ollypenrice

What I like about a clear, dark night is that you realize you have no need for a spaceship. You're on one already. Olly

Super result on one of the strangest and most beautiful of objects. What is different about this Ha region is that it is locally so smooth. Normally Ha objects are full of local structure but this is just a glorious set of sweeping curves. Olly

I don't know what PI will do with that master bias or superbias, though. What was being suggested earlier, and is well known in DSLR imaging, is simply using a master bias as a direct substitute for a dark. Olly

I have to say that I've felt this way about SCTs in the past, myself. The first good one I tried was hand made by Ralf Ottow and the second was the 14 inch Meade I inherited from Alan Longstaff. I must say that I do like the 14 inch. It gives far better stars and finer planetary detail than I was expecting. So I think SCTs are very variable. Olly

Lots of really helpful information here. Many thanks. Your scope now looks perfect. Olly

On the deep sky the difference between 8 and 10 inches is very significant. This was the first jump I made as a beginner. Visually it's worth it. As for handling them both, that's very much up to the person and the circumstances but I'd go for the 10 myself. Olly

Top tip for finding how well your flats have corrected an image: stretch it and then run it through the Equalize filter in Photoshop (in Image-adjustments.) This is a brutal process which exaggerates all contrasts and is great for finding joints in mosaics, residual dust bunnies, etc. Olly

But did you compare this with subtracting a master bias instead of a dark? Olly

Beware, this isn't showing how flat the field is, it's showing how even is the illumination. These are not at all the same thing. You may still find considerable field distortion on full frame with the 85. I certainly did on mine, as did a friend with his. (Elongated, comet-like stars at the long ends of the chip.) Nor do I think that a twenty-ish percent dropoff in illumination is all that good, really. It's fixable with flats but less would be nice. Compare this with full frame illumination on your TEC with flattener. Different world, but the TEC/flattener was designed to cover medium format film so 35mm is child's play. Olly

Many well known astrophotographers are using Photoshop CS2 and CS3. They are fine for AP. Masks can be used in both, though PI masks can be created with astro-dedicated tools. However, for me the key advantage of Photoshop is its Layers function and, with that, its selection tools. I don't have to spend ages trying to create a mask just where I want it: I can create a layer, modify the bottom layer globally and then use a selection tool (usually colour) to select what I want to modify in the top layer and erase it. What is more I can erase it at whatever percentage I like. If 100% is too much I go back one click, reduce the eraser percentage, and try again. Or I can expand-contract the selection, etc. If I can't get the selection tool to identify the pixels I want to erase I can, if push comes to shove, zoom in and use the eraser freehand.) Above all I can see what I'm doing as I do it! I can also do custom noise reduction in Ps, targeting highly specific noise issues in a particular image. (Zoom into pixel scale, look for patterns in errant pixels, think about how to select them and then about how to modify them. You'll always be able to find a way.) I do some things in PI (DBE, SCNR green) but once I've done those to the linear image I get the hell out of there into a program which speaks my language! I don't speak PI but, clearly, some people do. Olly

Course I am! I'm helping you buy that camera!!! I may be calling on you to help me buy a a new CMOS camera soon... 🤣lly

Not being able to afford it never stopped anybody on SGL from buying new kit... 😁lly Ps Pixinsight versus Photoshop? They are very different and the key thing is to pick the one which suits you and suits the way your mind works. I do far more in Ps than PI because I'm more comfortable in the Ps environment.

Firstly I think that many aspects of the processing are quite simply excellent. The background sky is beautifully flattened both in brightness and consistency of colour (with the colour good) and it is at a respectably light level. So many backgrounds are too dark, too blue, too red, too green etc. Not this one. Its great. Stars are small, tight and round. Not enough blue? Vlaiv, on here, has made the point that amateur imagers push for too much blue in spiral arms and I think he's right. I've certainly changed my own approach in the light of his arguments. Look at the Hubble team's colour: https://www.spacetelescope.org/news/heic1901/ As you've said, where the image starts to fail is in its small scale details. Would more data help? Enormously. Build in a large scale dither of about 12 pixels, take a lot more subs and use a sigma clip routine when stacking. I don't use a DSLR but think Alacant's calibration suggestions will work best. No darks, just bias. (DSLR darks are not adequately temperature matched.) Olly

This one, (I think!) Olly

Yup, I just measured a screen grab of your flat in Ps (Curves, sampling 5x5 average) and got this, which agrees with you quite well. On the face of it that should be a perfect flat but if it's over or under correcting then it's not right. Can we check that you made your master flats using dedicated darks for flats? If not you'd expect over-correcting. Olly

