ollypenrice

Since your days with CCD, pixels have become much smaller in CMOS cameras. With a 115 APO you are likely to be working at around 1 arcsec per pixel, which is a galaxy imaging resolution of the kind that used to require a long focal length to achieve. This matters because it makes a higher demand on your mount. To support your image scale a mount needs to run an RMS of not more than half that of the image scale, so that would be 0.5". A good EQ6 or HEQ5 (the top versions) can achieve this with care but not with much left in reserve, so I wouldn't go below that. Alternatively, given the small size of modern pixels, you might feel that a shorter focal length would still give you the resolution you'd like while being less demanding of excellent guided tracking. Olly

It worked perfectly, Alan. By the way, since the time bug of a couple of years ago when the Meade GPS stopped working, I simply stopped using the GPS entirely rather than risk a patch. Everything works better without it. GoTo is more reliable and setup takes a fraction of the time. Olly

So how are the squares created? Differences in brightness? Olly

I have this conversation endlessly since I'm an astrophotography provider. Firstly, many people misunderstand filters, so I begin there. Many assume that a red filter 'turns things red,' so I explain that, no, the red filter blocks the other colours so that the red component arriving at each part of the picture can be measured. When we have, similarly, measured the green and the blue we can weight each one correctly in a very good approximation of the human eye-brain's perception of the scene. I also explain that this is exactly how every digital image they have ever taken with camera or phone was created. They've been using the technique for years without questioning it and without knowing it. I'll then take a detour into the Bayer Matrix. I find that this explanation generally carries the day - or they just say 'yes' to shut me up. 🤣 Olly

@Grant Pixinsight's Photometric Colour Calibration requires focal length and pixel size. Any chance of this being supplied? It would also be interesting to know the kit being used but you might have reason to withhold that information and it isn't necessary for the processing. Boy, I love the luminance! lly

Doesn't this assume skies which are 1) perfectly consistent over time and 2) perfectly uniform in the distribution of the LP spectrum? Olly

These are not photos of the same thing. The first is a photo of the Parthenon, the second a photo of a retzina hangover taken from the inside... Back to your main idea, though: firstly, the whole point of the Warhol panel is that we do know they are the same image, and the same can be said for the M81/82s in the thread. It's not as if they were entirely unconnected with their subject, though obviously there is variance. 2) These are amateur pocessing jobs and contain both individual choices and errors. Only six of the nine have neutral dark grey background skies, so three of them are categorically wrong in terms of colour balance if the objective were to represent reality. To a first approximation we know that the background sky is dark grey and, even if someone wants to nit-pick against that, most imagers regard it as a desirable point of departure. Very, very slight deviation from parity in RGB is embraced by some expert imagers in full control of what they are doing and is done for aesthetic purposes, but most deviation from parity is simply done in error. I'll make so bold as to suggest that this is what's happening here. Note that, in the cases of the images with neutral skies, there is broad (though certainly imperfect) agreement on colour. Warhol would not be impressed by the degree of difference! 3) What is the role of the processor here, given that he or she is working with given data? Is it their job to process the data as it is supplied or to process it according to what they know about the objects? Bending the data to fit what you think are the facts is as unscientific as it is possible to get and would see you thrown out of academia! But then, checking the colours against the known astrophysics could be called calibration. This must go before the scientific high court for clarification! 4) IFN or not, and how much? Provided you don't invent any IFN, this is the choice of the processor. I've seen enough M81-82 images to last a lifetime but this excellent luminance data had something new (to me) in so far as the relationship between Arp's Loop, the IFN and Holmberg IX went. I made that a priority. I did struggle with the colour in this dataset. My regular calibration methods flatly refused to work and the blue channel was unlike the others. This may have been entirely my fault somewhere along the line and, in finding a way to get something reasonable, I have ended up with a shortage of blue at the bright end. I'm still playing with it but I know that what I have is not as representative of the astrophysics as I would like. This is also a competition, with the rules of engagement clearly identified as non-scientific. This just may influence the way folks approach it! Not me of course: I would never play to the gallery 🤣 Olly

The brackets I used turned out to be real junk once broken so you could see inside them. An arm coming out of a flat flange was just fitted in and then spread out behind to keep it there, not welded at all. If you know the construction of the item you're using you'll be fine. However, from your picture it looks to me as if you could have vertical panels fixed to the roof, running just below the corrugated top to down below the rail carriers then add a horizontal baton to the vertical panels on the inside to run below the rail carriers, which is how we've done it on all our sheds. You don't have to remember to chain anything down, it's got anti-lift in any position and it also seals out the weather very well. Olly

