ollypenrice

We once 'tested' a guest here to confirm that he really could see Mag 7. He could. (We had a star chart, picked a lonely mid magnitude naked eye star with the laser, and asked him to describe the starfield around it.) Another legend, indeed the legend, was E.E.Barnard. Two professional colleagues once called him to the eyepiece to ask him to confirm or deny a new possible double. His reply was something like, 'Yes, yes, clearly a double and the westerly component is itself a double.' (Source What Immortal Fire Within, William Sheehan.) One of my standard jokes here is, 'It's so faint that even Barnard had to wait till dark before he could see it naked eye...' 😄lly

The very dedicated image a star of the same spectral class of the sun (G2V) and experiment with exposure times until they get equivalent signal in each channel. They then keep the same proportion in exposure time for the main imaging run. Only a tiny minority do this and most, including myself, find that shooting at parity works OK because we have colour balancing routines available. Pixinsight's DBE or ABE work for me. I agree with Wim: just use one exposure time. The question of binning colour is a bit more complicated than that. Firstly the signal gain for CMOS cameras is nothing like the gain it offers with CCD. It may not be worth bothering. And then we have to consider the image scale of the system in arcsecs per pixel. Anyone imaging with a 'middle of the road' scale of about 2"PP might consider binning colour. If your scale unbinned is coarse, though, say 3.5"PP, binning may prove just too coarse and might degrade the final image. At the other end of the scale anyone imaging at 0.5"PP is probably chasing the impossible and would do well to bin everything including luminance. Finally some more advanced processing techniques use RGB-only stars, without the luminance over them. For this you need top quality stars so probably unbinned. Olly

Ah yes, seen elsewhere in the making! Olly

I found this very interesting since I've often wondered how production of specialist cameras was organized. In the past I've had great personal service from Rui, who takes us on this virtual tour, and I remain an enthusiastic Atik user. I've no vested interests other than as a happy user. Olly

It wasn't the star colour but the nebulosity colour which I felt had not been retained. When I blend short and long exposures on this target in Ps I have to increase, quite consderably, the colour saturation of the short subs which contain the nebula colour. Olly

Lots to like in the image. The HDR technique has preserved some perfectly discernible detail in the Trapezium but has allowed the colour to burn out to white. I don't know how to prevent that from happening in PI because I do my HDR blending in Photoshop. It must be possible, though. Olly

That looks OK but how does it stretch? Olly

My experiences have been positive and I'm very sorry that yours haven't. My more notable second hand purchases include two Mesu 200 mounts, a TEC140 and Takahashi FSQ106, two Meade 10 inch SCTs and a TV Ethos (from SGL's Moonshane so no worries about that one!) Given the number of troublesome refractors being reported when bought new, including some from Takahashi, I would want to inspect them first. It is easy to take a 'ballbearing test' kit along with you. I once did this test at a motorway service station half way between my house and the vendor's. The instrument gave an exemplary star test so I bought it. (You take a ballbearing glued to a black card to a considerable distance from the scope, illuminate it with a suitable beam placed nearby, and 'star' test on the BB in the usual way just both sides of focus. I would certainly want to return faulty goods. In my loft I have a number of items which I think to be of such poor quality that I wouldn't sell them at any price and when I'm selling something I don't much rate myself I do so at a very low price indeed with my opinion clearly declared and refund possible. Olly

Given the shameless mendacity of their words about the Hyperstar this does not surprise me. I won't say that bringing a refractor down to F3.25 is impossible but it strikes me as highly optimistic and very likely to bring other difficulties with it. Olly

I would certainly shoot the lum with the guided scope and the RGB with the unguided so as to drive any flexure artefacts onto the RGB where they will have little significance. And surely dithering during an exposure, especially an L exposure, will do far more harm than good. I solve the problem by not dithering on the dual rig and I can't say I miss it. You're combining two datasets with different residual noise which has a similar effect. Alternatively, with two filterwheels you could shoot through all filters on both scopes. This, though more expensive, would bring several benefits (provided flexure isn't an issue.) You'd be combining two sets of residual noise per channel and you could shoot L and blue at the highest elevation which can be a major benefit. (The best way to preprocess under this régime is slightly more laborious in that you should calibrate all files without stacking them and then sigma-stack all the calibrated files in a single pass.) Olly

For me 'one' is far better but the idea behind 'two' was sound. I'd go for about 80% 'one' and 20% 'two.' Olly

A very fine image. I find the background sky a bit 'colour busy' with some mottled green showing on my monitor. This should be an easy fix if you decide you agree. The stars could be cosmetically fixed in Ps using the radial blur feature within a circular selection but this is in no way scientific. I do prefer the less saturated images and agree that, once again, a refractor with small pixels can take on the big stuff. Olly

Once you've removed stars you can paste the linear image on top of the starless stretched one in Ps Blend Mode Lighten. Initialy it will be invisible. You can then stretch the top layer and the stars will begin to appear. Stop stretching at any point you like. Olly

This is great. The image has lost its red flush so letting the dust appear as dust rather than Ha. I would be tempted to try one tiny tweak (with no guarantee that it will work.) I'd see what happened if I gave the Colour Balance slider for shadows or mid tones just a nudge away from green and towards magenta. Olly

