ollypenrice

Earlier in the thread the term 'crushing' the blacks was used. I'm not sure if this is the way I'd put it. The usual term is 'clipping' the blacks, which seems more accurate to me. Clipping means, quite simply, cutting them out. 'Crushing' implies compressing them. Both sins are possible in post processing but I think 'clipping' is the regular culprit. Here's a healthy image with a healthy histogram. And, below, we have the same image black clipped. Compare the histograms to see that what has been clipped is the left hand side of the histogram and notably the flat line before the peak. This contained all the faint data. The temptation, which must be resisted, is to use black clipping like this to remove LP, gradients, noise, etc. Each needs to be addressed in its own terms. Clipping clips everything, most notably the picture! Olly

It's great to see her remembered in this way. I'm a keen reader of astronomy history so I do know of her work and her contribution but spreading knowledge of it is a great thing. Olly

Like Ken, above, I don't think the 3.3 will cover anything but a tiny chip. Mine didn't, for sure. I did once defork a 10 inch. It isn't difficult, but do it with at least one extra pair of hands. It helps to get a scissor jack between the tines just at the moment of separation. Don't force it too hard (indeed hardly at all) but use it to get a few thou of extra clearance. It might be worth raising the deadly of issue of F ratio as well. 😄 This has been done to death on here but, briefly, there is no equivalence between reducing F ratio by increasing aperture (the camera lens scenario) and reducing it by reducing focal length (the issue of this thread.) The real way to counteract the slowness of F10 is to increase the effective pixel size by binning 2x2 or 3x3. What matters is the ratio between area of aperture and area of (effective) pixel. FOV is a different matter but, here, the limiting factor will soon become the edge of field distortions which defeat the reducer. These scopes will never give a large corrected circle. Having gone through all this myself I concluded that the fork mounted SCT is best left as it is, in alt-az mode, and used for visual observing and planetary imaging. I found the wedge a nightmare to align and the mount far too springy and erratic for DS imaging. For DS a little Newt or small refractor, with a small-pixel camera, will do a better job more easily and the SCT will still be there to enjoy for what it does well. Four years on the 14 inch at the top of the thread is still exactly as it was then. Olly

The thing is that, with these scopes, you can obtain the widest possible FOV without a reducer by using a 2 inch back, diagonal and long focal length EPs. (The baffle tube imposes the final limit so the reducer doesn't get you anywhere visually.) There was an F3.3 reducer in the old days but this was intended for the tiny webcam chips which were popular in those old days. It didn't work for visual and nobody is using that kind of tiny chip any more. I suspect the same would be true of most faster reducers. What would be your intended use if you found a fast reducer? Olly

Ah, but the thing about Starnet is that you don't have to remove the stars and call it a day. You can remove them and put them back at a reduced opacity, which doesn't require the same signal strength as a full removal. I've only done limited experimenting but I put the stars back using blend mode lighten as a top layer in Photoshop. (Put the linear image on top of the stretched starless, choose blend mode lighten, and stretch the top layer to taste.) Olly

That's a glory, Lee! The Samyang lens is a stunning asset to astrophotography. In conjunction with Starnet for star reduction it must be the definitive widefield setup. 90 mins with a 7nm filter is incredible. Olly

The sub length question has been re-answered by CMOS cameras which work well with short subs. High read-noise CCDs do not. CCDs do, from experience rather than theory, benefit from long subs. Plenty of CMOS imagers are finding 300secs at unity gain to be effective. In the end the key number is the total integration time and even with the fastest setup known to man you will be scratching the surface with half an hour per filter. The fastest setup currently appearing regularly on SGL is probably Gorann's RASA 8 and CMOS rig. Check out his posts and integration times. Noise increases as the square root of the signal. What's less clear is what this really means in terms of a particular photo project. We don't stretch each project in the same way. Sometimes we want to wring the neck out of faint signal, sometimes we don't. Sometimes heavy handed noise reduction on certain parts of the image will be unobtrusive and acceptable, sometimes it won't. (How much structure is there in this faint signal? The more there is, the less acceptable NR routines will be.) The theory will only take you so far. You'll meet the reality when you start to work on your project but half an hour per filter will not produce a great image with present technology.. Olly

