ollypenrice

That makes more sense. In any event it's great data and has given rise to an interesting thread. Olly

So is this not a live thread? I thought it was, though it does say 2020 in Grant's post at the start. There just don't seem to be any old responses, though the year isn't stated on the posts, just the date. I think most people in this thread thought it was live. I'm rather confused, though I'm used to that... Olly

I just use a tube of builder's epoxy to fix a length of threaded bar into a hole drilled in the concrete. It has worked on a dozen piers or so over the years and never failed. Just be sure when you squeeze the gun that both components of the epoxy are coming through properly. You can check that by giving a short squeeze before putting the long mixing nozzle on. Olly

Samyang 135 lens? Olly

I don't think it would because, if there is no flexure between guidescope and main scope (and there isn't) the mirror must be stable. I think what I might try on our troublesome TEC dual rig is a flat plate bridging between all four tube rings and attached to them at four points using the self adjusting cup washers in the earlier link. I would then mount the guidescope on this plate, so it is equally connected to both tubes. This might make both scopes trail, or neither! Olly

You may be right but I don't follow this argument. It seems to me that, for a given moment, all mounts are identical. They point somewhere, centered upon co-ordinates x and y. The camera centered upon these co-ordinates will render a curved field onto a flat chip in accordance with the optics in use. You could take this image quickly (in principle) with a fixed tripod or an EQ mount or an alt-az but it will remain precisely the same image with precisely the same field curvature. Give this image to a standard mosaic software and it will extend that image's curvature outwards into the rest of the mosaic. In my view the mount design has zero importance because, if/when we have all the photons we need in one second, we won't need tracking mounts but we'll have our picture, and the geometry of that picture will be exactly as it was in the tracked version. If this were not true the tracked version would not have round stars. The whole point of an EQ mount is that it gives the same result (only deeper) than a short snapshotun-tracked. Olly

I would take a widefield with your lens, upsize it to the scale of the projected mosaic and use it as a template. I use Registar for this kind of task. Olly

I'm not sure that I'd call this field rotation. I think the problem is geometric. The sky we are photographing is, in effect, seen on the inside of a sphere but our final image will appear on a flat surface. This is a familiar problem in cartography. There are different projections of the 3D earth onto 2D paper maps, each with its own strengths and weaknesses. When creating a large mosaic you need to decide on your field geometry. If you just pick a random panel from your set the software will treat that panel's geometry as definitive and all other panels will work outwards from that one. The best you can do in this case is start with a panel in the centre of your image. A better idea is to take a widefield image covering all of your intended mosaic, centered on the same point. This can be resampled upwards to the size of the intended mosaic (it will look terrible but that doesn't matter) and it will become your registration template for all your mosaic panels. Your final mosaic will have the field geometry of your widefield image. Olly

I would say that a 20% overlap will make life easier. Automated mosaic software may well handle a two-panel quite easily but, as any mosaic grows, it becomes increasingly unlikely that automated software will succeed. This kind of software is very competent with daytime images but astro images are massively stretched and present a much bigger challenge. As for exposure time, how deep do you want to go? I was intrigued by the size of M31 on star charts. It was way bigger on the charts than on most images, so I decided to try a set of 30 minute subs to see if I could find the galaxy's outer reaches as seen on the charts. For mosaics I use Registar in conjunction with Photoshop. Olly

Thanks, that's perfect. Olly

I was interested in your bench test for camera tilt. Do you have more information on how that works? Olly

The mask cannot, so far as I can see, give a variable result in a perfectly collimated beam arriving at a perfectly tilt-free chip. The problem must lie in the optics or the plane of the chip. Have you tried rotating the camera rather than the mask? Olly

I thought the IFN and Bode's Loop were so good in the luminance stack that they deserved to be given priority. However, galaxy images need small stars for good comparative scale, so the processing priority was to try to haul out the Flux while going for the smallest possible stars. I used a set of custom Curves in Photoshop to lift up the IFN, de-starred the result in Starnet/Pixinsight and then used another set of custom curves for a star layer, added in Photoshop's Bend Mode Lighten. I went easy on the addition of the Ha after stretching it very hard with a sharply rising custom curve. It was added to the red channel of the LRGB in Photoshop's Blend Mode Lighten. Olly

Worth noting that the Hubble team didn't find M33 to be very colourful. Hasta La Vista Green is free/donation from Rogelio Bernal Andreo's website Deep Sky Colors and would be a good Ps addition for you, Alan. A little word to say that Rogelio, who has contributed so much to the imaging community, recently lost his wife. I'm sure we all wish him well and recognise his efforts to be creative and helpful to others. Olly

My guess is that the RASAs will prove mechanically stiff, the only question being primary mirror stability. Olly

Is that in effect a ball head with limited range? Olly

My experiences are contradictory. This just worked like a charm from the off. Two Tak FSQ106N scopes, one with Cassady T-Gad tilt pan adjuster, Mesu 200 mount, Atik 11000 mono CCDs, working at 3.5 arcsecs per pixel. When we switched to two TEC 140s working at around an arcsec per pixel we were plagued by slight trailing on the 'slave' scope (the unguided one.) Olly

With a side by side dual rig your saddle plate is at right angles to its regular orientation. It clamps a dovetail to which both OTA's are attached at right angles, meaning they are now pointing in the usual direction. To balance a side by side rig, begin with the counterwieght bar horizontal and the scopes pointing to the zenith. One scope will be heavier than the other so you slide the dual dovetail horizontally in the saddle plate till they are balanced. Now point them horizontally and move them fore and aft to balance, before returning to re-check the previous balance. It's inveitbaly a bit iterative. The counterweight balances as usual. FLO do tilt pan devices, large and small, for dual rigs and guidescopes. The one for guidescopes would carry a camera lens. With a lens it might be easier to mount the smaller unit piggyback on the top, needing less hardware so less expense and less wieght. Olly

Certainly the right principle but I think the washers may be made of something soft to avoid cracking the porcelain. Olly

How about these? https://www.cromwell.co.uk/shop/spindle-nose-and-workholding/fixing-components/fc25-17mm-bore-2pc-spherical-washer-set/p/IND4252860G?gclid=CjwKCAjw1JeJBhB9EiwAV612ywuGB3-wUoFeLSvyLeIvLvuv9H-fFkqIHwGNrCVVmAzJX7PMlg6tmxoC6NgQAvD_BwE They were spotted by Mark on the other thread. Olly

Well done that man! That's the thing to try. Many thanks. Olly

Maybe I'm missing something. What's the item marked 'Slob' above the rail? Olly

Thanks, but this won't allow the scope tubes to be slightly misaligned, which in reality they need to be for the optical axes to be aligned. Olly

If the balls and cups were made of aluminium with a high coefficient of friction I think they would help. The idea is to link the scopes together with additional fixings after the tilt pan adjuster has aligned them. The tilt pan is a short device, a fraction of the length of a long OTA, so the advantage of the braces we envisage is that they could go at the ends of the OTAs, so gaining mechanical advantage. Tomato and bro are using something like this to good effect but without the tilt adustable joints it takes a lot of setting up. High res dual rigs are well known to be hard to keep parallel with trailing showing up on the unguided scope side. Olly

I've followed your principle using the rim of a 7.5cm jam jar lid. (Cheapskate!) The dewshield is curved round out of thin alloy sheet from the hardware shop. You're right about it not dewing: we had a heavy dew this morning on all the kit but nothing at all on the corrector. An unexpected bonus. Olly