ollypenrice

In Ps I false-coloured the sun by 1) converting from greyscale to RGB and 2) going into curves to shape each colour's curve by hand to give the colouration I was looking for. Curves gives you more control than levels, which is locked into a logarithmic stretch. I just saved this as an action making it it a one-click job thereafter, though some images needed mild tweaks after the action. If AP has Curves then that should work. If it doesn't, go back to Photoshop!!! Olly PS Using Curves let me produce a colouration which went from a deeper orange at lower brightness to a yellower orange for the bright parts.

It would take you just two minutes to try a 5 minute sub over a 3 minute! The calculations will take more than that... However, I'm not going to argue against understanding the theory since science advances using both theory and experiment. The best test in terms of simplicity and time economy would be to compare 3x5 with 5x3, I'd have thought? Olly

I've done a similar comparison between a 14 inch ODK and a TEC140 on targets which included M51. My findings were similar, the take-away being that there was precious little to choose between them. Sometimes the bigger scope out-resolved the smaller but sometimes the refractor data was 'nicer,' notably with better stars. The reflector couldn't image the the Horse, Flame and Alnitak at all. It was blown out by internal reflections, though I could crop out a decent horse from between the patches of glare. Regarding speed, the SCT has to be potentially faster simply because it collects more light. What matters isn't F ratio but flux per pixel. Bin up the pixel size and the SCT will be faster. The question is, how to do that wth a CMOS camera? This all comes down to how effective resampling is and how and when it's done in the processing. I've no experioence of that. You present the images at the same screen size but how did you get them there? I think that's the key question. Great images either way. Olly

I've had plenty of guests react in this way, disorientated by the number of visible stars. I've also had folks step outside and say, 'Just my luck, there's a band of cloud,' because this was their first proper view of the Milky Way. As a backpacking student in Greece, fifty years ago, I fell asleep on a dock waiting for a night ferry. I woke up to see a sky so starry that I thought the end of the world was coming: I'd seen nothing like it. On telling my father about this he understood because he'd lived through the blackouts of WW2 when living in the Lake District. Olly

And I took my visual kit, years ago, to somewhere in the south of Europe, I'm not sure where, to find that there was a lighthouse outside turning observing a case of, 'Now you see it, now you don't!' lly

I love living at a dark site (SQM reaches 22) and not just for the astronomy, though that was the main reason for coming here. It's natural for it to be dark at night and natural to have the lunar cycle dominating life at night. And it isn't even slightly inconvenient, ever. You just keep a head torch by the door and in the car. When I go to stay with friends and family who live with round-the-clock lighting I realize how disturbing it is to be detached from reality in this way. France just banned the outdoor gas heaters used by bars and cafés. Quite right too, a ridiculous idea to heat the outside world. It's only marginally less ridiculous to light up the whole of the outside world. Olly

The phrase, 'The war between the corpuscularians and the undulationalists' made me laugh out loud! What a delicious pair of terms. I've read a reasonable amount about the rivalry between the particle and the wave theories of light without encountering them before and I must thank Theresa Levitt for introducing me to them. Her book, A Short Bright Flash - Augustin Fresnel and the Birth of the Modern Lighthouse, is proving to be an unmitigated delight and I recommend it to you. ( Trivial note: The 's' in Fresnel is silent.) Aren't lighthouses the last thing an astronomer would want to read about? Inverse telescopes projecting bright beams into the night sky? Horrors! But yes, those of us interested in astronomy should be very interested in Augustin Fresnel, creator of the prismatic lighthouse lenses which bear his name, because he was a pioneer undulationalist working at the same time as Thomas Young, he of the two slits. Fresnel's involvement in the debate came at the time of the fall and rise and fall again of Napoleon, and of infighting between Laplace and Arago in French academia. Lusty stuff! Beats a good private eye thriller. The book's title appeared in the Further Reading appendix of an astronomy history book I've just had for review, a case of one good book leading to another. Olly

As Alacant said earlier, I'd leave your black point higher. When you bring it in too far you get an unnaturally jet black sky - which is also distractingly glossy - and you clip out faint data. It's tempting to use the black point to reduce gradients, but don't!!! Your level of gradient struck me as quite low. Even though I live at an exceptionally dark site I get that level of gradient sometimes, though the fact that you have a horizontal band is odd. (That's if your LP is at the level shown in the M52 unflattened image. If so I wouldn't use an LP filter.) Olly

