In fact you are not. If both chips have the same size pixels the image scale is literally identical as is the level of detail and the final screen size of M33 on your PC. All that changes is that you have more sky around M33 wth the larger chip. Now let's imagine, as might well be the case, that the larger chip has smaller pixels than the smaller chip. This time the larger chip will give a larger and more detailed image of M33 than the smaller one. This is because it will put more pixels under M33 itself and when it appears as a fullsize image on your screen (meaning one camera pixel is given one screen pixel) M33 will be given more screen pixels and, therefore, be bigger. The size of the sensor has no bearing on the scale of the image and the person who coined the term 'crop factor' should be made to listen to an entire CD of yodellers in full voice! lly

The OP is in India where I guess the nights are very warm. This might be a big problem for any uncooled camera doing longer exposures. Olly

Most certainly! Olly

Yes and, somewhat later than my Nimrod pilot, we had a retired RAF navigation officer staying who knew our pilot and had flown with him. He told me about his training in astro-navigation. Fascinating stuff. I've just this minute finished David McCullough's Wright brothers biography and can honestly say that I've never read a more moving book. https://www.amazon.com/Wright-Brothers-David-McCullough/dp/1476728755 Olly

Or is it some kind of Icarus complex, or even Faustian? Indeed, in Marlowe's Dr Faustus, Faustus has mastered astronomy and, after his pact with the devil, has himself borne aloft on a flying chariot. Olly