ollypenrice

Bobro's earlier post explaining the relationship between pixel scale at the imager and pixel scale at the guider will answer your question. Focal length is just a shorthand way of discussing the matter. What I don't think has come up is the matter of flexure, particularly when imaging with reflectors. It is very hard indeed to remove all possibility of primary mirror movement in a reflector because if the mirror is held too tightly it will experience distortion. If the mirror moves slightly, so does the image. A separate guide scope cannot 'know' this so cannot correct for it. However, an Off Axis Guider uses the same light cone as the imager for guiding, so mirror movement (known as mirror flop, though this is an exaggeration!) will be seen and corrected by the guider. Under no circumstances would I guide a catadioptric with a guide scope. I've always used an OAG for this. The small mirror of the 150PDS might let you get away with it but, in principle, reflectors are best guided via OAG. You need to be sure that you have enough backfocus for an OAG, though. It pushes the camera further down the light cone. Olly PS You don't need to calculate pixel scale yourself. Here's the easy way to find it! http://www.12dstring.me.uk/fovcalc.php

It was a while before the penny dropped with me, too. Olly

That's true. The other reason is that it makes the image more repeatable if you want to come back to it for more data in the future. It takes ages to recover an arbitrary camera angle when framing. And then there are mosaics. I once tried to do a mosaic with an arbitrary angle optimized to the target. Never again!!! Olly

Dead easy, Steve. Have the camera running on a bright star. Open the crosshairs on your capture screen and put the star in the middle. Now slew slowly on whatever is the long side of the chip (which could be RA or Dec depending on Landscape/Portrait orientation.) Unless you're already orthogonal the star will now be above or below the line of the crosshair so you rotate the camera so that the star is back on the the line. Now 'slide' the star along the crosshair by slewing the other way to the opposite side to confirm it's remaining on the crosshair. Give the camera a final tweak of rotation if necessary. It takes two minutes at most. Olly Oops, beaten to it! As Freddies says, you can't have both optimal framing and orthogonal alignment but I find I hardly ever need to move from orthogonality.

Yes. But, thank God, Monique hasn't... lly PS, Seriously, filters really do start to get expensive in large sizes. The absolute killers are the square ones for the 36X36 chips. If you decide to look (and I wouldn't!) do so while sitting down.

One of our robotic scope owners has just installed a large format Moravian and it looks very convincing. Olly

What happens if you take a first and a last sub from a run and then align them on the stars and combine them? The stars wll align but what about the frames of the two images? If those don't align then that's because of your PA. I'm wary of the claimed PA measurements offered by the various software packages because I've seen good reason to doubt them, using the test I describe. Olly

I'd do a net search on the subject of customer service.

Ha is very deep red and I will always add it to the red channel. I don't like to add it to luminance, which is the lighting across the full range of colours. OK, I may use a whiff as luminance on some targets but it ain't right! OIII lies on the blue-green border (sometimes called teal blue) so I add it to both green and blue but in separate images which I then blend to taste. Others will use different methods. When I add narrowband I have the natural colour image on another screen and try to respect it when adding NB.

I have plenty of problems in processing for full size zoomed viewing, Rodd. It's just that I feel like a cheat if I don't! In this one the big problem was the noise in the colour layer, which is a technology issue, I think. I did far more NR than I'd like to do. I used custom techniques based on zooming in to pixel scale to see what was going on. Since I can find nothing wrong with the cooler on the camera I'll have to look at the usual suspêcts like the USB leads, etc. Olly

It's important to be clear that ALL electronic cameras are monochrome, including DSLRs and 'one shot colour' CCDs. We don't 'colourize' * and we don't invent colour. We measure it. In terms of colour there is no significant difference between a mono and a 'one shot colour' camera in terms of how the colour is obtained. Both use a red filter to block green and blue, a green filter to block red and blue...etc. In a 'one shot colour' these filters are placed over each pixel in a 'Bayer Matrix.' Red, green, green, blue - repeated across the chip. (Why two greens? Because it works in the daytime but is a very bad idea at night!) In a mono the imager shoots through an R then a G then a B in turn and combines them later - but the idea is the same. The same kind of software calculates how to weight the three colours of light. My main point here is to stress that there is no such thing as an electronic colour detector, as yet. They are all mono and they all use filters to work out the real colour. Olly * That is, in natural colour imaging, which is what I do in deep sky photography. I have invented colour from purely mono images in solar imaging. This is always declared. The colour in this image is pure invention:

That's the planetary, Paddy. I don't know if the bright star visually centred on it is the progenitor. I'd be surprized. It seems too bright. Probably line of sight. Olly

I only noticed it in SkyMap when planning the framing. It isn't a popular target, it seems. Here it is with 1.5 Hrs of OIII from guest Ton's own TEC140/GM1000/QSI683 Astrodon. 3x30 minutes, guided. (And with its poor neck wrung in Ps! It shows OIII is the way to go, though.) Olly