For me the main culprits are too dark a background sky and a mottled look to the background. This could be from noise reduction. It looks very shiny, giving the 'vaseline on the lens' look. In Photoshop I like my background to be at 23 and the histogram to show no clipping. Your histo shows no clipping but your background is just over half what I go for, so it's very black. A lighter sky looks more natural. I can see that you'd want a hard stretch to bring out the faint outer arms (which you have done) but that could be done using a stretch which flattened off earlier so as to give the main spiral less of a stretch. In terms of colour balance your cyans are very high and are dominating the other colours. I can't say where this comes from but it can be remedied or avoided in many ways, dependng on what software you use? It would be helpful to know that. Olly

Finding a blank album online was harder than I expected but I settled on the Hama 'Londres' large format. At 20 euros it cames nicely bound and well thought-out, with thick pages of convincing paper plus non-stick transparent sheets between them. I used double sided photo tape to fix the prints in place. The book is just big enough for A4 prints in landscape and easily fits them in portrait. In landscape you do have a fair curvature on the left-hand page so I planned the order of the photos to put the full-width A4 pictures on the right hand one. I was pleased enough with this product to order another for my non-astro stuff. Anyone foolhardy enough to call in next week will find themselves bored to death by fifty-odd astrophotos... 👹lly

Your logic seems sound to me. There's a quick way to test your PA. Take the first and last images from the same continuous run and place one on top of the other. Align them such that a central star from the top image is perfectly placed over its equivalent on the first. This will show the total rotation over the run. (To be more precise, the rotation will be around the guide star so, if you happen to know where that is on the image - if it's on the image - you could align the two at that point.) Olly

Lots to be said for it! The book looks great. My stepdaughter makes an annual album like this and they do come out well. Olly

Yes, I paid more to start with so as to avoid the dreaded cartridges. The printer to which you link uses cartridges, whereas the new generation of ink tank printers cost more to buy but about 10x less to run. I aim to do a lot of photo printing and the cost difference is very soon made up, not to mention the gain in convenience by not changing cartridges or running out of them. Basically, photo printing with a tank printer is so cheap as not to be worth worrying about. I think the business model with cartridge printers is to sell the printer very cheaply but the cartridges at a huge profit. I think these are fine for daytime photos where the lab has ironed out the calibration wrinkles so that skin tones, skies, etc come out about right. They probably have software linked to the printer to read an image and adjust it automatixally. Astrophotos are very different, with no obvious points of reference for an automatic calibration. There is also a lot of black in many astrophotos and a real need to get them to the right brightness and colour neutrality. I probably spent about half an hour per image adjusting them in Photoshop in order to get a high quality result specific to this printer and paper. The other thing is paper quality: I'm using Epson Premium Glossy and this is beautiful stuff. Odd, isn't it. Like you, I lift my black point when preparing print images for magazines, whereas this machine needs it lowering. It's just a CMYK machine and maybe doesn't generate the best of jet blacks. Fortunately I can still get a perfactly good astro background, though. I suspect a 'blacker black' would be nice for things like studio macro backgrounds. Olly

The fact that my missus is an artist with a use for printers was a big help in this regard... Olly

Wanting to print my own photos, astro and regular, I recently bought an Epson 15000ET ink tank printer and have been getting the hang of it. It isn't a truly dedicated photo printer but it can handle A3 and costs nothing much to run. A set of 4 genuine Epson inks costs about 40 euros and I've printed nearly 100 A4 pictures and used just over half per bottle, so about 25 euros. The quality of the prints I'm able to produce strikes me as excellent. I wouldn't ask for more and am delighted. Daytime clear blue skies need to be massively reduced in saturation and astrophotos need a global reduction in saturation. I made these notes as I went along and share them in case they might be useful, though they are specific to my own experiences. Colour calibration from Datacolour Spider is reliable but saturation can be a little too high. Reduce for printing. Working space is Prophoto RGB. Screen brightness is distinctly too high for print. Mouse over the image in Curves and the data point shouldn’t go over half way on extended features. Use brightness tool in Ps Brightness and Contrast. (This may be because my elderly screen cannot be turned up enough in luminosity to meet the recommendations of the Spider.) Print needs to be processed for higher contrasts. Background sky needs to be brought down to 15-17 (from 20-23) in Photoshop. Initially use contrast tool to do this because it darkens more than it brightens. This helps the contrasts as well. Signal just above the background will not be distinguished from background in print. Galaxy tidal tails, galaxy outer glows, dusty background structures etc. all need to be separated using kinks in Ps Curves. This will look way too hard on screen to be adequate in print. Background sky at colour at parity in Ps can look a little green. Black clip the green channel by 1 point, sometimes edit-faded to 0.5. Stars tend to come out smaller and better controlled in print. (Nice.) Print eats fine grained noise to an extent and noise reduction is less intrusive but shouldn’t be over-done. It's been fun going through all my pictures and reworking them for the printer. I now have fifty A4 prints to put in an album, with the working title of One Thousand Hours of Starlight. Totted up, that's more or less the total exposure time. Over 41 days: it makes you think! And that's by no means all that I have on the PC, it's just what I've printed. The average of 20 hours per image includes a good number of mosaics so there are plenty of 10 hour images in the mix as well. Seeing the pictures in print is an enjoyable way of looking and them and has refreshed them for me. I now need the right album, one without a film over the prints. Any suggestions? Olly Here's a picture of a picture.