Bryan, the 3.3 is only workable on the tiny old webcam sized chips. Olly

That's characteristic of edge artifacts. I always crop them right off before starting, otherwise you can't be sure what the histogram is telling you. Olly

'I knew I was pushing the seeing at under 1”pp but sometimes you have to ‘break the rules’ to see what’s possible!' I don't think you were breaking any rules. 1"PP or so is a routine image scale for galaxies. When Yves Van den Broek and I were running his 14 inch ODK we were at about 0.62, which was excessive and required the images to be presented at less than full size, but the only downside was taking longer than necessary at the capture stage. 'Correct me if I’m wrong, I read sometime ago you were bequeathed a 14” lx200, do you still have that beast? ' That's right, it belonged to Alan Longstaff who bequeathed it to me. It's going strong but strictly in visual use where the GoTo Alt Az mount is wonderful. 'My tube isn’t ACF and although I’d love that, I understand it’s harder to use focal reducers? ' You can't use the regular SCT reducers because they also flatten, which isn't necessary, but I have an Astro Physics telecompressor which works fine. Alternatively, with CCD at least, you can just bin up the pixels to match the FL. How this will play with CMOS, though, I don't know. Olly

Damn, that really is good! And it's a pleasure to see an unconventional choice of rig in action, as well. The combination of focal reducer and wise decision to bin 2x2 mean that you're working with certain numbers very close indeed to my own Bubble in the TEC140 with the same camera, unbinned. The resolution of just under 1"PP is nearly the same, and the F ratios are similar (6.3 and 7). However, you have more aperture and bigger effective pixels so you have more light per pixel, which is what really matters. To my eye there's precious little to choose in final resolution and I think your processing is lovely. The detail is delicious but there's no forcing. So... an instructive comparison. I have an early 10 inch Meade ACF which I've never used but this certainly inspires me to give it a run at last. Oh, and let's hear it for your EQ6! Well done that mount. Olly

We use rack and pinion gate openers after garage door openers pegged out on two sheds. They have proved reliable, even on our 4X3 metre shed. Olly

The camera has a chip window, no doubt, so it's quite unlike a lenseless DSLR. You have nothing to worry about. Olly

Humph, I didn't get one of those. I'll ask the dealer. Olly

This is a new Celestron RASA 8. Right in the middle of the corrector plate (the front optical elmement) it has... a large, cooled CMOS camera. Not a small scratch, a large and entirely opaque CMOS camera. So you really don't need to worry too much about your scratch. It's good that it looks black from the rear because it shouldn't create any internal reflections, either. I know it's nice to look at pristine optics but the truth is that grubby, marked optics give near-identical results. However, I'd bargain with vendor for a discount. Olly

Thanks Goran. I don't recall a distance ring coming with the Artsky adapter. Is this just one of those little spacers which fits over M42 threads? I've taken out the Celestron window since trying without it is the obvious first step. Olly

Hi All, First light on our joint RASA 8 project. Lots to do because this will be a robotic setup but we took an initial image image. The cables in front of the corrector are not yet organised so star shapes won't be good until they are. The Centre should come right when this is done. Centre. However, the corner crops upper left and lower right show stars extended like parts of the circumference of a circle. Here the four corners are presented as they lie in the full image. My feelings are that it looks like this... This kind of corner distortion is consistent with the chip being too far from the corrector lens... (It also arises from poor PA under autoguiding but this was an unguided 60 second sub.) What's puzzling is why only two diagonally opposed corners are affected. The chip distance may be too long. We are using the Artsky RASA camera adapter which should give the right backfocus for the ASI Camera but, for now, the Celestron clear glass filter is still in place. Removing it would mean our backfocus would be effectively even longer, in my understanding. So any ideas, anyone? Why two bad corners? I'm struggling to see how tilt could do this but maybe it could? Olly

Interesting! I couldn't make it work because I was holding the edge of the paper horizontal. This guy holds it pointing upwards with the paper arcing over and it does work. So back to roofs: high velocity airflow has low pressure, low velocity has high. Bad news for sheds, because the air velocity below the roof approximates to zero while that on the outside is the wind speed. And I think it gets worse, though I'm not a physicist: once the high pressure has lifted up the roof it's highly unlikely that the now unstable roof will remain horizontal. It will have an angle of attack with regard to the approaching wind. If the randomly created angle of attack is leading edge low, trailing edge high, it will work like a racing car wing and push the roof down (where it will rebound and come up with an alternative angle of attack...). But if the angle of attack is leading edge high, trailing edge low, both lift and drag will briefly increase, meaning the roof will lift higher and be displaced in the direction of airflow - away from its walls. It will then stall and come down fast, rather in the manner of a French guillotine approaching an aristocrat's neck... As pilots like to say, 'If you want to go up, pull back. If you want to come down, keep pulling back...' Also impresssively, a Formula One car could drive upside down along the roof of a tunnel from about 100mph upwards, I read. By that speed its downforce is already exceeding its weight. Your observatory roof wants to fly! Olly

