ollypenrice

Good reply, as ever, from Vlaiv. For a while I took to processing in Adobe RGB because the colour gamut seemed better and I felt it brought a slight advantage, particularly since I enjoy colour in astrophotography in the same way that some poeple enjoy fine resolution. (Not that I don't enjoy that as well...) But then I had to re-work the images in sRGB in order to share them. In then end I got fed up with this and decided to work only in sRGB! I'll be interested in any more informed discussion of this matter since I'm just being subjective here. Olly

Your pictures really are very good for the equipment used. Hats off. I don't agree with your view of aperture/ light grasp. It is not a simple matter of how much light you grab, it is a matter of how much light per pixel you grab. There are informative discussions on the forum including this one: When you say, 'The only thing going for my setup is sheer aperture, I can get in 30 minutes integration what would take a refractor or small reflector 2-4 hours I'm guessing,' I'm going to disagree with you. What matters is not the total light grasp of your scope but the amount of light per pixel it puts on your chip. (Talking of chips, if you have a million fine British chipshop chips and 10 people to eat them they will die from obesity. If you have a million chips and 10 million consumers they will die of hunger. It isn't how many chips you have, it's how many chips per person. Same in AP. It's not total light, it's light per pixel.) At 3.5 arcsecs per pixel I promise you my 106mm Tak is pretty darned quick. Below is a Heart Nebula image form this rig with just two hours' exposure. 20 mins per colour and 2x30 minutes in Ha. 4 inch refractor. A 20 inch can only beat this in the time (and of course it can) if you have big enough pixels. Big 'if,' however. Olly

Yup! Very good. 😁lly

In that case I have nothing to contribute since the aperture/seeing issue lies outside my competence. With regard to Mr Lord, I'll reply via PM. Olly

The 'smaller aperture beats the seeing' argument, if it is correct at all, is correct for the resolution of fine detail and will apply to planetary and lunar observing. I have no fixed idea of the validity of the argument and have never done back to back comparisons to test it. However, it would be absurd to suggest that a small telescope at 150x can compete with a large one at 150x on faint targets - and I rather doubt that anyone has advanced such an argument, have they? This is one of those threads in which I somehow feel I'm missing something - which I often do! Olly

The problem is that a Dobsonian is not an imaging mount. Although you have GoTo and tracking, you are tracking the sky right-left and up-down, while an object takes a circular path across the sky. Think of Orion who rises leaning to the left and sets leaning to the right. Only an equatorial mount can follow that curving path while keeping the object in the same orientation on the camera. Also the drives on your Dob will be accurate enough for visual observing but imaging requires an almost insanely precise motion. There are other problems, too. A larger chip will indeed cover more sky but how large a chip will your optics cover without distortion? Without a coma corrector it won't be very much. Since a Dob isn't naturally an imaging scope your best bet would be to try what you have rather than buy anything specifically for it. Maybe you have or could borrow a DSLR? A dedicated camera would be an expensive disappointment. The widely accepted order of priority in astrophotography is mount, camera, optics. If your mount is not pointing in precisely the right direction throughout the exposure it won't let the rest do their job. You won't get a good picture from a Hasselblad if someone is nudging your elbow. I'd rather spell this out at the beginning. Astrophotography is quite unlike daytime photography and doesn't just involve putting a camera in a scope and taking a picture. It really is nothing like that. For a great over-view this book is excellent: https://www.firstlightoptics.com/books/making-every-photon-count-steve-richards.html I know this may seem counter intuitive and hard to accept but we've all been through this! Olly

Camera or lens, it doesn't matter. Any calculator needs to know three things, focal length, chip size and chip orientation. Olly

There is a problem, potentially. If you have a polar alignment error and are autoguiding, you will get field rotation centered on the guide star. If your guide star is in the middle of your imaging field the middle of the frame will be perfect but the further away you look from the centre of the chip the more you'll see trailing. What you're proposing would be an extreme case of this off-guidestar field rotation, meaning that it would risk being obtrusive even with a low level misalignment. Warning: I'm not very good at these 'spacial awareness' things! Olly

Don't use a diagonal for guiding. I use the ST80 as well and have extended the backfocus by using a de-lensed Barlow which I had kicking about. The distance from the back of the focuser body on the main OTA (at the point where the draw tube emerges from it) to the back of the adapter needs to be about 15cm. I've just measured mine. If you don't have an old Barlow body you'll need to order an extender of the right length. Olly Edit, I crossed with Ken but our figures agree.

I agree with your main points but there is an increasing movement towards galaxy imaging with 5 to 6 inch refractors. Quite a few imagers here and eleswhere have chosen this path. Being plug and play they save time while reflectors are arguing with their owners! 🤣 Olly

Very hit an miss though. I've had a couple of pairs for sharing around and they're pretty grim! Olly

Can I be your Foreign Correspondant??? 🤣 Born in Wigan, lived in Southport, Parbold, Horwich, Chorley and Garstang. Went to school in Bolton, worked in Preston. Came to France in search of clear skies... Olly

I would stay well away from anything with a magnification of more than 10x. Personally I prefer 8x. More than this and you won't hand-hold comfortably so you'll be adding the cost and clutter of a monopod or tripod. Higher magnifications suffer more jitters when hand holding. You'll also be able to enjoy 7, 8 or 10x binoculars for wildlife. Olly

We should be careful here because beginners often feel that a smaller chip, framing an object more tightly, means that they have the object more 'zoomed in.' I realize that you don't think this but it's a mistake we see very regularly in discussions. Like most other imagers I'll crop an image of a galaxy so that, without clicking to enlarge, it will appear at an appropriate size on screen when opened. So as far as capture is concerned I don't think the FOV metric is important. It only becomes important at the presentation stage when a crop is all you need. Olly

I'm perfectly happy shooting galaxies with a focal length of a metre. I don't think I would lose much if I came down to 800mm provided I used slightly smaller pixels. You can crop images so that they instantly appear at, or close to, full size when opened on a forum like this. These are at about a metre. Olly

Key question but we need to be clear: is your LX200 on a wedge? Olly

That's good information because we never got any sense out of Full Half Radius (I think it was) on single stars using Nebulosity for capture with an ODK14 reflector. We reverted to Bahtinov Mask. I never knew why the equivalent of FWHM didn't work since I've always used it with refractors. Maybe this is why. Olly

Delicate, with a really well-judged black point. Tip-top. Olly

I focus manually so I select a small box around the focus star and get an almost instantaneous download. Olly (Yes, I'm a dinosaur and, quite probably, a brontosaurus even then!)

If you like you can Dropbox me the linear Lum Tiff and I'll compare it with what I'm getting from a similar setup. Olly

Just look down the objective. As you change each filter you'll see the new one roll in. It should be easy to see that your Luminance slot isn't an accidental vacant one. It isn't likely but I'd be inclined to check. Olly

It's a quick check. Don't take the wheel off, just look down the spout. Olly

This is the point. I can't argue with the speed and resolution of the CMOS cameras but the numbers don't tell the whole story. Olly

Blimey, I see what you mean. I've never had it as severe as that. How do you stretch the lum? Just a wild thought, but are you sure that's a Lum filter you have in there? It couldn't be a clear or something that passes lots of UV? Or even a filterless slot in your filterwheel? Unlikely but those stars are bizarre. I never see anything like that with my own 460/TEC140. Olly

Whenever the performance on paper of the KAI 11002 comes up it looks awful. But whenever I process data from my 11002 it is just so gorgeous to work on. The stars are right. The colour is right. There's no fighting, just happy, happy processing. Olly