OK, the place to start is your achievable guiding accuracy. Tell PHD your guide scope pixel size and focal length and it will give you your average guide error in arcseconds. Whatever this is, you need to double it and that is the finest pixel scale at which it is worth imaging. What is the longest focal length, with your camera, which does not exceed this limit? You'll find this calculator useful. http://www.12dstring.me.uk/fovcalc.php If your focal length takes you to a pixel scale below 2x your guide RMS in arcseconds you are wasting your time. You may get a bigger image of a small galaxy but that image will contain no more detail than a smaller one. This may be darned annoying but the truth often is darned annoying! I've imaged with a 14 inch reflector with an FL of about 2.4 metres and a 5.5 inch refractor with a FL of 1 metre. The big one was working at about 0.6"PP and the small one at 0.9"PP. The seeing, however, said '0.9"PP, over and out.' There is no significant difference between the results in my view. Olly

I think you're concerning yourself with the wrong numbers. It's an error to think that a mount's main concern is to carry the required weight. Yes, it does have to do that but what it must also do is deliver a tracking accuracy which supports the pixel scale of the setup. If you are imaging at 2.0 arcseconds per pixel the mount, roughly speaking, must deliver an accuracy of about twice that - so 1 arcsecond RMS. This is certainly going to be the figure under guiding since the Skywatcher mounts, good as they are, have periodic errors twenty or thirty times that, unguided. So... I think your 200PDS/Canon will be working at 0.89"PP. This is a realistic scale on nights of exceptional seeing and with a mount which can deliver an RMS under guiding of 0.45 arcseconds or better. This guiding figure is at the absolute limit of what's normally possible with your mount and the demands on the seeing are very unlikely to be met in the UK. This means you would do just as well, or better, with a considerably shorter focal length. Essentially you are chasing the impossible. You would probably catch just as much detail, and have a vastly increased field of view, if you went for a far shorter focal length. Aperture fever belongs in visual observing. It is a snare and a delusion in imaging. Olly

This is a photo I've never seen before. Rather a fine one. Olly

People I trust say there is no disadvantage with the Chromas and they are cheaper (or, at least, less expensive!) so I'll be trying their OIII, I think. Olly

You're posting your question in the imaging section and, to the best of my knowledge, these are visual filters. Mark and Louis link to photographs taken using these filters but they are not astrophotos - because they not intended for that. Or so I believe. Olly

And, in fact, from a given focal length the size of a given object on your PC screen at full size depends on the pixel size of the sensor. Smaller pixels mean bigger image when each camera pixel is given one screen pixel. (That's what we mean by 'full size' or '100%.') Olly

I think the idea that focal length is connected with sensor size comes from the unfortunate term 'crop factor' which Mike defines above but which is best kept out of astrophotography entirely. Just stick to meaningful terms like focal length, sensor size, field of view and resolution in arcseconds per pixel. Avoid Crop Factor and Magnification. Olly

Yes, that's a different environment and one about which I know nothing. Olly

My two penn'orth... The Hyperstar website, in my view, treads a very thin line between the 'just about justifiable' and the fraudulent. Read it very carefully and then read plenty more about the system from other sources. Note, for example, that M52 with the Hyperstar will be tiny and nothing like what you've posted above. Note also that the only place on the Internet where you will find 'easy' and 'F2' mentioned together will be on that site. F2 will never be easy for perfectly simple reasons. It has an incredibly shallow depth of field so finding and holding focus will always be difficult. Add to this the fact that the focus is by means of the crude moving mirror system so that compounds the problem. And a shallow depth of field means that any mechanical tilt in your system will have one side of the chip in focus and the other not. And then there's collimation at F2 on a system you cannot look through. You see reasonable and sometimes good results from Hyperstars but delete the word easy right from the start and replace it with very difficult. A Hyperstar setup can be built up at less expense than a top refractor setup of similar focal length but check out the forums and see what people choose to use... That said, the original non-Edge SCT is not to be dismissed for conversion. Vignetting. You need to know the corrected circle or image circle of the scope and the length of the diagonal of your chip. The chip diagonal must not be larger than the corrected circle. Check out the manufacturers' spec sheets to find them. We cannot tell much about the vignetting in your image because it's a stretch. Instead open your linear (unstretched) image in a program which will allow you to read off the ADU values (the brightness) of the background sky in the dark corners and in the brighter middle. Only measure the background, away from stars and nebulosity. If the corners are vignetted (darkened) by no more than 25% it should be possible to rescue them by shooting flat fields. I have a 23% fall off in the corners of one rig and flats fix it. I check these values in AstroArt but most of the stacking/calibrating programs should let you do it. Olly

Alas, the young lady in question cannot be expected to oblige all her admirers with more than a charming smile! Olly

What did the postman bring? He didn't bring anything. We have a new one and he doesn't like bringing things. He likes sticking a slip in the letterbox saying he tried to bring something but we weren't in (when we were) so we should collect it the following day at the post office. When we go to the post office he hasn't brought it there either so we are invited to come back the next day. Now you might think that this is a pain in the bum but that's because you don't know how nice it is talking to our young postmistress! 😍 😁lly