Yes. If you think of wings, they are not very big but they lift massive aircraft into the air. The highest wind speed recorded in Sussex (where SGL tells me you live ) is 82 mph. While this is given as a record, gust speeds in specific local situations may go unrecorded yet still be remarkably high. If the wrong gust goes over your roof, that's all it takes. Here on the forum we've been told of several flying observatory roofs, one of which was mine. It was chained down to two strong-looking steel brackets but these turned out not to be as well made as they looked and one of them sheared. The roof had survived a wild night and the wind had dropped by morning. I had a check then went in for breakfast. When I came out, despite the reduced wind speeds, one of the observatory roofs had flown off and damaged another observatory. It took four of us to lift it back on and, remarkably, it was fine, as was the setup inside. You just need to right gust... (or should that be the wrong one?) Olly

Nice process. I know what you mean about the IFN sometimes. Where it's really faint and featureless, as around Stephan's Quintet, I think that struggling to bring it out comes at too high a cost in terms of other processing priorities. In this field, though, Arp's Loop and Holmberg IX really are part of the action and add value so, if the IFN is in there with structure to boot, I like it. I've had a play with this and thought the luminance data, in particular, were exceptional. Olly

Because I haven't ranted on here about Crayford focusers in several years, and because I'm getting mellow with old age, I'll just say that I would always choose a rack and pinion focuser over a Crayford if possible. Of the many Crayfords I've used, including Baader and Moonlite, none has been without issue. Finding the compromise between sufficient smoothness and sufficient grip is the problem, made worse by the heavy cameras I mostly use. The Crayford was designed to carry eyepieces, not heavy cameras. There are ways to refine your guiding without spending a penny, too. You may know them but here goes. - Running the mount slightly east-heavy by adjusting the counterweights outwards when they are east and inwards when they are west tends to stop the RA from oscillating across any backlash in the gears. The worm will always be pushing the wheel. - Using the Guide Assistant in PHD and applying its recommendations is often conducive to better guiding. - Experimenting with different durations of guide sub is interesting. I've found no one duration suits all mounts. Three or four seconds is fine with our roller-drive Mesus but I've gone as short as 0.5 sec with EQ sixes and an Avalon I had also thrived on subs shorter than those recommended by the maker. It used to be argued that very short subs gave a good graph because the scope was locked onto the stellar image, but that the stellar image itself was moving around due to the seeing, so the true guiding was not so good. This was called 'chasing the seeing' but, more recently this claim has been contested. I have to say that short subs seemed to work for me with the EQ6. Olly

I didn't know about this and am sorry to hear it. I'm lucky enough to know that Steve is every bit as affable and well informed in the flesh as he is on screen or in print. Speedy recovery. Olly

I think you had tighter focus for the first image but the second is still good. I would just give the colour balance a slight tweak since your background sky is a little green. Flats should be exposed so that the histogram peaks about a third of the way to saturation, so a third of the way from left to right on the graph. A bias is, by definition, taken at the camera's shortest possible exposure with no light getting in. If you match your light-free exposures to your flats they are called flat darks and if you match them to you lights (ie your pictures) they are called darks. Olly

Perhaps someone else could come in here. I'm not imaging at the moment and can't remember what PHD tells you about the RMS units. From memory I thought it gave the RMS in arcsecs if you'd given it the guider pixel size and FL, as you have. (To be quite honest my guiding has been so good for so long that I take little notice of it and have a rusty memory of troubleshooting. The joys of using Mesu mounts!) If it's giving you the error in terms of pixels at the guider then you just have to mulitiply the error according to the difference between guider pixel scale and imaging pixel scale. (If your guider runs at 4"PP and your imager at 2"PP then the error 'seen' by the imager is twice that of the guider. In this case you'd be looking for an RMS at the imager of 1 arcsec, requiring an RMS at the guider of half that, so 0.5 pixel at the guider. In the end, if the stars look tight and round then you're getting decent pictures. However, round stars are not a perfect indicator of optimal guiding because, when the RA and Dec errors are equal, as they may well be with an autoguider, they can be blurred equally in all directions. When calibrating the first Mesu we had here we alwys had round stars but, as we refined the PHD parameters, they became significantly smaller. Olly