iantaylor2uk

Thanks for that, but I don't really understand why the calibration near 0 Dec is better than recalibrating near to the object you want to image.

I took this the other night (14th Jan) just before I imaged some DSO's - processed to make it look rougher than usual

When you calibrate use 1 sec exposures but once guiding I find it is better to up this to 2 or 3 seconds, so you are not chasing the seeing. For my set up I also tend to have the RA and DEC aggressiveness set to around 50%, but you'll have to play with these to find what's best for your mount. I tend to use the ASI Air Pro for polar alignment and even though that may report it as 3', the PHD2 logs usually report it as higher (6' or so). I find anything less than 10' works OK.

What I meant was that in view of the original poster not being able to see any decent images, dithering is lower down on the list of priorities in my opinion. Clearly the most important things to do are: (1) frame the object properly, (2) get the focus right, (3) get the gain settings and sub length right so you can preview the image to check it looks OK, (4) get the autoguiding working, and then take a series of images, stack them to see how they come out. If there is walking noise, you can add dithering later on.

I wouldn't bother dithering at all until you start seeing good images - you can worry about that later

You should be able to see the histogram in the acquisition software, and you should be able to use the preview function before you start taking hundreds of images, to check it looks ok

Each sub has a lower dynamic range if you use higher gain, but you choose the sub exposure time so you don't fill the well, and you will get the dynamic range back when you stack lots of frames

If you put the gain up, you will have more signal and lower noise, so that should help

What acquisition software are you using? I tend to use the ASI Air Pro, and the images you see on the screen are automatically stretched, so the object would have to be very faint not to show up on the screen.

The length of time you can go with your subs with a CMOS camera depends on how dark your skies are - if you have very dark skies, then 5 or 10 mins are fine. For most of us with light polluted skies, you are better off with shorter subs, since you are picking up light pollution from the sky background. Also the other rule of thumb is that the narrowband filters usually need 3x the length of subs compared to standard UV/IR filters. So if you're doing 5 mins with a narrowband filter, you really shouldn't be doing any more than 90 secs with just a UV/IR filter. There is a very good video (from Robin Glover) on YouTube and there are Excel spreadsheets around in which you can input your camera, your settings, and sky background, and it tells you the optimum length subs to use. I find stacking a few hundred subs is quite quick in DSS - although it appears other software is not as quick. Obviously you can use the preview screen to check you are not overexposed with the sub length, gain settings etc that you use. The other benefit of using shorter subs is there are less that have satellites flying through so you have fewer to throw away, and some people with lower level mounts may prefer shorter subs to avoid guiding issues.

Good point - although the ZWO 071 camera uses quite an old sensor (IMX 071) it does have quite large pixels (4.78 micron pixels) and I have been impressed with it, despite the older sensor technology. However I am tempted at some point to move to the newer ZWO 533 camera. Matching the pixel size of the CMOS camera to your scope is well worth thinking about before you choose the camera.

Hello Dave - I started astro-imaging back in 2000, where I used a Philips Vesta webcam to do lunar and planetary imaging, and I was then one of the few people in the world who had a SAC-7 long exposure camera (that I imported from the UK). I also had a few Atik CCD cameras. What put me off back in those days was the tiny sensor size - it was often a struggle to get the object onto the sensor (especially as I didn't have a Goto scope). I now use a ZWO 071 which has an APS-C sized sensor, and if I use that on a WO 81 refractor, the field of view is almost 3 degrees by 2 degrees! Speaking for myself, I use either 30 secs or 60 secs exposure with a UV/IR filter, and always keep the same gain value and cool the sensor to the same temperature (I tend to use a high gain of 200, and cool the sensor to -5C). If I use the L-enhance filter I tend to use 2 min exposures. If you keep the same settings, you can use the same dark and bias library. Taking flats is usually quite quick and easy to do using an LED light panel. You can definitely get good results with some of the modern CMOS cameras, but if you are used to CCDs, it will be a change, particularly with regard to taking shorter subs and stacking more of them. I would also encourage you to play with the gain. Although many imagers seem to prefer unity (or lower) gain, there are a few of us who prefer higher gain and even shorter subs (the benefit of higher gain is also lower noise). The image below is just from 2 hours of 2 min exposures (using gain setting of 200 and the L-enhance filter) on the Rosette nebula, using the ZWO 071 camera (on the WO 81 f5.9 refractor)

If you use Nebulosity, there is an auto colour balance feature which corrects it - I assume other software could do the same thing

I have setting circles on my Losmandy G11, which are very good (and large) and used to use them before I moved to the Gemini-2 goto system. What I generally did was find a bright star close to a DSO I wanted to image, and adjust the setting circles so the RA/DEC of the star was correct, and then moved the mount to the position of the DSO I wanted to go to. With an APS-C sized sensor and small refractor (such as a WO 81) the field of view was large enough you could usually find most DSOs using this method. However, it is much easier now I have Goto! I think it will be more difficult with some of the tiny setting circles on smaller mounts (I remember a HEQ5 mount I used to have 20 years ago and the setting circles were pretty useless on that mount).

With the new ASI Air Pro firmware, you seem to be able to keep an existing calibration (or you can delete it and start a new one). If I aim towards Orion (which is roughly south and at zero DEC) and calibrate there, should I then use that calibration even if I move to a different position. For example, when I moved the scope towards M97, and recalibrated, it came up with a message that the calibration was quite different to the previous one (which I assume was because of the different RA/DEC position). Should I simply have used the original calibration (from when I pointed at Orion) rather than recalibrated?

I find the L-enhance filter is good to use when the moon is up

I think disks only form if the gas cloud is rotating - there is a paper online at: https://adsabs.harvard.edu/full/1976ApJ...210..757S which looks at why globular clusters are spherical. If a gas cloud forms a star then the star is obviously spherical (neglecting rotation), although any other smaller bodies formed around the star are usually in a disk around the star.

Imaging with the Tak 102 TSA tonight

This YouTube video is useful: https://youtu.be/Ror5OXKV8Sg

The rotator still works with the flattener attached

It just pushes in and then you rotate the adjuster on the flattener which forces the three rubber bands onto the inside of the tube, and acts as a self centring mechanism when it is properly tightened. There is a video on YouTube, I'll see if I can find the link

I have the L-enhance filter and have found it to be excellent on the targets it is optimized for. Some people have reported star halos on the L-extreme filter.

I use my WO 81 GT IV scope with 1x Hotech flattener with an APS-C camera (a ZWO 071) and it is pretty flat right out to the edges - you can play with the spacing to get it right but I use a spacing of 55 mm from sensor to shoulder of flattener. Some photos taken with this combination can be seen on my blog. The recent one of the Rosette nebula was good, although this has been cropped down a bit.

I use a 1x Hotech flattener (which in fact can be used for any scope between f/5 and f/8) on my William Optics 81 GT IV and have been happy with it - although it isn't a reducer (so focal length is the native 478 mm).

On my ZWO 071 cooled camera I generally use fairly high gain (200 on that camera, which has a max of 240) and 2 mins when using the L-enhance filter, but would generally use 30-60 seconds if I'm only using a UV/IR filter. This works fairly well for me.