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Definitely used with lens, and yes, the focus was better. Not sure how much difference it will make on a wide angle lens? Here's the only image I can find of the mask in action (pre final adjustment):

Afraid I can't answer your first question about whether it would be worthwhile, but how to get one is a bit easier. I tried to get one for my 200 mm Canon lens and failed miserably: there seemed to be no commercially produced ones available. So instead I used one of the on-line websites to generate a graphic, inputting lens diameter and the other details the site required, then printed it on to OHP transparency film and attached it to a cardboard ring which fits over the lens hood. It works OK and was cheap enough to make that I don't mind if it gets damaged occasionally.

I think I can see the problem, perhaps worth raising over on the user group? https://groups.io/g/DeepSkyStacker

That's not quite how I understand it, Neil. May be into terminology differences here, but I think that only calibration frames in the Main group are applied universally. So if you load lights, darks and flats into group 1 (confusingly, the second group tab) then they only apply to that group. So with multi-session stacking perhaps one should only load offset files into main and put everything else in groups 1, 2, 3 &c?

It does have the appearance of walking noise and your comment about the gradual object shift tends to support that. Are you able to dither between frames? If so, you may find that helps particularly if you then stack with sigma clip in DSS (needs at least 20 subs and more is better). There are software solutions as well but it would be better to eliminate at capture time, if possible. Another question to ask is how tightly bolted down is your guide scope/imaging scope combination? I found the rather floppy focuser on my ST80 was very good at generating this sort of gradual drift.

What fabulous data, thanks so much for the opportunity. Processed in StarTools 1.7.417 according to Ivo's ( @jager945 ) work flow, and then 'adjusted' in Photopaint.

You might want to consider this from the other end, as it were. How accurate is your guiding and how is your seeing? The results might not be any better imaging at 0.33 " per pixel (your first option with the C9.25) if the seeing never gets below 1.25 arc secs or the mount won't guide at better than 1.0 " . On the other hand, if your mount guides at .5", you have nice stable skies and use a focal reducer then imaging at .5" on the good seeing nights might give very satisfying results?

As far as laser collimators to avoid like the plague, I bought a cheap (~£20) Seben Laser collimator a few years ago and it rates as possibly my worst astro purchase to date. As delivered, The on/off switch didn't work properly and it would randomly switch itself on in the box resulting in a flat battery every time it was taken out for use. The index mark for the intensity settings and the off position was way out of alignment. The laser dot is a spread out, diffuse kidney bean rather than a dot. It was miles out of collimation and the collimation grub screws (minute 2 mm allen headed bolts) were buried under black mastic which was awkward to remove. I did eventually get the switch to work, but only if used on the two settings either side of 0, but I have never managed to get it collimated satisfactorily. However, I know other users have had good experiences with them so perhaps mine was just a bad example?

You might want to loose the eyepiece tube in order to get close enough in to reach focus. Here's what I used to do with my 200p : Hope that makes sense?

I think they may be caused by diffractionspikes/reflections from Shedar (Alpha Cas) and Achird (Eta Cas). Something in your imaging train (Skywatcher's Coma corrector is one possibility) may be picking up on these. Here's your image overlaid on the surrounding area and the directions of the stars and the strength of the spikes is quite persuasive:

I image with an 8" reflector. Started out guiding with a finder scope, switched to an ST80, but finally ended up with an OAG. This was a game changer as it dramatically improved the roundness of stars and guiding accuracy, generally. My back garden is fairly wind protected but if wind strength is above 10 mph from the east or 12 from the west, a lot of subs get lost...

You can get car adapters for most laptops. They are designed to plug into a cigarette lighter socket. I run my (secondhand) Panasonic Toughbook when out in the field for about an hour and a half on its own battery then either plug it in to the car for a recharge or into a spare lead acid accumulator. It doesn't take too long to acquire a full charge and I don't have to stop imaging.

If you can get a circuit board and the pre-programmed IC from Tom, I'd say definitely build one. However, Tom has posted a notice saying that during the current pandemic he doesn't want to send out boards and ICs. I built one a couple of years ago and it is still in use on my home modified EQ5 mount whenever I use my 200 mm lens.

