ollypenrice

After @gorann spotted that he had the data to combine a Bat image with a Fireworks image, Paul Kummer and I found that we could do likewise with one more filler panel. Here's the result. I also found, at last, my 10 year-old CCD Tak106 OIII linear data for OU4, the Squid. Despite having 24x30 minutes of OIII, the signal was weak but brutal stretching using the X-suite software allowed me to get far more out of it than I did originally. It was added to green and blue in Photoshop using Blend Mode Screen, heavily clipped. (I usually use Blend Mode Lighten.) This is the only narrowband in the image. Paul drove the scope and stacked up the data, I did the post processing and the OIII. The galaxy has also been very gently enhanced using a TEC 140 image. The big version is here. https://ollypenrice.smugmug.com/Other/Emission-Nebulae/i-ct23L2q/A Thanks to Goran for the good idea. Olly

An SGL member on here may remember a club visit to my place during which, on their first night, she was sent to find me because nobody in her group could find Polaris. She reckoned they'd asked her to do it because she was the only woman in the group. If you're used to a dark sky, and can routinely see Ursa Minor, it is very easy to pick out Polaris but you have to be used to it, as you say. Olly

I'm doubtful about the wonders of computerization. It very easily turns itself from the solution into the problem. Olly

That's very kind of you, Goran, but unfortunately it only extends our Fireworks panel downwards by a small amount. The overlap with ours is considerable though we go over to include the open cluster. I think it will be well worth another panel. Olly

We almost have it, with just one small block missing in the bottom right hand corner. It looks great. Thanks! Olly

Great idea! We probably have most of this FOV as well. Hmmm... Olly

Don't forget that plate solving isn't compulsory. You can derive the RA and Dec co-ordinates of your image centre from a planetarium loaded with your specific field of view and then go to a nearby star, re-centre your mount on that star, then simply drive to the co-ordinates manually. Olly

There's a lot to be said for being undersampled when starting out since the undersampling will absorb errors in tracking and focus. You'll also catch more light per pixel which speeds things up. Up to 50mm or so, you're in very tolerant terrain. Going up to a hundred mm increases most of the difficulties. One of the instruments I use is a 135mm Samyang but it is not an easy setup to use. Quite the opposite. Olly

Regarding focus, the usual advice is to focus on a star located at one of the four points where the one third lines intersect. (These are four imaginary lines, each parallel with one of the edges of the chip and one third of the way over to the opposite edge.) This is supposed to give the best compromise between centre and corners. Olly

Sampling rate is an important consideration in astrophotography. It defines how many arcseconds of sky are projected onto each pixel. While this will affect the resolution of detail in terrestrial photography, it is far more important in AP because stars are very awkward things for lenses to control. All the lenses above will leave you undersampled (pixels too large to separate details which can be separated by the optics) but keeping to the longer end will give a smoother result. That argues in favour of 50mm. Prime lenses have fewer elements than zooms, generally meaning cleaner stellar images. I'd always go for a prime in AP. The Canon Nifty Fifty on your list has form in AP - and very good form. How its Sigma rival compares, I don't know, but the Canon has done good things. While it would be nice to shoot astro shots wide open to get more light in the time, it is usually difficult in reality because stellar distortions creep in. Rather than stopping down with the diaphragm, which will create artifacts around stars, you can consider stopping down with a front aperture mask instead, either cut from card with a compass-cutter or made up using stacked filter rings. Another factor, though, is tracking. Do you intend to use a tracking mount to 'unwind' the rotation of the earth? There are many available, the cheapest option being a home made 'barn door tracker' - it will Google. On a fixed tripod the case for a shorter focal length arises from its ability to expose for longer without trailing. Why is a shallower depth of field easier to focus? I can see that it 'snaps into focus' more readily but, conversely, the focal plane is much shallower and, therefore, more critical. I'd have thought that the two characteristics cancelled each other out, at best, and more probably mean that a deeper focal plane/slower F ratio is easier to focus. Olly

Long subs are strictly for cooled cameras. My standard narrowband sub with that rig was of 30 minutes. Olly

Not the RASA, no. Tak FSQ106 with an Atik 11 meg at 3.5"PP - so a lot of light per pixel but only 50% QE. Olly

For many years I've preferred my own experiments over the widely accepted orthodoxy. The orthodoxy is, let me stress, usually right but there are exceptions. Two examples: 1) in CCD imaging I found that a 30 minute sub did go deeper than 2x15 minute subs. 2) Dither is no big deal with a cooled camera. You don't need to dither if it creates new problems. In post-processing I reject far more of the orthodoxy but that's another story. Olly

