ollypenrice

Like vlaiv, I would go for a rack and pinion. I host a number of robotic scopes and find that the R and P focusers never give any bother but the Crayfords regularly do so, including reputable ones like Baader and Moonlite. When you're counting steps or supporting a heavy weight against gravity you want more than friction to keep things consistent. (Yes, I use friction drive mounts but they are in balance, not being tested against significant resistance.) Why it seems to be so hard to make a focuser is something of a mystery. Olly

This is just for the record to help anyone who might have a similar problem. The Celestron RASA motor focus on a setup based here stopped giving decent focus and eventually drove the scope right to one end of focus then stopped doing anything. On investigation I was relieved to find the grub screws locking the motor shaft to the focuser shaft were no longer fully tight, so I tightened them and we ran the focuser calibration routine. It failed to complete the process and subsequent attempts also failed. In the end it stopped responding to requests to calibrate and we feared a bent focuser shaft as quite often described on various forums. If you get to this stage, don't give up. We found that when the motor failed to respond there was no indication that it was fighting to turn a jammed shaft. No vibration, no heat, no sign of energy at work. Also, the focuser shaft turned easily with a spanner on the flats so we continued to suspect a control, rather than a mechanical, problem. Rebooting the USB, changing ports and replacing the power supply had no effect. We then wound the focuser shaft round for maybe ten full full revolutions with the spanner to get it away from the end of its travel and tried the calibration command again. We also loosened and re-tightened the two screws bolting the motor unit to the scope. It worked and the focuser then did a couple of excellent autofocus routines so, for now, all is well. We can't be certain which of the operations was the critical one but we don't think it was loosening-tightening the motor screws. (I broke the golden rule of one test at once!) We think the step in bold type was the breakthrough and that the problem may have started with the loose grub screws. Olly

Your badly flattened version, in the problem corner, shows channel brightnesses in descending order, of red, blue, green, in the background sky. they are miles out of balance with green being far too low. This suggests that your panel has a corner which is brighter in green than elsewhere, no? Olly

First thing is to look at the stars. Good ones are small and round, which yours seem to be. It's quite normal to see only the inner galactic core. If you saw much more than that, most of the image would be over exposed. Do you know about stretching images in post processing? Stacking is quick and free so I would suggest experimenting with how far down the 'quality food chain' to go. Olly

Well, my skies are sometimes SQM22 and I get one image a year, at most, which has no colour gradient worth correcting. They nearly all do. Who knows where they come from, but since I get them on an OSC and they follow the camera after the flip, I'm going to say that the camera is my prime suspect. It can't be the sky or local light sources if it doesn't follow the flip. ABE or DBE offer a quick and painless fix nearly all the time. I generally go for 1/3 of full capacity for flats but when I've been higher, up to 2/3, I haven't seen any problems. The single flat I use when being lazy is the luminance flat. Firstly it nearly always agrees with the others and, secondly, the image will be illuminated by the luminance by definition, so that, too, flattens the other channels to some extent. Olly

Sorry, me again! You ask, ' ...why is background with gradient? How come that pixels on the left part of the image have stronger red response than pixel on the right part of the image, while blue/green response is opposite?' That's a very good question but I don't think it has anything to do with flats. At least, it doesn't in my case because I get this kind of gradient regularly on all sorts of cameras but, most obviously, on the two OSC cameras I've used. It's extremely pronounced on our ASI 2600/RASA data and, after the flip, it follows the camera and not the sky. However, we have yet to shoot any flats in the RASA so it can't be a product of flats. Nor did flats remove similar gradients from my OSC CCD. I don't expect them to remove the RASA gradients either but perhaps they will. I'll post on this when I've done so because it's an interresting problem. Olly

