bobro

Lovely image well executed with the SA. Wish this nebula was higher in the sky at my location. Yes, as @geordie85 says, looks like walking noise - resulting from pattern noise appearing to move across the final image due to drift in the subs. A couple of things you could try with the subs already taken to see if the final image improves: 1) If you are using Average stacking in DSS, try Median (or perhaps Kappa-Sigma) stacking to help reject too high/low pixel values. 2) Try stacking with many more dark, flat and bias (at least 30) to see if cleaner calibration frames help. For future narrowband imaging longer sub exposure and a higher ISO would help reduce the read noise component. Better polar alignment would also help reduce sub drift and so the appearance of walking noise, although a longer exposure makes drift more likely. Dithering together with Kappa Sigma stacking is a very good method of dealing with walking noise. To dither automatically a control program (such as Astro Photography Tool) tells the camera when to expose and then, following the exposure end, tells a guiding program to dither, with dithering complete before the next sub starts. It's not clear from your post but I think you are not using a program for exposures but just the camera (or perhaps intervalometer). As you are using guiding, if you wish to dither manually you would need to stop guiding, press the SA buttons to move the mount a little in RA and DEC, and then restart guiding. This doesn't need to be carried out after every frame - every 5 frames should be ok for a 30 frame total. Use Median or Kappa Sigma stacking - this will reject pixels too far from the norm, resulting in a reduction of various types of image noise in the final image. The SA only supports RA guiding/dithering. Dithering in RA alone can make walking noise appear as it may produce a similar effect to drift. Therefore the best solution is likely to be manual dithering in RA and DEC. Hope this helps.

The overlay is from nova.astrometry.net where images can be uploaded for platesolving. Once you have posted a number of times (can't remember exact number - 10?) you will be able to attach images.

Some very aggressive use of curves and down-scaling to reduce noise starts to reveal some stars. Thought you might like to see the constellations you captured. Longer subs will certainly help.

There's real hires and detail in the image and I'm looking hard for the wispy bits. Isn't there any way for it to rise above the black clipped background and show even more detail? A bit of cheating perhaps....

To give a slightly detailed answer to your questions. Your exposure for the sky looks spot on (exposure for the buildings and Moon are ignored). The reason for the graininess in the image is due to the way photons (light) arrive from the sky - photons don't arrive in a smooth fashion but more like buses, which may come all at once or with long gaps between. The result is that adjacent pixels on the camera sensor collect different number of photons for a given exposure time, resulting in differences in brightness that we perceive as grain. With buses, if we averaged the arrival rate of them over a day it would work out ok. Same thing for light - the longer the exposure the smoother the grain appearance as the arrival rate of photons on adjacent pixels starts to even out and the image becomes smoother. ISO makes no great difference as it only amplifies the pixel capture readout (including random photon arrival variation noise) received on pixels but makes no difference to what the pixel captures. The key way to reduce apparent image grain (ignoring post processing of images and thermal sensor noise) is to increase the number of photons captured as this will even out noise. The simplest way is to use the lowest f# with the setup that produces an acceptable image as a wider aperture lets through more photons in a given time. The most used method is to increase the overall exposure time - with a single image this may mean reducing the ISO to prevent sensor readout saturation whilst increasing the exposure time. When imaging exposure time results in the target starting to become saturated or stars become linear (star trails) in the captured image, multiple stacked exposure are the way to achieve the aim of lowering noise whilst keeping an acceptable image as this increases the overall exposure time. A different perspective compared to one shot photography, but that's the way long exposure imaging works to reduce noise. Getting a little more technical, modern DSLR cameras tend to have small pixels. Whilst that is great for images taken in bright light, in astrophotography the small pixels accentuate the noise due to random photon arrival rate over the small pixel surface. To counteract this, images are binned, which means combining the images capture from adjacent pixels, resulting in lowered noise due to the effective larger surface. This can be done in camera by using a lower resolution or in post processing. Hope this is useful in some way.

