ollypenrice

Yes, it is a full-scale revolution. Imaging will never be the same again. I've always loved broadband imaging for its natural colour, but envied the NB imagers their tiny stars and ability to stretch harder as a consequence. I now feel liberated from this constraint and rijuvenated as a consequence! The dusty targets are opened up, as are widefield vistas to short focal lengths. (The main reason for doing vast telescopic mosaics was to control field star sizes. I'll be happy to do them with shorter focal lengths now.) Olly

Very responsive to star removal-replacement since the starfield is dense and bright. Olly

Or maybe exit pupil? Olly

Reflectrion nebulae in Monoceros. TEC140, Atik 11000 CCD (remember those?) Mesu 200. 11 hours, LRGB. Olly

Depends what kind of spider. Some Newts, including Obsession and David Lukehurst, use a spider shaped as our cables are shaped. The net degree of artifact is the same as a cross-vaned spider but it is spread around the entire circle. (I have this on the reliable authority of optical genius Ralf Ottow.) It is possible to use a RASA with a cross-vaned cable equivalent but, personally, I really don't like star spikes. Olly

That didn't work for us. It gave slanted outer halos rather wittily parodied by vlaiv! Maybe I did something wrong but, with our system as illustrated, we get pretty good stars across the chip - even if they are not Takahashi/TEC 'good.' But, hey, for refractor stars the trick is to use a refractor!!! 🤣 If it works for you, stick with it. Olly

It's very crisp, but for constructive analysis I'd say you've retained red star colour but lost blue. I also think your black point is rather low. These two issues may well be related because most of the star colour is to be had around their edges where they are not too bright to be burned to white. If you clip those edges you lose star colour. I'd try a re-stretch from linear while leaving the black point much higher to see what that did for star colour. Olly Edit. Blue stars are hotter (therefore brighter) at a given distance which is why it's easier to burn their colour to white than it is with cooler red stars.

Very good indeed. I've edited this post because I scrolled down too fast to see your revisions. Olly

Well done! Lovely management of the very extreme dynamic range in this part of the target. Olly

Agreed. Olly

We find no artifacts with this arrangement... You do have an issue somewhere along the line, though, as shown by the blue stars. They show fantail-like flares radiating out from the centre. This may come from your cable arrangement but there was an SGL thread fairly recently about this. It seems a large batch of RASAs came from China with a defect producing this effect. Celestron are looking into it. Your colour looks fine to me, as does most of the image. I'd be wanting to soften the transition in brighteness atthe core, though. Olly Edit! crossed posts with Adam, there, who beat me to it.

We don't find any need for a dewheater with the RASA. We have a short dewshield as deep as the camera and the camera's heat and fan drafts do the rest. Olly

While I wouldn't bet my life on this diagram (which is for chip distance on flatteners) your defocused image looks consistent with the chip being too close, perhaps? Your stellar elongations look radial from the centre to me. Olly

So the system works by having the lens drop onto three upward pointing 'bottom-position' screws and is then pinned to them by screws pointing down? In that case it wouldn't matter if one of the 'bottom position' screws were bottomed out, but if more than one were bottomed out you'd lose a dimension of adjustment? I'm not the engineer! Olly

The B-mask is a focusing tool, not a collimation tool. I'm not sure from your wording whether or not you have this clear, but perhaps you do. There are four things to get right with fast systems. I'm familiar with the RASA, not the Hyperstar, but there will be lots of common ground. 1) Check that whatever camera you are using for collimation is tilt-free. If it isn't, you're doomed! Have a look at the video at the end. 2) Collimate looking at the zenith, as Tony says, and with the star dead central. Out of focus circles must be concentric. I'm not sure exactly how the pairs of antagonistic collimating screws work but you need to be sure that they are not at their limits of travel. The lens system must be 'suspended' between them and able to move both ways. It sounds as if this might not be the case for you. 3) the spacing must be right. 4) The focus must be nailed. A mulit-star autofocus routine is best for this. On the RASA we use Celestron's own autofocus motor and find it very good. Olly

