cfinn

All very good caveats, thank you. I did a couple of independent checks of that calculator a while back and it all seemed to check out ok. I completely agree on the QE, I was generous on that front, and you are right to say I have ignored transmission losses. You are also right to say that there is a choice to make on how close you want to get towards noise free sub. My view is that we are well into diminishing returns by +5%, but I accept some may want to get even closer than that. Even with all of that considered, I do think this proves that it is possible to swamp read noise in subs of less than 10s in duration with latest CMOS cameras, which is what makes this whole exercise quite feasible in my opinion. I'm also operating at f/7, which is hardly ideal. Furthermore, the relationship between optimal exposure time and read noise goes as read noise squared, so even small reductions in read noise as CMOS technology develops will translate into big gains, making sub exposures of 1s or less extremely viable. This means the bigger problem in my view is the compute power necessary to process all those tens or hundreds of thousands of subs!

Yes. Running the calculation through here the sky background in luminance (300nm bandpass) will be roughly 2.6e/px/s. I have captured a screenshot so you can see the parameters I have chosen. Following the reasoning here, the optimal exposure time to swamp read noise is roughly 10 x read_noise^2 / sky_background_rate. So that is around 5s.

@rorymultistorey I could potentially be of some help here. Currently working with an Esprit 120 - 840mm focal length @ f7 and a QHY 268M. Not the best scope for the job (a fast 10”+ aperture scope would be ideal), but the camera is probably as good as it gets right now. If I operate in high gain mode at maximum gain, then I am down to 1e read noise, which in my suburban skies (SQM 20) means I am swamping read noise in luminance after around 5s. My image scale is 0.92”pp, and if I can get my EQ6-R guide RMS down to 0.4” or so then I can support that image scale, which would critically sample a FWHM of 1.5-2” or thereabouts. The question is then just whether or not I can achieve that FWHM with a choice selection of 5s subs. Your video certainly suggests a meaningful resolution improvement should be possible! Charles

I use and can recommend the Baader LRGB set. They are completely parfocal with my Esprit 120, which is a real time saver as I still focus manually with a Bahtinov mask. Can’t comment on SHO yet as I don’t have a set, but I am planning on getting the new ultra narrowband 3.5/4nm filters when they are released. Baader claim they will also be parfocal, which would be fantastic.

Hi Bogdan, No problem. Good to know you have better results with the extra counterweight. The physics suggests this should be the case but nice to hear it works in reality. Charles

I use an EvoGuide 50ED with ASI 120MM Mini riding on top and it works great. To attach it I have a 15” universal dovetail and use a pair of Losmandy female to Vixen female clamps. I’m including a photo below so you can see what this looks like. It’s very rigid, which you want to minimise differential flexure, but comes with the disadvantage of adding more weight. Note that I also have a Pegasus Pocket Powerbox Advance attached using a pair of additional clamps. All of this together with the imaging camera, filter wheel etc gives a total weight of nearly 17kg, necessitating the use of the counterweight extension shaft on my EQ6R. This gives a bigger moment of inertia and amplifies the guiding RMS in RA slightly, though I am still sub-arcsecond with careful balancing. To counter this, I have a couple of additional counterweights on order to negate the need for the shaft extension and achieve balance closer to the mount. Just something to be aware of. An OAG will definitely save on weight but may be harder to use (accounts vary) and gives a narrower FOV, which means fewer guide stars and less opportunity to use recent advancements in guiding (multi star). Charles

Welcome to SGL! Your findings make a lot of sense. The chart I made based on your setup/location was assuming the use of the ASI 294MC as you say, which has an effective bandpass of roughly 100nm since every pixel essentially has either a red, green or blue filter over it. Using the mono variant with a luminance filter effectively increases the bandwidth to 300nm and you benefit from higher QE as well, so for broadband targets (galaxies, reflection nebulae) you can get to the same SNR in less than 1/3 of the time. For these kinds of targets I don't think there is benefit to be had using a light pollution filter because you will cut out almost as much light from the target as you do from the sky, particularly as street lights move to LEDs. Different story of course for emission nebulae. I think your experiments show that the best way is to go mono to maximise bandwidth and QE and get as much integration time as possible. Sub-exposure length is probably best kept quite short as well, so that you don't saturate pixels with the sky glow and preserve dynamic range.