It's very good, no question. But it is also, by a mile, the brightest DSO in the sky. When I last imaged M42 I used 11 second subs for this, the Trapezium region, and 15 minute subs for the outer nebulosity. That gives an idea of just how bright the Trapezium is. Olly

Yours is the first of the two scenarios. This is actually good news because the other source of elongation, besides tracking, is tilt - and that's a pain to sort out. Yours is the nicer kind of problem to have! lly

General points... Since planets can usually be seen from home, and since small apertures cannot reveal small, faint extended objects anyway, my thinking has always been that compact travel scopes should be able to do what small scopes do best - and that's widefield. I wouldn't make higher powers a priority. I'd also be reluctant to use a light-consuming complex EP in small apertures. Olly

Another way to think of it is to remember what 'magnify' means. It means 'to make larger.' When you look at the moon with naked eye the moon's image on your retina is a certain size. When you use optical aid, the size of the image on your retina will be made larger than it was before, and by a definable amount, 10x, 50x, 100x etc. This is all nice and easy: we have an image on a retina and that is what we magnify. Now point an imaging telescope at the moon. It collects and focuses light and projects an image onto the chip. Is this image 'magnified?' Magnified over what? There is no definitive baseline size equivalent to the image on the retina so we have nothing to 'magnify.' We certainly don't magnify the moon since our image is a fraction of the moon's size. The only magnification to be found in photography is in the macro world. Some macro lenses can project an object's image onto the chip, an image which is larger than the object itself, and this is, quite reasonably, known as magnification in macro photography. In fact, all telescopes compress the moon's image to a tiny fraction of the moon's real size. This is really negative magnification! Olly

93x3 minutes this time. Paul does this bit so I always have to look it up. Your wish is my command! Some of the cosmetic processing was done after the crop so I've made no real effort with the bright stars. I think the crop has the best of it. Olly

I do like this region not to totally swamped by Ha signal, leaving the details, like the trunk, picked out in silhouette as they are here. Olly

This is indeed a loss. I had the pleasure of Steve's company as a guest here, a few years ago, and endorse everything said. Olly

This may come as a surprise, but I have never dithered. Caveats: I've only ever used cooled cameras. I've never bust a gut to finesse polar alignment, though, in the CCD days, it needed to support 30 minute subs. Now it only needs to support 3 minute subs. Given the short exposures needed by CMOS, and their tiny pixels and low read noise, won't a small polar alignment error provide any dither that your system actually needs? If it really needs any at all? I would forget the clamour of the orthodoxy and received wisdom and just try not dithering and getting more good subs. Olly

ELP is right, but you white point is also clipped. Here's your image with its histogram in Photoshop. A healthy histogram looks like this. (I'll use my own Heart Nebula here.) On the left to right axis the histogram goes from dark pixels to bright. On the vertical axis it shows the pixel count at a given brightness. What we see in your histo is no thin line on the left. That's because you have 'black clipped' your data, cutting out pixels with slightly more light than the background sky. This leaves you with a jet black background sky and your faintest signal discarded. On the right hand side we see, also, that the line exiting the graph is not dead flat either. Your brightest pixels and your slightly less-then-brightest have been cut off before they can be distinguished from each other. This is, without doubt, the most common beginner error and is easily fixed, but only by stretching from scratch. When it's clipped, it's clipped. It is very tempting to try to fix sky gradients by clipping them out. My advice is never to do this. Gradients must be fixed by gradient removal tools which are now many and various - and very good. Olly

My favourable (and very short) review of the ST80 as a guidescope goes like this: Running an old school, low sensitivity CCD Lodestar in an ST80, I imaged commercially for around ten years, and about 250 nights per year, without dropping a single sub to guiding error. The mount was a Mesu 200. Olly

I like this object, the third in our recent run of four. Capture and pre-processing by Paul Kummer, my post-processing. RASA 8/NEQ6/ASI2600MC Pro. This is cropped to about 25% of the full frame. Olly

No guidescopes have alignment rings. What they have, if they have rings, are misalignment rings which were used in the days of insensitive autoguiders which needed to be pointed this way or that, relative to the imaging scope, to find a workable guide star. Those days are long gone but manufacturers insist on still providing them, leading to the modern myth that guidescopes need to be aligned. Actually I think they are just there because these scopes are based on findersscopes which do need to be aligned. A rigid mounting is much to be preferred. By the way, I've started using one of these mini finder guiders a couple of years ago and consider it a darned nuisance. It de-focuses itself a couple or three times a year. When I was using locked-up-solid ST80s for guiding I never touched them in ten years, scraping out the worst of the spider webs every three years or so. The worked perfectly. I'd be delighted to see the back of the pretty little anodized pest which replaced them! Olly