Median stacking is a simpler method that just picks the central sub pixel value based on the number of subs sorted by pixel value. Doesn't quite give the same noise rejection as Kappa Sigma but is better than an average value which includes a fraction of each sub. Worth a quick try perhaps?

The default Kappa Sigma setting for Lights (image attached) normally works pretty well. I understand a minimum of something like 6 subs is required, though I have never gone that low in number. Per Channel Background Calibration is used to adjust to differences in sub brightness so that Kappa Sigma works effectively. As long as the aircraft trails (lights) are not in the same place, Kappa Sigma stacking should remove the offending pixels. If you wish to post the subs on a file sharing site and put a link here, I'm sure you will get help with using DSS. Bob

Kappa Sigma stacking in DSS will reject pixels that are too far from the norm and works very well at removing aircraft and satellite trails.

I find flats a little tricky to judge due to uneven screen illumination of my laptop, so I look at the sky background values using GIMP. Some gradient likely came from the sky background (not very dark!), though I tried to remove it. No doubt purchasing a 130PDS would improve things a lot, especially as I shortened the main tube on my Meade reflector to allow a DSLR to come to focus. The side effect of pushing the camera outwards doesn't help the focuser. I still have the 150PL to check out with the new 130M camera. Even with x2 binning it's likely oversampled with my setup at 0.8 arcsec/pixel, but let's see how it goes.... Bob

Thanks @JRM and @Astrobits for the responses. The application is imaging. Whilst I wouldn't go to the expense of purchasing a quality secondary mirror for the scope in question, I do have doubts about the current mirror. Of course the way to find out is to replace the mirror - as I enjoy a bit of diy and tinkering I'll look out for a SkyWatcher mirror on Astroboot and do a comparison. Bob

My first attempt at LRGB using a recently acquired uncooled Altair 183M (thanks @Adam J for advice). It certainly takes some thought to work out how to process the image. Plenty of issues to work on - a loose RA coupling didn't help guiding, CC spacing not correct yet and registration wasn't made easier by the lack of a filter wheel - next purchase! Still, it was great to see colour added to the initial lum image taken whilst waiting for the RGB filters to arrive. All subs 120 sec: 77 LUM, 22 each of RGB. Processed in DSS and GIMP.

This level of imaging is way above my experience, so all I can say is great to see such registration, colour and round stars from corner to corner. Plus an interesting red fringe to M57. Keep well!

Some time ago I changed the primary mirror on my Meade 130 Polaris for a mirror from a SkyWatcher 130P as images suggested the main Polaris mirror wasn't great with obvious aberrations. The result was an immediate improvement in the quality of captured images. However, the secondary mirror remains the original from the Meade. I wonder if there is a way to test the secondary to determine its quality, assuming it has a significant involvement in image quality?

The Quattro 10S is quite a heavy OTA (although within the stated capacity of the EQ6-R) so will tax the mount to some degree. Perfect OTA balance isn't always a good thing, especially with a heavy load. When I started with imaging I had a reflector on an (overloaded with extension bar) EQ2 mount and would see similar images to yours if the setup had not been adjusted to be east heavy. I tried west heavy but is didn't work. If the setup is near perfect balance it can stutter in RA, producing images such as yours. When east heavy the motors keep pushing the scope in RA and provide smoother movement. Not really connected to periodic error - just a question of taking up the mechanical slack/backlash. The good news of course is the image you posted with no trailing - a sign of great things to come!

In essence that's what I meant Adam. I've been comparing images taken with my Canon 1000D (on 650/130 and 150PL reflectors) and new 183M on the 650/130 reflector. The smaller sensor/lower resolution of the 183M (in conjunction with the scope optics) seems to show brighter stars as larger for the same image fov. So yes, the image scale is very important and a good CC may make a useful difference. I'm pondering how an M13 will look with the 150PL (skies allowing). I think that brighter stars will look large but with the image scale M13 will also be larger. Then there are the aspects of arcsecs/pixel and slower scope to consider, so it may not be worthwhile.