My explanation is this: most of the time I find it makes no difference whatever. I fully accept that others might find big differences between flats in different filters but, quite simply, I don't find that. Maybe one image in twenty throws up a filter with its own dust bunny and, when it does, I make a flat for it. I know the theory and if I have a guest who wants to make 'flats per filter' we just do them, but when I'm imaging for myself I only do so if there's a need. It will also depend on the kit. I find dust bunnies are not produced by dust on the filters but elsewhere, probably the chip window. And I also find that the RASA, at F2, doesn't seem to produce dust bunnies at all, probably because they are so far out of focus. Does exposure time matter? I'd have thought that what matters is that the flat in use be properly exposed. The linearity of modern cameras is what makes flats work in the first place, no? Some people like to do things by the book, some are pragmatists who base their decision on seeing the results. I'm a pragmatist but I don't insist that this is the best way to be. It's just a welcome shortcut on a night when you have 24 hours of data to calibrate in the morning. (OK, that's a one-off record but we did once manage it with three scopes. ) Olly

Superb, and the star technique is spot on. Olly

Very nice image, natural and clean. I have to say that I thought the stars pretty small for a telescope of such modest aperture. As vlaiv says, seen at a reduced size the image looks excellent. You also have a good, flat, neutral background sky and that's so important. Olly

Certainly. Autoguiding is the life-blood of astrophotography and is remarkably easy... Olly

The problem here, though, is that the guy in the video is able to slide the stars downwards behind a fixed foreground. We can't do that with the OP's image because lots of sky is visible below the horizontal branches... Heh heh, that's lovely, Francis. Chapeau, as they say over here! Olly

In the end the best I could do, which isn't very good, was to paste one onto the other (foreground on top), align them and use the colour select tool to pick the bright sky, after which I erased it. You have the Coathanger Cluster showing particularly sweetly. One of my favourites! Nest time, pick a simple foreground!!! Another thought: take a single, long sub for the foreground then, as quickly as possible, start the astro subs. Align the stacks onto the first sub when stacking the astros. Olly

That was my idea was well, Alan, but it increases the distances between foreground objects so the foreground image still fails to cover the sky! I'm now playing with another idea... Olly

Hmmm, you've made life hard for yourself by choosing a foreground which has sky both above it and below it! 😜 (Those branches sticking out into the sky on the right hand side.) Without that, you could just paste the foreground image on top of the sky image, and move the foreground up a bit to cover the blur. However, I do have an idea... I'll try it and get back if it works! 🤣lly

I'm not sure your drift procedure was correct. These are the key steps: 1) Orientate the camera along RA and Dec. To do this, take a 5 second sub while using a slow slew speed. This will produce star trails. Rotate the camera and repeat until your trails are parallel with one side of the chip. It doesn't matter which. 2) Find out which way your image is appearing in terms of north, south, east, west. (Point south and turn off the tracking. Take a few short shots. The stars will move west between subs so now you know east-west. Lower the angle of the scope till you see the horizon appear. It will appear on the southern edge of the image first so now you know N-S. Mark this on a piece of paper! 3) Observe a star close to the equator and in the south. If it drifts south, move the mount west. If it drifts north, move the mount east. Ignore movement N-S. 4) Observe a star around 20 degrees up in the east, or as close as you can get to that. If it drifts south, raise the mount. If it drifts north, lower the mount. They key thing is that you need to be clear about N-S-E-W in the image you're using. In analyzing mis-shaped stars, anything with two blobs close together, or overlapping each other, is likely to be produced by backlash in the drives. What's happening is that the star is being imaged on one side of the backlash then on the other but is flopping quickly from one side to the other, leaving little or no trace of its journey. If one blob is bigger than the other, it means the mount is spending longer sitting on that side of backlash than on the the other. If you have evenly thick trails which get longer as the subs get longer then you have a problem with periodic error. Olly

A simple trick which is particularly effective with OSC data on Ha-rich targets is to use Photoshop's Selective Colour (Image-Adjustments-Selective Colour)) to lower the cyans in red. This brings up the Ha signal remarkably well. Here I just used it on a screen grab but it would be far better, of course in an uncompressed format. If you want to get clever you can use the modified version as a luminance layer over the original, allowing you to see more faint Ha without changing the colour, or you can apply it fully as luminance and paritally as colour, or whatever. After de-starring in Starnet, how did you re-apply the stars? I tend to do it in Ps by opening the starless version, pasting the top layer in linear form on top, and changing the blend mode to Lighten. At first the top layer will then be entirely invisible, but if you give it a simple log stretch in Levels the stars will begin to appear. You can stop stretching at any time, meaning you can keep the stars at any size you like. Olly

