ollypenrice

...but CMOS has undercut CCD and astrophotographers will pay what it takes... lly

I've no idea what a smartphone is but this sounds like good news for astrophotographers... 😁lly

I must confess that, after an excellent dinner at my club, I found myself obliged to take the Clapham omnibus to my London chambers. The only available seat being next to the authoress of a well-known etiquette book, I beseated myself next to her and, unfortunately, bortled loudly. I looked at the lady earnestly and said, in my finest stage whisper, 'Think nothing of it, my dear. They'll all assume it was I.' Olly

Certainly... ahem, excuse me... certainly not. It was Hubert (our bloodhound.)

Various things can cause a gentlemen to bortle. Too much ruby Port at his club, of an afternoon, particularly after Champagne with the fish, can bring it on. These ghastly, gassy foreign beers can cause one to bortle as well - and I'm sure that imbibing Coca-Cola would be a dead cert, though a gentleman wouldn't do that. I also remember one chap's 1932 Blower Bentley bortling at Balmoral after getting a partridge stuck in its fuel filter. Olly

I know, you have to laugh! However, the oddest one I've had concerned the satellite images on Sat 24. I once had a perfectly clear night when the satellite showed solid cloud over my entire region. A week later I then had the reverse. This is inexplicable. A friend helped organize the world hang gliding championships, locally, and they employed their own weatherman from Meteo France to attend the event and set up his equipment to give super-accurate short term forecasts. I later asked how useful this was and my friend leaned back in his chair, stroked his beard and said slowly, 'Well... we won't be having him again.' lly

If your question is partly about the reaction of amateur astronomers to SCTs and Maks, I'd make these observations: - Cats used to be relatively inexpensive because other designs, in the past, were much more expensive. They have spherical mirrors which are easy to make. They were also given a boost by well-funded advertizing from Meade and Celestron. Then fast Newtonians and good refractors became vastly less expensive when the Chinese started making decent ones. This leveling of the price playing field opened up more choices to us all. - The rise of digital deep sky imaging favours other designs. Large apertures are no longer needed and long focal lengths are often a limitation. As pixels get smaller, focal lengths can and will follow. Also, even those not interested in deep sky imaging will have an eye on the potential re-sale values of their purchases and notice that the imaging-unfriendly SCTs and Maks take very big hits on the used market. Olly

The main thing is to give the lower brightnesses a much harder stretch than the higher, so the nebulosity is stretched harder than the stars. One way to do this is to stretch in Curves rather than levels. Levels does stretch the faint stuff harder than the bright but you can do so more forcefully in Curves. Firstly I got rid of the LP gradient in Pixinsight using ABE for speed. DBE is better. I then went into Photoshop. You can just use a regular log stretch in Levels at first, bringing the background sky up to about 22 in Photoshop values. Once the background is at about 22 don't stretch it again. Pin it at 22 and stretch above it using a curve like the one below, which will avoid stretching the bright stars too much. Only do repeat iterations of small stretches like this. I also ran several iterations of Pro Digital Astronomy Tools 'Make Stars Smaller.' These were formerly known as Noel's Actions and are worth having. A good way to get more out of your Ha data in one shot colour or RGB images is to go to Ps Adjustments-Selective Colour and lower the cyans in red. (Top slider goes left.) This is spectacular. The methods I'm suggesting will always work better with more data because you are stretching the hell out of the faint stuff! Olly Edit: As Bruno says, you could make a starless version with Starnet++ but then put the linear original on top in Ps layers, change the blend mode to Lighten and stretch the linear image gently. Only the stars will appear on the starless one. I'd have done that but my copy of Starnet has vanished from this PC!