The 55mm is taken from the knurled flange on the CC. Depending on the camera, you may need an adapter/spacer (sorry, on phone so can't see your sig/equipment list). There's a good one for Canon cameras, although a little expensive!

That's interesting, thanks! I've got several (slightly embarrassing ebay over order) step down converters sitting idle in the electronic spares box and was wondering about wiring a separate power supply to the fan. I'll do some experimenting.

Thanks for that, Dave, always great to get member's field experience.

Many thanks, Julian. Just the info needed!

Thanks Gina and James. I thought satellite, but couldn't find a suitable candidate - operator probably!

So, warmest night of the year and my home made cooler for my Canon DSLR had to be shut down! Why? Well despite almost unbelievably good guiding statistics in PHD2, I was getting elongated stars, even on 5 second exposures. Eventually I put my hand on the camera and felt noticeable vibration which completely disappeared once the power to the cooler (peltier + heatsink + cooling fan clamped to the back of the DSLR) was removed. Now it's not the greatest cooling system ever, but it does drop the EXIF reported camera temperature to anything between 12 and 15 °C below ambient. Perhaps more importantly, it keeps it constant at whatever it reaches and I'd really like to be able to reinstate it. So, does anyone know a source of reliable, vibration free, cooling fans? This one came (I think, it's a while ago now) with the cooling kit and heatsink and such but perhaps I should go elsewhere this time?

Anyone able to help me identify this object? Parts of the trail showed up on three adjacent 30 second subs at 00:29 and 00:30. This is the middle one (unprocessed apart from size reduction and conversion to jepg) and the trail should be 30 second worth of movement. The central (very) faint fuzzy is NGC 6946 (Fireworks galaxy). Can't find a nearby satellite on my admittedly fairly elderly version of Stellarium but assume that's what this is?

The distance between the flange on the Skywatcher Coma Corrector and the sensor needs to be 55 mm (your second statement). But, I have used the SW CC for some years now and while it flattens the field on my 200p, it suffers serious reflections if there's a bright star in or near the frame. I recently upgraded to the Explore Scientific CC, which although longer and heavier (and more expensive), does a much better job of suppressing unwanted reflection artefacts. The Skywatcher version reduces your effective focal length to 909 mm (according to calculations from All Sky Plate Solver) while the ES one increases F/L to 1065 mm. Not really significant but of mild interest.

Afraid I don't know enough about your particular camera, but for my Canon 200 mm lens I use a home made Bahtinov mask and live view or connect to a tablet or laptop and focus using that. The free version of APT (Astrophotography Tool) has some built in focusing aids, if that will work with a Nikon?

I had something similar on my Canon 700d a few years back. On advice from the camera supplier, (Juan Fierros - www.cheapastrophotography.com) I disabled live view and the problem went away.

I hope your replacement guidescope solves the problem, but if it doesn't, you might find that getting rid of the scope rings with their plastic tipped screws and clamping the guidescope firmly to the dovetail with, for example, clamshell clamps and then clamping the dovetail to both the main scope's rings will help reduce the mechanical flexure. There are several occasions in your guide logs where the DEC performance is actually better than the RA - I haven't looked at all the logs, but here's one example: When the Dec goes wrong, it's large (the largest peak below is almost 20 arc seconds) and pretty rapid and, as the DEC motor was not running, not down to software. Here's an example: There is still some backlash (it takes 15 pulses before the small deviation at 00:53 comes back within limits, for example) but this doesn't seem to be the main problem.

I see your point, but like you, I am not sure if the distance at which the secondary (which is flat) intercepts the light cone is significant? Anyway, with a slightly different collimation and a change of connector to the focuser, last night's 'quick 'n dirty' (pre-nautical dark) shots were a lot sharper and the field a lot flatter: What I should have done, of course, was to rotate the camera through 90° and compare the results. I may be blaming the scope for something that is camera caused. Next clear night, I'll give that a go.