Just ran another calculation to back this up (as if further proof were needed!). The chart below shows the integration time necessary to reach increasing levels of signal to noise ratio on a target having surface brightness of 25 mag per square arcsecond in Los Angeles (Bortle 8/9). This is with a RASA at f/2 and specs to match the camera in the description (ZWO ASI 294MC Pro). The dashed vertical line shows the integration time of 49.2 hours, which suggests a SNR of roughly 5 should have been reached on IFN of that surface brightness. If you want SNR of 10 you would have to go for 150 hours! Having said all of this, it's pretty mind blowing any of this is possible because at this point we are talking about imaging something that is 10 million times fainter than the sky background. I think this should bolster owners of the RASA that much is possible with such a fast instrument, even from the middle of the city!

I quite agree. My suspicion is that it gets lost in processing all too often, due to the fact that it is usually very spread out and could be mistaken for background in background calibration.

Lovely image!

Here is the discovery paper in 1976 by Sandage et al. In the abstract it states the brightest portions are ~25 mag per square arc second in the V band. Note that these are the brightest portions only. Much of it extends to much fainter surface brightness. I am not completely surprised by this, otherwise there would not be reports of detecting the IFN visually. It is also a question of signal to noise ratio. My calculations above are for a SNR of 5, which is quite small. I suspect many detect the IFN but then lose the signal in processing what with background calibration, removing light pollution gradients and so on. To get great images like Goran's will require much higher SNR, which is where faster f ratio combined with darker skies really helps you. Binning will of course compensate for slow f ratio, which you mention you have done above. The argument for the RASA is clearly that you can cover a much larger area of sky than you could with an f8 RC with large pixels or aggressive binning. One other thing to say about my calculations are that they are for a mono sensor with a luminance filter. For an LRGB image you would need to multiply the total integration time by 2 (assuming same integration time per channel) or for RGB/OSC cameras multiply by 3!

Absolutely, I would agree with that. If capturing the IFN is your primary goal then the RASA definitely seems to be the best choice.

The more I think about it, if you want to go this deep for not an extortionate amount of money then a RASA/HyperStar or a fast Newtonian are your best/only options. You could use a small refractor with a focal reducer, but then I don't think you can get to f4 or below without problems, plus you start to under sample and lose detail. You could also trade focal ratio for big aperture and large pixels (e.g. CDK, RC), but then you have a very long focal length and would need a very large corrected field and a large sensor to maintain a decent image scale, so things start getting expensive quickly. Also the bulk would mean you couldn't travel to dark skies if you live (like most of us) in a light polluted area. Despite being a refractor fan, I would probably conclude that the apex of value, performance and versatility for average seeing and variable weather is probably an 8-10 inch, f4 corrected Newtonian Astrograph, such as this. With modern CMOS sensors this gives you an ideal sampling rate of 1"/pixel, you get good sky coverage with APS-C sized sensors and good SNR on faint targets in reasonable amount of time. You would need four 5" f7 apochromatic refractors imaging simultaneously to match it! The downside of course are those diffraction spikes and Newtonians are a bit cumbersome. They don't cut the clean, slender lines of a nice refractor and they are slightly less easy to use. So I guess if money was no object, I would spend £20k for a Riccardi Honders astrograph such as this!

Quite a striking difference!

Seeing some amazing images of the IFN (integrated flux nebula) in the deep sky imaging section of the forum, I thought it would be interesting to run some calculations to see just how difficult this is to do. The IFN is a component of the interstellar medium at high galactic latitudes that is illuminated by the integrated light of our galaxy. It is incredibly faint, with the brightest sections having a surface brightness of just 24.5 magnitudes per square arc second at visual wavelengths. Let's suppose that we are in a Bortle 5 area, with a sky brightness of 20 magnitudes per square arc second. This is a fairly typical scenario, and the best that many people can realistically hope for. In this scenario, the IFN is more than four orders of magnitude fainter than the sky background! Many now image with CMOS sensors with small pixels, so let's suppose that those pixels are 3.8 microns in size. We'll assume we are using a mono sensor with 80% quantum efficiency on average, and a luminance filter with a bandwidth of 300nm. We'll also assume we are using sub-exposures long enough to swamp the read noise of the sensor, so the dominant source of noise is photon shot noise from the sky. Under the above scenario, the chart below shows the total integration time in hours required to capture targets of varying surface brightness with a signal to noise ratio of 5 using telescopes of different focal ratios. The dashed horizontal line indicates the surface brightness of the brightest portions of the IFN, with more being revealed at fainter magnitudes. At f7, we require a total integration time > 13 hours just to capture the brightest portions with a SNR of 5! The benefit of faster focal ratios is very evident here, which explains why many of the best images I have seen come from fast systems such as RASAs and Newtonian astrographs. The other way to improve the situation will be with larger pixel sizes (or binning) or seeking out darker skies. Charles

For sure!

