ollypenrice

For a subjective comparison I once imaged the same object at the same time with the same model of camera but at two focal lengths and resolutions. In a Tak 106 at 3.5 arcsecs per pixel I had the Astrodon 3nm Ha and in a TEC140 I had a 7nm Baader at 1.8"PP. Under normal circumstances - ie with the same filter in each rig, the TEC would give considerably tighter stars and more structural detail in nebulosity. In this case, though, the images looked remarkably similar. The Astrodon filter had lifted the subjectively perceived image quality as dramatically as that. I'm afraid I don't have the original data any more but am confident in my recollection because it was so surprising. Olly

Quite the opposite, I think. The significance of F ratio arose in the camera lens world in which a diaphragm allowed the aperture to be varied while the focal length remained unchanged. Problems arose in AP discussions when F ratio was reduced not by increasing aperture but by reducing focal length. tooth_dr's comparison is interesting precisely because FL (and pixel size) are fixed (almost) and only the aperture has changed, as is the case with camera lenses at different F stops. When the F ratio is reduced by increasing the aperture then we know we have more light so we can be sure the exposure time will go down. They key player, here, is the extra light. Olly

Here we have the antidote to the F ratio myth conversation because we have different apertures at the same focal length, meaning that the F ratio becomes a truly meaningful number. The increase in speed is pretty spectacular, as we'd expect. Personally I'm less bothered by the slight softness of the Epsilon stars than I am by the spikes, but people feel differently about this. For me spikes say, 'This is a picture,' which stops me from mentally losing myself in space, so to speak. Nothing can be done about this and not everybody feels it anyway. Setting aside the seeing on the Epsilon's night, it is always going to be much harder to nail focus, tilt and collimation on a system as fast as this. To my eye you're doing very well with the instrument. Tweaks won't hurt but they don't hit me as a screaming necessity. If you need to pixel peep to see it then it isn't a valid problem in my book. So, dual refractors or fast reflector??? (Remembering that, by definition, doubling your refractors only makes a difference of one F stop. I always find this a scary thought!) Olly

No max exposure time provided you have matching 'darks for flats' and you don't exceed about 2/3 well depth. Olly

Indeed. (For one thing you do need the LMC to be above the horizon!!! ) Olly

I'm not sure that the multiband filters would work with the steep light cone of F2. Certainly check that first. I've never tried them myself. My experience of the expensive Astrodon 3Nm Ha versus the less expensive Baader 7Nm is that the Astrodon gives tiny stars and higher contrast in the nebulosity. The difference is considerable. On the other hand, for LRGB I'm perfectly happy with the Baader. The cheapest RGB filters are absorption filters. They are OK at entry level but certainly not as good as the interferometric ones. As you are aware, the angle of the incident beam at F2 is difficult for filters to handle. Olly

The LMC is gorgeous! Such colour and luminosity. Olly

Lovely gentle, natural rendition. Olly

So would I. The Moonlite strikes me as being more of an exercise in prettiness than function, though it's super smooth for Dob users where it will remain horizontal. Mine started slipping, which isn't surprising given that the roller drives shiny anodized aluminium. The Steeltrack has far more grip but the Diamant is the top choice from Baader. If using robotic focus you really don't want any slippage. Little competes with the FT R and P but the price is out of proportion with the cost of the scope in this case. Olly

That background sky is way, way better on my monitor. Nice one. I might just give the green a tiny lift to kill the slight magenta caste of the image but that is a great tweak you did. Olly

Academia can, indeed, be a small world. While reading a book about cephalopod intelligence last year (Other Minds, Peter Godrey-Smith) I came across a reference to research by a childhood friend of my father's, Peter Dews. I'll do my best with your paper but I'm afraid I'm a chemistry ignoramus. Olly

It's the first thing I noticed, that Ha. Very good that you picked it up. And, bless me, my first click when Googling jumping snails found you right away! On the face of it a less gymnastic animal would be hard to imagine... (There's an English joke about a Council road builder who, late in the day, stamps on a snail and kills it. When an outraged lady asks him why he performed such a gratuitous act of cruelty he replies, 'I'm sick and tired of the damned thing. It's been following me round all day.' Olly

I'd missed this thread but it's a real thriller on many counts, notably for the images and the location. Both are rather mouth watering! But the performance of the travel kit is also pretty inspiring. Great thread, Goran. Now off to Google jumping snails... lly