Everything I do, all the decisions I take, are based on experiment. I'm happy to listen to the theory and it's often informative, but it's only as good as the initial conditions it's given. Experiment will never lie. Olly

Because you haven't been looking at mine? 🤣 I do it absolutely all the time. To cheat you have to break the rules. Where are these rules? Natural justice and common decency certainly state that we shouldn't steal other people's data and that, if we use it with permission, we give a credit. In the write up it seems helpful to others to state the 'when and where' of the capture so I usually list that (if I can remember, but sometimes I forget to include it in my files.) I think it would be crazy not to use all the good data I have on a target. My rule is to make the best image I can. Olly

Shrewd! 😁lly

When we started using a dual rig here, dithering became too difficult to entertain. We didn't want to use the same exposure times in both rigs. With set-point cooled CCDs did this matter? Nope. Did we see a significant difference? Nope. Did we conclude that the loss of dithering was underwhelming? Yes. However, with some cameras it does make a big difference. Just don't turn it into a religion. I think that, with a lightweight mount without motorized Dec, you would need a supernatural polar alignment not to be getting natural Dec dither in any case. Olly

We know that larger optics cannot produce an increase in surface brightness and I remember being told off, years ago, for suggesting that they did! OK, so let's agree that large optics cannot increase surface brightness but only the surface area of the object's image at its original brightness. But, technicalities aside, what of it? Red supergiants have high absolute magnitudes despite low surface brightness because they have large surfaces. Subjectively, we call Betelgeuse a bright star despite its low surface brightness because we can easily see it. It is subjectively bright enough to see. Since seeing is, by definition, a subjective experience I don't think we should be unduly influenced by the term 'surface brightness.' We certainly shouldn't be deterred from buying large scopes because of it. I'm minded of this after reading of Stu's good fortune in this thread:

There's always good work for a large Dob. When I read your statements about more weight being needed behind the mirror, I thought of my own years with 20 inch. On mine, I fitted a counterweight arm outboard of the mirror box and extending below the mirror, but the low profile of your scope wouldn't let such an arm clear the ground. It's admirably compact. Enjoy the view! Olly

Interesting. I think the high imaging post counts are partly due to the fact that the technology is ever-evolving, sparking the need for new exchanges of ideas on how to make it behave! The good old visual telescope has a tendency just to work... (When did you read, 'I went out to the Dob last night and when I pushed it west it went east, and then, when I left it alone, it started spinning round on its ground board and flew off into the hedge?') 🤣 Another perspective: a dark site is even more advantageous to visual observers than imagers (who can shoot in narrowband) yet, when I opened an astronomy-centred guest house at a dark site nearly twenty years ago, it was the imagers who did most of the booking. The overwhelming predominance of imaging bookings has never changed. Olly

Sorry, I used the wrong term, I'm afraid. Obviously the increased FL reduces the FOV but what I meant to point out is that it reduces the size of the corrected circle as well. I was being careless. I've no idea how the corrected circle will hold up if you use a Powermate. I can see why you'd want a longer focal length in order to increase resolution, but why would you want a reduced field of view? You can reduce it by cropping the image. When the resolution in arcseconds per pixel is a constant there is no difference between the effects of optical vignetting (ie corrected circle reduction), reduced sensor size or cropping in post processing. They will all give you the same image. BTW, since the scope is a Petzval with a fixed corrector lens it may not co-operate with a regular Barlow or Powermate. TeleVue make a similar Petzval so there may be information on their site regarding this combination. Olly

I once took some darks on my Tak 106/Atik rig. I had a snug-fitting metal objective cover over the scope, a closed electric filterwheel and, for good measure, put the Ha filter into the lightpath - and shot these darks in the closed observatory. I then compared them with darks taken with the camera off and its metal, o-ring sealed chip cover screwed on. Set point cooling was running for both. These were 30 minute darks but the off-scope ones were 'darker.' How it did it I do not know, but light got in somehow. Not much, but the difference was measurable and consistent. However, you should be able to get panel flats if you shoot them in the dark. I can see twilight flats being a problem and vaguely remember someone else posting about this with a Newt. I'd try a direct question putting '130 PDS Flats' in the title. Olly

Yes, a shorter FL lens would serve you well with this camera. Olly