I don't pulse guide but I wonder if this is backlash? If so, running one-side-heavy (as you do in RA to reduce backlash,) doesn't work well because, towards the zenith, the imbalance approaches zero. The usual trick is to run slightly misaligned and guide in Dec in only one direction, the direction needed to correct the misalignment. You find this by trying both then swapping after the flip. It does work reasonably well. Obviously, if this is caused by backlash then reducing that mechanically is the best bet. Olly

What's the catch with catadiptics? The focal length. Dead simple and just that. For the rest, they have a lot going for them (apart from the dewing issue which a dew heater and camping mat extender will fix.) I don't think the smaller SCTs give the tightest stars but my current one, a 14 inch, actually gives great little stars, nicely coloured. However, there are objects which are very disappointing in the 14 inch, as compared with a faster Newtonian. The Veil, for instance, is far too heavily cropped and lacks apparent brightness. Indeed all the extended objects suffer at the enormous focal length. Even a forgiving F6 Newt of the same aperture would make a huge difference. These are comments about visual observing, by the way. I don't image with an SCT largely because of the F ratio in an age of cameras with small pixels. Olly

It's not unusual for refractors to struggle with blue stars at the best of times. Also, any high haze will exacerbate the problem. However, the halos are in two forms, inner and outer, and the fact that they have hard edges argues against their arising from blue bloat. They are more likely to be from internal reflections when they have a hard edge, I'd say. Olly

I'd add a very important '7' to this list. Focus. I'd be very surprised if those stars could not be made smaller by better focus. Olly

We have actually had two brief skirmishes withdistant night club lasers here but the big hitting professional observatories stamped on them immediately. Olly

Great in a night club! (Not that I recall ever going to a night club...) 😁lly

Too slow for DS imaging? And unlikely to have either a large enough corrected circle or be free from artifacts like tilt, reflection, etc. It has the right numbers and it has a rack and pinion focuser. Looks pretty tasty to me, on paper at least. Olly

Brain fade. There's a lot of it about, these days, between my ears... Thanks, I'll fix it. lly

You should have seen them before the repair processing, Vlad! That's as good as I could get them without spending even longer on fixing those halos around medium bright stars. It's a RASA thing, certainly, and the harder you're stretching the surrounding region the greater the problem is. It's also true, I think, that this dusty region does produce genuine stellar light scatter at source. You're also right that the tight cores are odd, but that's the RASA again. You get a tight, bright stellar core surrounded by a rather flat and much fainter extended glow. I was just talking to Tom O'Donoghue about this. Note how small and tight the brightest stars of the main cluster are as they sit in the blue nebulosity. They are smaller and tighter than my refractor M45 stellar cores but I think those RASA outer glows are swamped by the blue nebulosity. No optic is perfect. On the other hand the RASA has got us as deep as this in three ours (per panel.) On balance I like it. But the halos are inherent to the data, they are not processing artifacts. Harry, you're back! I was just beginning to get worried about you and was on the point of dropping you a line. I am slowing down a bit for sure, though! 🤣 Olly

Most kind, Sir!

Why seven??? 😁 Because it proved much easier to make a 6 panel then to shoot a seventh with the main cluster in the middle. It helped Registar calibrate the four panels around the cluster image into a seamless block to which two more panels could then be added. When the faint stuff is going to be stretched till it screams a mosaic needs to be seamless. Joint project with Paul Kummer who owns most of the rig in our robotic shed. That's a RASA 8 with ASI2600 OSC on Avalon Linear Fast Reverse. Paul handled remote capture and also did the DBE on each of the panels. I did the post processing. Initially I was worried that we hadn't gone very deep in 3 hours per panel but it turns out there was plenty of signal buried away in there and, despite the hard stretching, this has had no noise reduction whatever. It wasn't an easy processing job, though, unlike the recent Veil we posted. As ever, the Equalize adjustment in Photoshop proved invaluable, firstly to diagnose the accuracy of the mosaic blending and redo it where necessary. Then I used an equalized image as a luminance mask for an extra stretch to bring out the faint stuff. Olly

