The Admiral

I wondered that, but it doesn't bode well for further down the line ?. Ian

Unfortunately, then, that is why Linux will never gain traction! And I for one don't want to be spending my evenings trying to sort out computing problems. Perhaps Linux has some way to go then before it will appeal to the likes of me, sadly. Ian

As much as I'd like to 'get into' Linux, I've fallen at the first hurdle! I've downloaded the Mint Cinnamon iso, and now just need to go through the verification process. Simple innit? No! It should be, but I'm rather confused, especially as the user guide doesn't exactly tally with the on-line description. And it does seem rather convoluted to my non-computer-geek brain! What I fear, and which is off-putting, is that this may just be the start of something a lot worse. Ian

If you are happy pressing the button of a remote release, that's up to you, but an intervalometer of some kind makes life easy. I would not recommend just pressing the camera button though as that is likely to induce shake and give you star trails. Incidentally, you can't guide properly on an Alt-Az mount. Ian

Well, I know we share the same sentiments Steve, and your interjection comes as no surprise . But alas I guess you are preaching to the converted in this thread. Ian

You might not say that if you were in the UK living with the incessant poor weather Ian

Pleased to hear it and that you've decided to go for one, I'm sure you won't regret it. I bought mine not for any theoretical gains but purely because I wanted to be able to better image some of the larger DSOs. There's the added advantage of also flattening the field so I get stars reasonably sharp across the whole FoV. Without it, I'd have felt the need for a field flattener anyway. I've always used it for deep sky imaging, even for smaller objects. Ian

Oh I do hope you don't really feel that, we wouldn't want you to feel unable to ask questions. This sort of thing happens, and if anything just reveals our own uncertainty . Please, keep asking! The best approach is always to give something a try, beats any theoretical discussion. In fact, we wouldn't be doing Alt-Az imaging if we had religeously followed what experts say Ian

Now that's the sort of gear we want for Alt-Az imaging Hmm, on second thoughts, a 20m focal length wouldn't work well with my Nexstar SE mount Ian

Ken, I've had a thunk . The noise associated with the number of signal (target and sky background) photons collected by a pixel will go as the square root of that number. If the number is increased, by whatever means, be it by using a reducer or by increasing exposure time, the associated noise will rise slower than the signal. In other words, the SNR from that source must surely improve with increasing signal, irrespective of any improvement due to dominating the sensor noise. If that wasn't the case, then surely there would be no merit in taking long duration subs, or taking many subs and stacking them? Am I talking sense? What a minefield! Ian

OK, that makes sense Ken, though for the limited exposures used in Alt-Az imaging with a DSLR, I imagine signal noise is unlikely to swamp sensor noise, but I may be wrong. Though do you think that raising the signal by using a reducer would be beneficial to seeing deeper, i.e. getting signal off the bottom of the histogram, given that we're normally a long way from being sky limited? Also, I'm not sure how software binning compares to hardware binning when it comes to noise though, but a minor issue so far as the argument is concerned. DSLR users have no choice but to software bin of course. So far as reducing the effects of mount tracking issues is concerned, it strikes me, despite my earlier comment, that that advantage is only gained if there isn't significant cropping after the event. Ian

I think we are getting into a different realm here by introducing another telescope with a much larger aperture. The thing is, the La Palma scope has a FL of 200 x 10 = 2000cm, whereas a 10cm f/7 scope has a FL of 70cm, and therein lies the main difference. We shouldn't be comparing f-numbers, but focal lengths and apertures. Perhaps that is what is meant by the 'myth'? For what it's worth, here are a few notes I made to try and get my thoughts straight (assuming I've got the maths correct ). The title may, or may not be, a red herring! Is the F ratio Myth a Myth v4 .pdf It's the conclusion that is important here. If you like, the act of 'concentrating' the photons can be thought as being analogous to using a magnifying glass and the sun on a piece of paper. If the glass is not focused, all the photons coming through it are spread over a large area and there is no effect. As the glass is brought into focus, so the intensity of radiation in the illuminated patch gets higher, even though the same number photons are passing through the glass, until at some point the paper combusts! We are clearly not burning the sensor in imaging , but to my mind we are increasing the intensity of light on each pixel. That's my way of thinking about it anyway, though not saying it's the correct way! Ian

Difficult to answer really, it's not something I've attempted to demonstrate, principally because with Alt-Az imaging the exposure is constrained totally by field rotation and by tracking quality. This as you probably realise depends on both altitude and azimuth, and I've taken the line of using just a 30s exposure for everything as a matter of operational convenience, unless I particularly want to reduce it to deal with wide dynamic range. Some folk have managed to get decent results with a minute or more exposures, in preferred directions, but my mount wouldn't be able to achieve that I don't think. Given the constrained exposures, I have taken the view that a reducer would improve things, based on my, perhaps naive, understanding of theory. You would not use an fr to reduce exposure time; you would maximise the exposure time within the imposed constraints. I believe it will improve image quality, and I can convince myself that that has been the case . Not only that, but the effectively reduced focal length means that the effects of mount tracking deficiencies are less obvious, enabling me to keep a higher proportion of subs. It has also allowed me to image more of the Rosette Nebula and M31 for example, and given me a bit of room for cropping off the field rotation artefacts. Hope that helps, Ian