Nice colourful and sharp cluster Neil. Interesting contrast with my M13 - a recent first light B/W image ( 30 min) with an Altair 183M (non-cooled) and coma corrector not yet installed. Your larger DSLR sensor (I guess that is what you used) provides smaller and sharper brighter stars. Bob

Yes, but one night when you look upwards and the sky is super clear........

Quite lovely and the (candifloss) nebulosity is great. Rotated left 90 degrees provides a view easier to appreciate on a computer screen. Makes me want to dig out the 135mm lens I purchased a while back to try a bit of wider field - but never got round to doing!

Quite amazing what can be achieved in processing. Whilst not astronomically correct, I'm drawn to the starless version - sometimes less is more!

Would be interesting to see the full frame.

As @david_taurus83 suggests, practising with polar alignment and bright star targets such as M13 (or M3) is a great way to tune your setup and can get you ready for winter when the bright Orion nebula etc comes into view. Imaging galaxies unguided is harder as they are much dimmer. I recently destroyed my guide camera by plugging too high a voltage PSU into a hub, forcing my setup back to EQ5 unguided. I have to say the subsequent images weren't great - even at 30 secs. There's something I haven't quite got right with my setup, so it depends on guiding to keep it on track. If you can get your EQ5 tracking well unguided that will be a great start. Good luck! Bob

The dual axis motors are in fact stepper motors, which allow the simple handset to accurately track a target in the RA axis direction in order to keep the target in the same position in the fov. I initially used the dual axis motors with AstroEQ before upgrading to motors better suited to goto. As mentioned above, there is no goto functionality with the simple handset - a synscan handset and motors are required for this. A major drawback of the dual axis motors is the painfully slow slewing speed due to the motor gearing - takes an age! That means it is often best to unlock the clutches and point the mount manually before tightening the clutches for final positioning using the simple handset, followed by tracking.

What doesn't seem to have been discussed earlier in this thread is that the ASI294MC is a colour sensor, whereas the ASI120MM is mono. Hope I'm not missing something here, but here is a stab at an explanation (I'm just starting with understanding the differences between mono and colour imaging). Due to the colour filters on the 294MC, photon capture rate will be reduced by a factor of 3. This is equivalent to the difference between stacking a single sub and three subs or alternatively the difference extending the overall imaging time by a factor of 3. Taking into account the relative f#s of the two scopes and the different pixel sizes results in a 1.6x higher arrival photon count per pixel for the 120MM compared to the 294MC (as noted above by @vlaiv). The end result is the 120MM captures 3x1.6 = 4.8 times more photons per pixel than the 294MC (assuming same QE). For the same overall exposure time the SNR will be higher for the 120MM due to more photons being collected. @microbe increased the gain of the 294MC image to try and match the target brightness of the two scope images taken with the same imaging time. This will increase the apparent on screen noise of the image. Part of this noise will include read noise and thermal noise, which will be a higher portion of the 294MC overall noise due to the lower photon capture rate.

Having spent all of my working life in design, production and sale of relatively small market though high margin specific electronic products, the answer lies in the route to market- base component costs aren't so much more than a mass market product, but intrinsic cost comes from the design, production, marketing and selling costs of a lower volume product. These costs are increased by a lack of sales competition e.g. can you find different selling, or even temporary promotional, prices for a given astro product? We have to accept that our interest lies in a small and specific sector and that means cost to us. In addition we like to have good service from our suppliers and that adds to the selling price.

It's a slightly chilly and damp 12 deg C down here - hence more comfortable testing indoors! (Plus working on the coma corrector spacing.) When first powered up and already at ambient, with the fan running the sensor settles at 2-3 deg C above ambient within 5-10 minutes, so not much time lost there. Good ideas about building a dark library - thanks. Bob