The image is nice. I think of LRGB this way: a colour filter blocks about 2/3 of the visible spectrum light, a bit less maybe, but about that. An L filter passes all of it. So it takes an hour per colour filter (3 hours) to get as much light as you get in 1 hour of luminance. In practice I find I don't really need 3 hrs of RGB to match 1 hr of L, I really need 4 hrs or so. The theorists disagree but I'm not a theorist, I just measure what I actually get. Then we find that we don't need as much colour information as luminance. The RGB can be processed for low noise (essentially it can be blurred) and high saturation. The strong L layer can be processed for sharpness (the opposite of blurring) and depth of faint signal. As a general rule we are not expecting much colour from the faintest signal anyway. When combined, the weaker RGB signal will work well with the stronger L signal. I think that in your M33 you have a lot of colour noise in your background sky which it would be very easy indeed to remove since there is little real colour information in the background anyway. You could lose the colour 'detail' in the background (which is an artifact anyway) without smoothing the luminance so the sky would not look artificially noise reduced. Keep an L-RGB mentality throughout the processing! Olly

I'm a firm believer in leaving the camera aligned with RA and Dec, either in portrait (long side aligned with Dec) or landscape (long side aligned with RA.) It is dead easy to align the camera this way. Align the camera by eye with the dovetail first then just slew slowly in one axis during a shortish exposure of a few seconds and look at the angle of the star trails you produce. That's your camera angle. Rotate till the trails are horizontal or vertical as desired. If it's orthogonal you can add data years later because finding the RA/Dec angle is easy. Finding a random angle wastes vast amounts of time because there is no routine way of doing it. Olly

I don't do this in Pixinsight but in Photoshop so I can't help but... ... I agree with this. I often push the L too hard myself then have to go back and re-do it. In Ps I add the L to RGB in partial iterations, reducing colour noise and increasing colour saturation between each one. I think your rework is a very big step in the right direction. Olly

Good stuff. I think the luminance certainly out-guns the colour so more RGB data would be worth having. The background sly seems a bit 'colour busy' yet the brighter signal seems bleached out a bit. How did you combine L with RGB? I think that might be the secret. Olly

Michael, who replied above, has steadier hands than I do. I regard 10x as my hand held maximum mag and nowadays prefer 8x - but this might not apply to you. I'm happy with 8x42 but have used 10x50 as well. It's a question of slightly less shake versus slightly more light. Neither is life changing in my view. Olly

Looking carefully at our version taken from my dark site I don't see anything in ours which isn't in yours. You have all the faint outlying patches that we have. Of course there may be more but, if there are, we ain't got 'em! Olly

I think the last one is the best and a very good rendition indeed. Somehow this is a target which leaves you wanting more from it but I suspect that this is simply because the heavy dust obscuration is denying us access to a significant part of the target's light. A small detail but the background sky along the two vertical sides is somewhat red or magenta as compared with the background around the galaxy, which is perfect to my eye. Should be an easy fix if you agree. Olly

Are you calibrating your flats with flat darks? (With CCD you can just use a master bias for this but wth CMOS you must make darks at the same settings as your flats.) Uncalibrated flats may well over-correct. However, I've had this problem arise out of nowhere and evade explanation so there is no guarantee that flat darks will work. Olly

By the way, Ha regions imaged in RGB or OSC can be considerably enhanced in Photoshop by going to Image, Adjustments, Selective Colour and lowering the cyans in red. Sometimes the effect is nothing short of spectacular. I don't know Affinity Photo but it might have an equivalent. Olly

Woo Hoo, I can see a whole new set of variables appearing here! Just what we need! On the left, posh Japanese 4 inch F5. On the right, budget Chinese 4 inch F5. Which is faster? Now we have to look at the image circle (say 80mm on the left, 40mm on the right) and the sensor size. The imager with the small sensor says, 'They are the same.' The imager with the giant sensor says, 'The one on the left is twice as fast.' They are both right if the target needs a 2-panel with the smaller circle and smaller sensor. After all, a larger sensor collects more light, does it not? It just goes to show that no one number, in isolation from the rest, can tell you what you need to know. (Even Vlaiv's 'aperture at resolution' which is the best candidate so far, in my view.) Olly

What a great start! Olly

A very attractive result. But what is real? This nebula is red to the human eye. I think that's quite simple. Exactly what kind of red? Aaaahhh. lly