The Admiral

My, possibly spurious, understanding is that when you perform an alignment what you are doing is getting a fix on where stars are at a point in time, so that the 'model' is calibrated. But Polaris doesn't change position with time, and so is rather less than helpful in getting a 'fix'. I would not recommend using Polaris for alignment. I also try to align on stars which are approximately 90° apart in azimuth, as it strikes me that using orthogonal axes would improve the precision. I choose bright stars between 40° and 60° altitude as well as separated by about 90° in azimuth. In addition I always level the mount so as to minimize the corrections that have to be made to get the model 'fixed'. I normally just used a 2-star alignment. Ian

What mount are you using to track the heavens? The maximum exposure I've used with my NexStar Alt-Az is 30 seconds without getting too many streaky subs, though others have managed to use a little longer. At the end of the day though you will be limited by physics, in that you will get field rotation. Lowest towards the East and West, and worse as you increase altitude towards the zenith. I used to limit it to 60°. Ian

Glad that you've managed to tame your mount. I think the backlash setting just makes the correction movements take longer so that the mount can take account of the fact that for a short time the drive will be turning yet not actually moving the scope, if you see what I mean. Ian

I'm a little perplexed about using an ASIAir Pro with a DSLR which is not one of the listed models. I'm thinking for example of the Sony or Fuji cameras, and others, that are used for imaging. Unfortunately I've not been able to find any up-to-date manual for this product, all links refer to the original ASIAir manual it seems (unless someone can point me in the right direction). If this is the case, then that is a very poor situation for a new product. Now I fully understand that the merit of the device is to interface with the imaging camera, be it a ZWO camera or one of the listed DSLRs, so that the whole gamut of features can be utilized. But if the camera is not included is it still possible to control the device? I've wondered if the little port called "DSLR" is a simple jack terminal which can provide shutter control, as would be implied here: Looking on the ZWO forum it doesn't appear so, as others have recently asked the same question. Unless they are still referring to the original product. I haven't managed to get approval to post a query on the site yet. Has anyone got an ASIAir Pro and who can clarify the position please? Ian

Can you provide link please? I've found a link: Ian

I should perhaps add a couple of other things. As happy-kat said, the tiny zig-zag movements that the scope makes to keep track can lead to a number of frames showing movement, and as a result sometimes a good proportion can't be used in the stack. Another thing is that field rotation is minimal in the Easterly and Westerly directions, maximum in the Northerly and Southerly directions, and increases with the altitude. However, I do appreciate that you are concerned with the overall drift of the target within the frame with time, which the above won't affect. It may be perfectly adequate for visual observation however, and perhaps that is what the mount is primarily designed for. The scope itself is an f/12 design which will require long exposures on feinter deep sky objects, more than is really usable for alt-az photography, and will be the cause of your difficulty with plate solving. I am sure it would be more than adequate for the Moon, planets, and globular clusters, and possibly some select deep sky objects. Most of us Alt-Az imagers tend to use rather faster optics, say f/6 or faster, often in the form of camera lenses. Looking on the Celestron site I see the mount does three tracking modes, Alt-Az, EQ-North, and EQ-South. What are the latter 2 modes for? Is it for use on a wedge, not that I would recommend getting one? Ian

There is an often cited myth that you can't do astrophotography with an alt-az mount. Not true at all as you will see if you look at Happy-Kat's link. For sure there are some limitations, like field rotation and exposure duration, but work within those limits and you'll be OK. If you already have the mount, don't rush out and buy an equatorial (or a wedge), spend some time with it and hone your skills and see if astrophotography really appeals. I agree with all the points mentioned by happy-kat. My mount used to keep an object more or less centred throughout the session, so it should be possible (I just used the handset to run the mount). Keep your exposures below 30 sec to start with. Also worth checking that your mount is set to sidereal tracking and not planetary or lunar*. Leveling and inputting the exact time and location is also critical, including whether daylight saving time is being used or not. Good luck. Ian * unless, of course, your target is a planet or the Moon ;<)

This is a timely post as I've just ordered this reducer/flattener for use with my Altair 102mm, with a fl of 710mm (the same as the Z103)! Pity that the light skies will make setting up a little problematic. I very much like the idea of having a built in adjuster, it should save a lot of messing with spacers. The current info on the WO page is at least consistent That's my understanding too, but you've thrown in a bit of doubt with the statement about the labels being reversed! I suppose it'll come down to sucking it and seeing, but I'd be interested to read something on the basic principles at play here. Ian

Hmm, confusion reigns! Ah, I think I'm confusing the ASI120-MM/MC, which appears to be discontinued and is not supported, with the ASI120MM-S/MC-S, which is the larger USB3 version and is supported. My mistake, sorry about that . Ian

Thanks for all your advice. It looks very clear that the ASI120 will serve my needs, and save me a bit of cash . This hobby is expensive enough! Ah, I have been confused over whether it is supported by the ASIAir Pro. However, I see that according to the ZWO website the ASI120mini version is supported, but not the larger USB3 version . Even better then, a bit of future proofing! Ian

I'm am going to be taking my first steps in guiding and experience the joys of guiding for the first time in the coming season. I shall be using a 50mm f/4 guidescope. It looks as though this will give me ~ 3" per pixel with the ASI290 and a FoV of about 1.6° x 0.9°. With the ASI120 it will give me ~ 3.9" per pixel and a FoV of about 1.4° x 1°. My current imaging system gives about 1.4"- 1.7" per pixel with Fov of between 1.9° - 2.3° x 1.3° - 1.7° depending on the reducer I use. I would prefer not to have to align the guidescope in order to find guidestars. It would seem that either camera is suitable, but what benefits would spending almost double on the camera bring, other than that the ASI120 mini is not recognised by the ASIAir Pro or PrimaLuce Lab's Eagle Core, should I ever get around to using them? Is there any detriment to using a colour camera rather than a mono? Ian

It seems I belong to an increasing group as well then :<). Ian

For example, this site https://www.photonstophotos.net/Charts/RN_e.htm#Canon EOS 600D_14 where the curve is for the Canon EOS600D, as it is better known on the UK. If you hover your cursor over the data points you can see what the read noise in in terms of electrons rather than its log*. However, the 'dynamic range' worsens as ISO increases (https://www.photonstophotos.net/Charts/PDR.htm#Canon EOS 600D), which means that the sensor is more readily saturated by bright objects at high ISOs. So there has to be a compromise. This is one of the problems with AZ imaging, that individual sub exposures are of necessity limited and the fact that you need to take many of them you also want to keep the read noise down. It's not clear that from your message that you are stacking lots of image files, but if not, then you should aim for dozens, if not hundreds . You probably are though, as I see you have used calibration frames. Ian *Edit. Below I've plotted the the read noise against ISO, with the ISO scale linear rather than the usual log scale, and it shows more clearly where the read noise levels out.

Lovely rendition, and has been said before the core is nicely tamed. These multi-bandpass filters seem to be gaining traction, how well do you rate the Altair? Can I ask what you are using to process please? Ian

True, they have their drawbacks, but the lack of an anti-aliasing filter and good Ha response has given me some pleasing images. I had my X-T1 available anyway, so why buy something else? I wouldn't necessarily buy one if I had nothing else to use, but good enough for what I need at the moment, and would only be replaced by a proper astro camera. Ian