That's not my view, but happy to be proved wrong . Ian

Sorry to feed the headache , but wouldn't using a reducer increase the brightness, as well as SNR? Surely, that's how the higher SNR is achieved. Is it not the equivalent of using a shorter longer FL eyepiece in visual observing, where a lower overall magnification gives you a wider field of view and a brighter image? Ian

Well I for one would like to have seen some pictures of it actually in use, carrying some equipment and on a tripod, rather than floating in space! It took me while to work out what goes where (assuming I've got it right), and would fully have expected to see a dovetail clamp somewhere. I would agree it is not exactly selling the product; you shouldn't need to ask questions in my view, that isn't marketing. But as you say, a curious first post, but not necessarily wrong for being that! Interesting user name! Ian

Well now Ken, this subject raises a lot of contention, and often has 'you know who' wading in with the 'f-ratio myth' . He said it again in a recent post I noticed. However, I'm afraid I have to say I don't buy it, or at the very least, I don't understand where I am at fault! If the FoV is increased using the same equipment as before, it follows (to me!) that the photon flux/pixel from an object must necessarily increase, because all the photons being emitted by that object will be squished over a smaller area of sensor, i.e. fewer pixels. Now if exposure time is governed by SNR, won't the use of a reducer allow the target SNR to be reached quicker, simply because the pixels will be 'filled' at a faster rate? I accept that the total number of photons entering the telescope will be governed by the objective area, but it's not the total number of photons that we are interested in here, surely? What am I missing? Ian

I don't think the make of reducer is critical, so long as it's suitable for the focal length. I see that the Lunt white light 'scopes are made by APM, so anything suitable for those should be fine. Just make sure that the back focus is adequate if you intend ever using a filter wheel. For the Photoline one it's ~56mm at a FL of 715mm. It's also useful if the reducer can attach to the focuser by screw thread, thus ensuring stability of the optical train. Mine doesn't, and I use the Baader Click-Lock screwed onto the focuser to hold the reducer central and square. Ian

I went for the 0.79x 4-element Photoline from TS Optics, but it isn't cheap. https://www.teleskop-express.de/shop/product_info.php/info/p5965_TS-Optics-PHOTOLINE-2--0-79x-Reducer-4-element-for-Astrophotography.html You pays your money and makes your choice. I've been satisfied with it, but I'm sure there are cheaper ones that others have found to be OK. It covers an APS sensor with little vignetting. As for adapters, each set up is different so it's hard to advise. In particular I use a mirrorless camera so the camera mount-to-reducer distance is different to when using a conventional DSLR. FLO, 365 Astronomy and TS can together provide an enormous range of fittings, though be wary of the higher postage charge from TS as it's non-UK. Do Lunt make a reducer specifically for their 'scopes? Ian PS. Ah, are you in Canada? Those suppliers might not be appropriate if so.

I use a reducer on my Atair Wave f/7 102mm. It flattens the field as well, but you will need to finely adjust the reducer-sensor distance in order to get round stars across the frame. That can be a bit fiddly, and you may need to source a mix of adapters. Otherwise, I'd go for it. You will be able to cover more DSOs in the FoV, which is useful as you will need to crop off the outer portions of the frame because of field rotation. Ian

Even better! So long as DSS can recognize it. And yes, that is one of the issues with Alt-Az imaging, the large number of short exposure subs required, and hence storage space and processing time. I see that there is a beta version of 64-bit DSS now available, which should speed things up a bit. M104 has come out very well too! Ian

It is true that over time the images will rotate, but so long as the field rotation isn't significantly visible on a single frame (i.e. each exposure duration is short enough), DSS will automatically de-rotate them and align them. To my mind the slight problem is that I guess the images that you get from your 'phone are jpegs, which is less than ideal, but it will still work. The issue is that you will still need to significantly enhance the contrast of, or 'stretch', your stacked image, and the 8-bit jpegs will not take kindly to that process. Still, you've done it with the image above, so it should be better with a stacked image. Give it a try, that's the only way to find out! It's a while since I used DSS, but I guess start off with the default settings. Have a look at the User Manual on the DSS website, or alternatively there are a number of on-line help videos and tutorials. It's not a difficult piece of software to use. Ian

That's come out rather well for just a single frame. You could try taking a lot more and stacking them in an external application. Ian

I think you are probably thinking of Filroden who used his Esprit 80 on an Evo mount, before he went over to the dark side . I use a 102mm APO on the earlier, and inferior, Nexstar mount, or at least I did when I was actively pursuing imaging. Ian

Because Sony's primary concern is with conventional photographers for whom it isn't an issue, and it allows them to improve the appearance of images by reducing the apparent noise. In doing so it probably also allows them to advertise a higher maximum ISO, which, for some reason, seems to be the thrust of competitors' advertising. Just my 2 penny worth. It does beg the question, though, why they couldn't allow the user to turn it off if required? Probably not as simple as it sounds though. Ian