I think you should begin by looking at your flats. The vignetting in the images is very easy to see - bright middle and dark corners. Your stretched flats should look like this and, if they don't, there is something wrong with them. Assuming they do look like this you could open them in a program which lets you mouse over the image and read off the ADU values. This will let you express the brightness of the corners as a percentage of the brightness of the centre. A fall-off of more than 25% is going to be a problem in terms of S/N ratio but the vignetting will still go. The second image is inverse-vignetted in green. If this was calibrated with flats it seems that the green has over-corrected, giving that magenta inner part. Could you post a stretched JPEG of your flats? Work-around: I don't think Pixinsight's DBE would have much trouble flattening these but how much signal there is in the corners I don't know. Are you using the reducer-flattener in the optical train? It's pretty much obligatory, I think. Olly

I don't believe expansion has a father. Like most great scientific theories it has several. Of those who contributed to it, the greatest (for me) has to be Einstein because, without him, the observers would have been looking at galaxies racing away from us - which isn't expansion. Expansion is unthinkable without a conception of relativity. I suspect that Einstein's role in expansion is similar, in some ways, to Galileo's role in heliocentricity. If we strip away all the oft-repeated nonsense about Galileo's spat with the church and remind ourselves that Aristarchus came before Copernicus, we might ask what Galileo really contributed to heliocentricity, given that he got De Revolutionibus suspended (not banned) by the church? Well, what he contributed was a demonstration that a rotating earth was physically possible, and that is an enormous contribution. The heliocentric model had been around for nearly 2000 years but had been consistently dismissed because it was perceived as being physically absurd. Tycho said the earth was, 'a hulking body, unfit for motion.' Galileo demonstrated that it wasn't. Similarly Einstein demonstrated (indeed had already demonstrated) that the expansion or contraction of space was physically possible. Olly

No, the last thing I'd want to use with a 3.5 metre focal length is a Barlow! That's a stunning picture... OPlly

It sounds like we are seeing the same thing, Peter. I'll stop worrying about it! Olly

I tried three eyepieces, all recently cleaned. Initially I wondered if the Ethos I started out with might have too much lens 'complexity' for the planets so I tried a simpler Meade UWA and then an even simpler orthoscopic, all of similar FL. The soft spot was the same in all cases but the Ethos gave more visual room to avoid it. Maybe it's my eyes. Olly

We had great seeing last night and I was using the 14 inch Meade SCT on Jupiter. It became clear that, right on axis, the view was inferior to that just off axis. There was a very slightly darker, softer central spot. I was using a Moonlite focuser which has a curiously long adapter, putting the star diagonal a long way back. I wondered if this was causing me to see the shadow of the secondary. I do have a regular Meade 2 inch back to try but I wondered if this was a known issue? Olly

I've put this notion on 'hold' for the moment, Dave. The offset between focusers would be considerable so it isn't clear that an observer would be able to move easily between eyepieces. If that weren't possible there would be little point in having the small scope on the same mount. Olly

I'm converting one observatory from imaging to visual, though it's a bit small for visual. Still, with the classic observatory chair and a rotating focuser you can observe in all directions and at all elevations. It is not possible to get behind the telescope on the west side - but you don't need to. By rotating the focuser you can sit at right angles to the scope. This chair can go right down to the ground or high enough for children to use when observing at low altitudes, so no parts of the sky are excluded. I made this chair about 16 years ago, in a larger and all-steel format, to live outside and be used with a long refractor we had. For many years it's been relegated to serving as a stand for our mailbox but I re-used the oblong steel main frame and cut off the rest, replacing those parts with smaller wooden ones. I just need to find a cushion wand we're good to go. Medium high. Low but can go lower. Olly

Thanks. Very ingenious. What I'm now wondering is how to make the pixel selections you suggest... Olly

We find ourselves in a strange position as deep sky imagers these days. CMOS pixels are getting smaller and smaller and there's no hardware binning possible. This is great for creating widefield images at short focal lengths without the need for mosaics, but how do we best exploit the optical resolution of large apertures and longer focal lengths? How effective is software binning and what's the best way to do it? Olly