Very good. Just one thing, though: the nice bit of video graphics allegedly showing the merging of broadband red, green and blue-filtered images produces an image in the Hubble palette rather than in RGB, I would say. A digression on the subject of eyes... If, while talking to someone, we roll our eyes to one side and back again, with our eyelids open, the eye movement is fairly innocent. If, however, we close our eyelids during the eye movement, open them with our eyes looking away, close them while rolling them back and then open them again, the effect is 'very disturbing' for the other person. I suspect that Dr Becky knows this... Olly

Unless we introduce mechanical stabilization, a 10x binocular is what it is in terms of image stability. Maybe an increase in field of view would give a tiny impression of greater stability but really the instability lies in the hands of the observer and the ability of their eye-brain to compensate for it. Most people - and this certainly applies to me - get a better experience from reduced magnification as they age. I used to like 10x binoculars but now I don't, I much prefer 8x. (This is hand holding non-stabilized instruments.) Comfort in the hand and the right eye-relief do matter but, in the end, not everyone benefits from 10x. I now go for 8x42 rather than 10x50. Weight works both ways in that ultra-light binos lack the 'anvil effect' which damps out small vibrations but, of course, too much weight is tiring to hold well. Olly Edit: Chris's review above rings very true indeed for me. I have a different make but entirely agree with what he says.

That's very good, with a proper 'teal blue' for the OIII and a red gamut which I think could distinguish convincingly Ha fom SII. Olly

Although the RGB mapping is a given, with the Hubble, you can still adjust hue to adjust the nature of each primary colour channel. Photoshop's Selective Colour gives three way control more than just red, green and blue, in fact. Olly

This is a good way to spend 5 minutes. The HP's key feature, its underlying rationale, is that it 'stretches' the colour data contained in OIII, Ha and SII across the full range of visible colour spectrum. As these data arrive from nature, they are compressed into the right hand half of the spectrum, so it goes OIII Ha SII. This leaves most of green and all of blue, over to the left, unused. All the HP does is move everything to the left. SII makes the smallest movement into the middle of red. Ha makes the jump from deep red to the middle of green and OIII jumps from the green-blue border to the middle of blue. Although the HP is not a natural colour palette it does, in this sense, respect nature in not inverting the captured wavelengths. Any wavelength which, at capture, was longer than another at capture will remain longer in the final image - and so with any which was shorter. Often in amateur AP discussions there seems to be an underlying notion that the Hubble Palette is just a way of colourizing a picture, but that's not so. It has a solid physical foundation. (I still don't like it but that's because I think nature is the greatest painter in the universe...) Olly

I agree with Vlaiv and think that, if you don't like green in astrophotos, don't use the Hubble Palette. As Vlaiv points out, in most cases the Ha, mapped to green in the Hubble palette, has by far the strongest signal. So what do you expect? There is also a rationale behind the HP which is often overlooked and that is that, looking at where the true wavelength of each emission lies, the HP simply stretches the separation between them but leaves them in the original order left to right. (In that sense it has something in common with the stretching of brightness where, again, the original order of captured brightnesses is respected.) I thought this was well worth five minutes: Olly

Do you aim to use the 70mm scope on the Adventurer mount? If so I think you'll find the focal length excessive since 420mm really requires motorized drives in RA and Dec and, at some stage, autogudiding. Olly

Because these scopes are not suited to deep sky imaging they don't command high prices. (People get started then become interested in DS imaging and look for something better adapted.) I still have a big SCT though, indeed a Meade. Olly

It shows very clearly in my 2017 Ha-OIII-LRGB version... My image's best fit is with your 2021 rendition. Olly