Don't expect to be able to judge by eye the fall-off in a linear flat. It's really not a big fall-off. The most extreme vignetting I deal with involves a fall off of 23%. Something like 10% is far more common and doesn't look like much until its stretched. The best thing would be to measure the ADU values in the corners of the flats and in the middle to get an idea of the real fall-off in the corners. Olly

Regardng the whited-out flat in SGP, is it being given a screen stretch here? That would explain its having 27K ADU while looking white. If you save it as a TIFF and open it unstretched in PI, what does it look like? Did you shoot dedicated darks for flats? The CCD trick of using a master bias as a universal flat dark won't work, but I think you know this. Asking just in case. Olly

What I'd do first is measure the level of vignetting without flats applied. Take sample ADU values from the background sky in the corners and in the middle (which you won't be able to do in this image 😁). This will tell you the true level of uncorrected vignetting. Actually I guess you could just measure it using the flats, in linear form. That should work if the flats are correct but a sub of fairly 'empty' sky would be best. In my most vignetted rig I have a 23% fall-off corner-to-centre. This sounds catastrophic but flats do correct it. That will tell you the level of vignetting you're dealing with. My experience of ABE is that it will not work on galaxy images. Galaxies have faint outer glows, sometimes scarcely visible in the image, and ABE will measure the sky within these glows and conclude that it's too bright due to some kind of gradient. It will then pull down the background where that outer galaxy glow was, and create a dark ring around the galaxy. When using DBE keep well away from the galaxy and look on the web for very deep images which will reveal where the faintest glow will be on your own image. Keep the markers away from there as well. Olly

The problem is that your guide graph is looking OK but your picture isn't. It's the picture you hang on the wall! If those short trails either side of the star are from a Dec oscillation, we're not seeing the usual pattern which arises from this. The usual result is that the mount spends most of its time in one of two locations, either one side of the backlash or the other. It spends little time flipping between. The result is that you get two stellar images, one offset from the other. It can look like an 8, for instance or like o-o. What you've got is -o- which means that the mount is spending most of its time in the middle, as it should, but is having brief out-and back excursions in one axis. This is very unusual and makes me wonder about backlash as the cause. It might in principle arise from rapid spurious commands to nudge one way and then the other on occasion. Remember that most of the guide graph is an interpolation. The only real information the software has is where the mount was at the point the sub was taken. It will then draw a smooth curve between those points but, in real life, the mount could do a jig between those points and the software wouldn't know. The two good subs you found: were they at the beginning or end of the sequence? I'm just wondering if you might have spent some time shooting through a washing line or power lines etc. It wouldn't be the first time... Olly

Unfortunately this will not replicate the well-known technique of running the mount east-heavy to avoid oscillation across drive backlash. When the mount is pointing at the zenith the effects of being camera-heavy reduce to zero because the heavy end is directly below the light end. You'd need a weight off to one side. There is an alternative way to reduce Dec backlash, though, if you're unable to tune it out: run the mount slightly out of polar alignment and disable Dec guiding on one axis (the axis not involved in correction for polar misalignment.) You have to identify this by trial and error but it will always be the same thereafter unless you do a meridian flip, when the other axis should be activated. At least, with this method, the guider will not spend its time nudging the mount back and forth across the backlash. I've used the technique myself on a troublesome mount and it does bring a big improvement, though fixing the backlash is the best thing. To the OP: when analysing tracking problems it is vital to know the orientation of RA and Dec on the image. It's good practice to align your camera along RA and Dec when framing allows it, as it usually does. Align the camera roughly by eye to start with and then take a sub of a few seconds while slewing slowly on one axis. This will produce a star trail at the angle of your camera. Rotate till the trails are horizontal or vertical. Convention has it that the long side of the chip along RA is called 'landscape' and the long side along DEC is called 'portrait.' Keeping the camera orthogonal also makes it a hundred times easier to add more data at a later date. Olly PS It isn't necessary for guidescope and imaging scope to be parallel. Guide scope rings were not invented to let you get your two scopes parallel. Quite the opposite: their purpose was to allow the guidescope to be moved off axis in search of a guidestar when guiders were very insensitive. These days its best not to use them and bolt the guidescope down hard, off axis or not.

Thanks. I did! And I understand your answer. Super result in any case. Olly

It's very fine. I'm sure about the rationale of adding RGB to a false colour palette, though. I'd have thought RGB added to HOO would be more natural? Olly

Wow, that's gorgeous. Removing the stars gives a more three dimensional look so that now the Veil is just 'hanging there' and looking very spherical. Olly

Nice and, I think, new to me. Olly

I agree, though adding a layer of glass can't be ideal. Probably won't make much difference though. You'll have a slightly different focus position if that affects the rig in any way. Olly