The very high end refractors have always been there. The outstandingly good budget ones, now plentiful, have not. This is the age of the affordable telescope so let's not get hung up on how expensive the high end stuff is. The real story is about how good the cheap stuff now is. And if you're short of cash, why go for a refractor in the first place? There are insanely good budget Newts available these days and blessèd be John Dobson for showing us how to mount them. The Apos and semi-apos debate is a bit of a red herring since 1) the difference is very slight, 2) the individual scope rather than it's generic type is more influential of its performance and, 3) the issue is not the same in visual observing as in imaging. (In imaging you are far more likely to be undone by the focuser than by the level of apochromatism. Other key things in imaging, more important than colour correction, are flat field size and thermal stability of focus.) And then there's personal taste. Some people are upset by the Tak FSQ's 'inverse lighthouse beams' on bright stars and some are not. I'm a 'not.' Olly

Currently you shoot at F5 and a focal length of 750mm. F5 is good. Any faster and you'll almost certainly spend a lot of time struggling to get it all to work. So is there something about 750mm that doesn't please you? Is it too long or too short? To my mind it's a slightly 'in between' focal length, a bit short for galaxies and a bit long for nebulae. (I used to feel this about a 1000mm FL a few years ago when pixels were larger, but small pixels have made a metre a good galaxy FL nowadays.) If you want shorter, then the small modern refractors are excellent, as is the ultra-short Samyang 135 lens, which is also fast without being a hassle. Or are you not happy with the image quality of the present scope? Olly

I can't pretend to understand the over-correcting flats problem because we were once plagued by it using an SX camera/Nebulosity/MAC computer (none of them mine. I was just the man on the ground.) We never got to the bottom of it. The flats procedure is different for CCD and CMOS. With CCD you can shoot flats then use a master bias as a dark-for-flats. It will work fine. With CMOS you must make dedicated darks-for-flats at the same exposure settings as your flats. As I understand it bias are not useful in CMOS imaging. I don't think your final image tells us anything about the effectiveness of flats/no flats because it is black clipped, so the faint brightnesses which are corrected by flats are absent from the image in any case. Above all, don't throw anything away. I'm sure you have a good image in there somewhere. One work-around I tried with our over-correcting rig was to make a stack with flats and a stack without. I would then average the two of them to get a compromise before working on the gradients in software. You have my sympathy: the over-correcting flat is something which has still defied every explanation I've read so far. One area to look into, though, is the flushing of images between captures. I don't remember the details but it seemed a promising line of investigation when we were fighting this problem. Olly

You can only damage the mirrors and corrector plate by scratching them or breaking them so, if you haven't done either (and clearly you haven't), then they will be fine. They cannot be 'slightly cracked.' Optimism! Olly

As others have said, very few scopes cover full frame and a small apo is unlikely to do so. You'd need to check the corrected and illuminated circle which will need to be at least 44mm. Given the limitations of uncooled DSLRs and the cash value of a full frame CCD I wouldn't entertain the idea of modding one when you can get a far better cooled CMOS for a reasonable price. Modding DSLRs made some kind of sense when the alternative was expensive CCD but that's no longer the case. When you say, 'Will my images from a 61 APO be too small on a full frame ?' I think you may be falling into an error with its origins in terrestrial photography and the term 'crop factor.' This term is used as if the size of the sensor had a bearing on the focal length and/or image resolution. It has no bearing on either and should only be used in terrestrial photography for comparing lenses and formats, never in astrophotography where it leads only to confusion. In a given telescope one number alone tells you how large an object will finally appear on screen: the size of the pixels. The scope's projected image on the chip is a constant size whatever the camera. The size of the output image depends on the pixel size, smaller pixels producing a larger final image of the target. (The more pixels you put under the objects projected image the bigger that image will end up.) In general full frame DSLRs have larger pixels, I think, so this may be exactly what you don't want. The amount of sky around your target depends on the chip size but, if you don't want it, you can crop it. Olly

One of the many nice things about AP is that you can choose what your image is about. I would just do as you've done and post two versions. This applies to lots of targets: do we want Ha in a Double Cluster image? I do and I don't so I made both. I don't feel, in your case, that the stronger IFN detracts from the galaxies in any case. Very good image! Olly