In this article they acknowledge they weren’t halo free and have put in significant R&D time to develop new coatings which supposedly are (nearly) halo free. The proof will be in the pudding...

No idea. And no idea on price either. They say a “moderate increase in price” compared to previous version, so still considerably cheaper than the high end brands I would imagine.

So Baader have announced these new “ultra-narrowband” filters (3.5/4nm) with bold claims about finally solving (or at least drastically minimising) the problem of halos. I will be looking to buy a set of narrowband filters come August/September so it would be wonderful if this is really true, because Chroma/Astrodon are definitely out of reach for me and I’m sure many others. Hopefully they would be parfocal with my Baader LRGB set too, which are themselves completely parfocal. https://www.baader-planetarium.com/en/blog/new-baader-cmos-optimized-ultra-narrowband-and-highspeed-filters/

I see! Goodness, I think I will revisit this when I get the chance so I can more accurately describe and understand what I did. I’m keen to ensure what I’ve done is reproducible and makes sense.

Hi Olly, Perhaps you can help me understand this better. I was essentially trying to reduce/remove the nasty green halo from the bright star bottom left and also to remove some purple mottling in the background, which amounted to playing around with the selective colour sliders for both green and magenta in Affinity Photo. It seemed to do the trick and also helped me get to an overall colour balance I was more happy with. Do let me know if there is a better description for this process or if there are better ways to do it. It was the one part that was just trial and error for me so I would love to develop a more theoretical understanding. Charles

Here is my entry, last minute once again! Workflow as I remember it: 1. AstroPixelProcessor: combine LRGB - light pollution correction - star colour calibration - histogram stretch 2. Affinity Photo: selective colour to reduce green and magenta - curves - levels adjustment 3. Starnet++: remove stars 4. Topaz DeNoise AI - noise reduction on starless image 5. Affinity Photo: add starless image from last step as a layer on top of image from step 2- set to overlay - apply high-pass filter to bring out medium-scale detail - repeat with another layer to bring out large-scale detail 6. Topaz DeNoise AI: noise reduction and sharpening on image from step 5

Hi, I use the following to transport all the components: https://www.astroshop.eu/carrying-bags/oklop-carrying-bag-skywatcher-eq6-r/p,62811 https://www.astroshop.eu/carrying-bags/oklop-padded-bag-for-tripods-eq6-neq6-azeq6/p,56173 https://www.astroshop.eu/carrying-bags/oklop-padded-bag-for-counterweights-2x5kg/p,56174 Really nice quality for the price. Very happy with it. If you prefer, there are also some options that reuse the packing foam for additional protection such as this: https://www.rothervalleyoptics.co.uk/geoptik-pack-in-bag-for-skywatcher-eq6-r-pro-mount.html I hope this is helpful. Charles

Hi all, just thought I would share an update on this. The replacement unit arrived this week. All in excellent working order, except for the focuser which couldn't hold any weight. I found the four grub screws underneath the body of the focuser that control the focuser tension and a couple of them had become a little loose from shipping. I tightened them up gently and all is now completely fine. The dew shield assembly on this unit is also better than my last. It slides along the tube much better and does not sag as much as the previous one, so that is a bonus. Those blue shock absorbing balls in the case also stay in place a lot better! I had clear skies and temperatures of 4C ambient last night, so I took the opportunity to do a quick test on a bright star (Capella). A visual star test at high magnification looked good - collimation spot on and nice even energy in the diffraction pattern either side of focus. I also used this as first light for my new QHY 268M with a single 30s exposure in luminance. The stellar halo is not free of artefacts, but I think that is to be expected for a star of this magnitude. It looks reasonably well controlled, and the stars across the rest of the field appear perfectly round with nice clean halos. So, for the time being I am satisfied. The real test of course will be next Winter season, but right now I am just pleased to have a telescope back to enjoy using once again. I purchased a Herschel wedge as well, so I can get some good use of it during the summer with some solar observing. Charles

Ready with my Esprit 120 and new QHY 268M. And that trail of light in the sky is an airplane contrail catching the evening sunlight!