I'd always prefer to guide a reflector with an OAG because of the potential for mirror movement. Essentially one problem, these days, is that very small pixels and very long focal lengths are not good bedfellows. You could have the same real resolution and a much wider FOV if using a shorter focal length. The required guiding precision depends on the sampling rate of the imaging rig and is affected both by FL and pixel size. In the old days pixel size didn't vary all that much so focal length was a good shorthand but now cameras vary in pixel size by a factor of more than three times so FL alone is no longer useful. Olly

I agree with your reservations, Mick, so please don't apologize. I've struggled with this image at every stage of its long evolution and this is as good as I've got it. It has two problems. 1) It would be deep by narrowband standards, I think it's fair to say, yet it's a broadband image in a dense starfield, so the stars are always going to be a challenge since they are not going to be narrowband-sized. I was hoping to de-star and re-star the image but Starnet++ would not handle the file sizes and our internet won't handle the StarXterminator download! 2) I've never liked the colour, particularly the red. The RASA data improved it but it's still too pink. The RASA 8 (this one and others, I believe) doesn't produce the tightest stars. There's the F2 light cone, the large central obstruction and the need to route the cables in front of the corrector which all play a part, I guess. Actually getting round stars across the chip has taken considerable effort with tilt and collimation and they are still not perfect. Finding optimal focus is, at present, a matter of compromising between corners. More work on collimation, tilt and cable route may help. Going deep in broadband in such a dense starfield is difficult. Excuses excuses! 😁lly

Yes, in that case I think The Lazy Astronomer is right: your frames won't overlap perfectly. There's a free/donation plug-in for Photoshop which I think you'd find very useful. Hasta La Vista Green is available on Regelio Bernal Andreo's website, Deep Sky Colors. It would help dispense with the green bias in your image. To check your background sky colour in Photoshop is easy. Choose the colour sampler tool in the eyedropper menu, set the sample to 3x3 or 5x5 average (in the top menu) and measure the background in different places. I like a background with all three colours the same and ideally prefer 23/23/23 RGB in Photoshop's units. Olly

...and another thing is that your 130 is a known quantity in terms of optical performance. You don't say how happy you are with it but, all being well, it should be an easy scope to use with no time lost to collimation or other issues. Plus easy maintenance. (Wipe it down with a damp cloth! 😁) Reluctantly I've come to the conclusion that Vlaiv is right in estimating 1.5 arcsecs PP as a probable minimum. I've been working at about 0.9 but I don't think I'm gaining anything over 1.5 or so. What's your guiding RMS in arcseconds? Olly

Are you sure that you will really obtain more DS detail from a scope of this focal length than with your 130 refractor? I wouldn't bank on it. You will need very good, stable seeing and very good tracking accuracy. I've found no significant difference between the final detail caught with a TEC140 at a metre FL and that caught with a 14 inch catadioptric at 2.4 metres FL (on a mount running at around 0.3"RMS.) The Vixen would obviously give you a bigger image but would that bigger image hold up at 100% and, if it didn't, would it be any better than an image shot at lower nominal resolution? I wrote an article on this very comparison for Astronomy Now and concluded that I was limited not by FL but by seeing. My site is pretty good. Pixels are now getting so small that the rationale behind long focal length is thrown into doubt. Jeremy is right that the figure on this scope is finalized by having a thicker aluminium coat around the edge of the mirror, which does have implications long term. However, I think the main question is, Will this really give you more detail? Olly

Aperture is not so important in AP because the camera can collect light over time, which the eye cannot do. A quick list of priorities would be: - Choosing a focal length at which you want to image. See what your camera's APS-c chip will frame at different focal lengths. If you have any doubts about your mount it would be best to keep the focal length short so that less tracking accuracy is needed. - Colour correction, particularly at the short wavelength (blue) end. - Focuser quality. It needs to be a properly on-axis, tilt-free focuser which will hold the camera's weight. - Image circle must be large enough for your chip unless you are willing to crop. In your situation I would think about the mount first and whether or not it will autoguide well. Until you have an accurate, well guided mount you are, to be brutally honest, rather wasting your time. Olly