A histo peak at half way is fine. Olly

Just for info, in AP 'deeper' is generally used to mean 'fainter.' And another note: you won't want more focal length for M31. It is utterly enormous. Check out Gorann's recent rendition at 400mm in which he didn't quite fit it all in. When I did it at 530mm with a full frame camera I still needed to make two panels. For any chip of APSc size or above you will need a rear lens element of some kind. This can be in the form of a Petzval or Petzval-like scope design in which the rear element(s) are in the main tube, or it will take the form of a flattener on the drawtube. Some manufacturers list flatterers which are not reducers, AKA 0x flatteners. You'd need to browse through your options with this in mind. Olly

You're right but the trick is to gather the cables together close to the camera and run them all to the mount end of the counterweight shaft, to which you attach them. from there they can drop down low and won't move too much during a run. I host six robotic setups and that's been working well for us. Flips are routine. Olly

Let's start with a target which will fit on the chip at either F ratio. So... - We lower the F ratio as in terrestrial photography by increasing the aperture. Now all pixels, including the target's, get more light and exposure time is reduced. You have captured more object photons. This is not controversial. But... - Now we lower the F ratio by reducing the focal length, leaving the number of object photons exactly as before. We cannot get something for nothing. What we can do, and have done, is concentrate the same number of object photons onto fewer pixels, causing them to reach the desired S/N ratio more quickly at the cost of a smaller image of the object. If this is what you wanted, go ahead. You could also capture bin the image, or software bin it or resample it downwards, to get a very similar result. Let's now consider a larger target in which you actually want all the new parts of the target in the wider FOV with the reducer. And, yes, your new, wider field image will indeed reach the target S/N ratio more quickly in accordance with the terrestrial F ratio calculation with which you're familiar. What this boils down to is that there are really only two sound reasons for using a reducer. 1) to increase the FOV because you're interested in capturing all of it. 2) to turn useless oversampling in longer FL systems into productive time-saving. As pixels get smaller this second reason will become ever more pertinent. Olly

That's great. The professional data bringing in the dusty ring around the Ha is new to me. I'm often surprized by how rarely we see this wonderful and huge nebula in images. It has a fine structure and looks as if it's being swept by a westerly particle wind to my eye. Olly

If ever you have a few very 'difficult' stars - and you will - there is another trick to try. Make a copy layer and apply, to the top layer, a fully feathered eraser large enough to cover the whole problem star and its bloat. Nothing will change because the images are identical in both layers. However, make the bottom layer active, open Curves, and put a fixing point at the nearest bit of background sky before the bloat. Add a fixing point below that. Now you can pull down the curve above these points, playing with its shape, till the problem star behaves itself! Olly

Good result. I'd want to have a look at the dark halos around many of the stars. This often comes from deconvolution. If you didn't feel like going back to early on in the processing an easy Ps fix would be to use the clone stamp set to 'Lighten' and take a sample from next to the star to apply over it. Before: After: Olly

Mine may be a minority opinion but I hold it none the less, and spend a lot of time with beginners. Buy a good mount and autoguide from day one. Keep to a short focal length and, if you possibly can, miss out the DSLR stage and go straight to a cooled CMOS camera. Olly

No, this isn't a WiFi connection. The idea is to connect the camera via cable to the gimbal carrying it, so as to have the start-stop record button right next to the gimbal controls. The cable connections are all there and there is a mode button on the gimbal to toggle between normal shooting and video but this has no effect that I can see! Thanks Alan, I think it will be something like this causing the problem. I'll go and see if I can find an equivalent in my menu. Olly

I'm stuck! I want to use my 250D for video on a new gimbal which arrived recently. If I plug in the remote cable to connect the camera to the gimbal and use the gimbal's 'half press' equivalent, the camera responds and focuses. But it won't start recording with a long press. Suspecting the gimbal I tried a Canon remote shutter button (with the same cable connection as used by the gimbal) and it behaves in the same way. Focus yes, shoot footage no. I have gone into camera settings and enabled 'Remote' but I still can't initiate capture other than with the little red button on the camera body, which is not practical with the gimbal. What am I missing? Cheers, Olly

No idea. I never gave it any thought since I didn't know it was there! As you say, an OIII test would be interesting. lly

A marriage made for the heavens in any case! I do prefer V2. It's Feb 9th but could this be image of the year already? 😄 Olly