Yes, that's the one. In fact our present guest shot 1.5 hours of OIII in his own TEC last night and we've just blended it. It's nicer. I'll update the pic when I get a chance. The SCT project went on 'hold' because the idea was to mount it on one of our robotic guest's mounts, a GM1000, but he had so much trouble with it that he took a refund and is now using an Avalon which is too small for the SCT. However, I have a second Mesu coming next month so I'm aiming to give it a run on that. Actually the ODK data was very soft. I guess it was down to poor seeing but I can't remember. The resolution was much the same as in the TEC data. The problem was poor S/N in the TEC colour because the camera isn't happy for some reason. I like to have equal amounts per channel of L,R,G and B becaue it makes processing easier. However, with the usual Photoshop thuggery you can work with two or three times as much L. I perfer not to when possible, though. There are no ODK stars in this. They would have added spikes. There was no star reduction here other than on the handful of bg ones, which I pulled down in Curves near the end. Olly

LukeBL posted a nice NGC4725 recently. Here it is again in a wide field which includes PNG339.9+88.4, the tidally distorted galaxy NGC4747 and the Sbc galaxy NGC4712. Plenty of faint fuzzies, too. NGC4725 is a barred spiral with prominent ring structure. One of my favourites. I've a suspicion that this camera isn't cooling properly so the colour was a real fight and the Ha I shot last night is scrap. The PN shows well enough in LRGB like this and I've emailed a query to Atik to ask if the FITS header temperature is a record of what temp I set or whether it is a measurement of the real temp. (It was showing -25 but I'm not convinced.) The day was saved by using old ODK14 data for colour in the three galaxies. Once we have the technology sorted the addition of Ha and OIII for the PN would be nice. TEC140 and ODK14. WIdefield L 4Hrs, RGB 1.5 hrs per colour. Similar exposures from the ODK14. Enough excuses, here's the pic! Best seen in the larger size. Click on the image and full size button is lower left. Olly

As I said, I'm not aware of anything to beat DBE. That means what it says - I'm not aware of it. I don't claim that it doesn't exist! I'm not in the business of trying every new bit of software that appears on the market and I suppose that I'm mainly motivated to try new software when I see an image that breaks new ground. If I think, 'How did he or she do that?' then I'll be in there, full of curiosity. I'm hardly a PI evangelist, I'm manly a Ps imager, but DBE is profoundly impressive. If you have an alternative, tell us about it. Give us some demos. I'm all ears, I promise. If there is something out there which is more sophisticated than DBE - great. Olly

Just think about what your imaging hardware costs. It's a lot. Now think abut how much of the final image quality comes from processing. It's also a lot! It's more than a lot, it's going to be something like 80%. You can't process rubbish so the hardware is very important, but a good capture is no more than the collection of good raw data. You still have to do something with it. My captures are no better now than they were ten years ago but my processing is a lot better. DBE in Pixinsight has no rival of which I'm aware. I still think the software giants are PI and Ps. Look at the images 'out there' which you like the most and see how they were processed. Olly

I don't think there is much doubt amongst historians that Hooke did have a role in giving Newton a cenceptual, though not a mathematical, leg up on his way to his theory of gravitation. Of course Newton would never admit this. When Newton said he had seen further because he stood upon the shoulders of giants he was, in fact, taking a swipe at Hooke who was deformed and dwarfish. There's a geat Hooke biography byu Stephen Inwood which I have read and another by Allan Chpman which I have yet to read. Olly

No point in my repeating my enthusiasm for this mount. Hope you're as happy, Harry (and Steve.) Olly

I have to say that it looks very like PA to me. On what basis do you say that PA seems good? The simplest test is to take a 'first' image and a 'last' and combine them, then look at the edges of the frames. Any disparity in the alignment of frames, assuming reasonably round stars, comes from PA. Olly

An unusual look and feel to this one. Sh2 126, 2 panel HaLRGB, twin Takashashi FSQ106/fill frame CCD, Mesu 200. The 'Breaking Wave' extension around Gamma Cass, down to the Pacman. 3 panel, twin Taks. NGC 2170 using twin Tak and TEC 140 data. 35 hours. Beverly Hills here we come. A joint effort with Mr and Mrs Gnomus. HaOIIILRGB, 35 hours. And finally IC447, LRGB TEC 140. An eleven hour quickie! As ever, the kit is co-owned by myself, Tom O'Donoghue and Yves Van den Broek. Thanks guys and guests, a good year. Olly

The speed of the system is not linked absolutely to the F ratio rule in AP when using reducers. If you are interested in everything that fills the frame then the rule applies in useful way. If your real target is a small discrete object framed by dark sky which doesn't interest you, then the F ratio rule does not apply. (It becomes the F ratio myth.) The photographic F ratio rule is predicated upon varying the aperture. A focal reducer leaves the aperture where it was and alters the focal length, so it brings in no new light from the discrete object. The two situations are entirely different. I wonder what the effect of binning really is on this CMOS camera. From what I've read it may bring no increase in signal to noise, in which case why bin at all? More homework needed! I like many aspects of the camera so I'd better get studying. Olly

Sorry, I hadn't noticed the dates - but I've had worse!! lly

Imagine what would happen if they put Robert Hooke on the pound coin while they were about it... Oo-er missus. Olly

Firstly you need to be slightly misaligned. I've never actually set out to misalign, I've just exploited the misalignment that was there already! (By not endlessly refining the PA but settling for a 'close enough' alignment.) You then disable the guiding in Dec on one axis and see if that's the axis you do or don't need to guide on. If the mount guides in Dec that's fine. If it drifts off target you need to re-activate the direction you disabled and disable the other one. Write down the result of this test (if you're anything like me...) and reverse the disabling after the meridian flip. Olly