I wanted to compare 2 filters this evening as I have recently purchased the new Antlia Triband RGB Ultra filter. The original filter I had for the one shot colour was the ZWO Duo-Band filter.
M42/M43 Filter Comparison
As can be seen above the difference without a filter is quite dramatic (Top – No Filter, Middle – Antlia Triband RGB Ultra, Bottom – ZWO Duo-Band). There is more broadband light captured. The red nebula is less apparent and the background sky is much brighter.
Running Man Nebula Filter Comparison
The above image from the running man nebula, NGC 1977 demonstrates that without a filter a reflection nebula comes through best (Left – No Filter, Middle – Antlia Triband RGB Ultra, Right – ZWO Duo-Band). The ZWO filter gives a more green image over that of the Antila, which in itself reduces the reflection nebula but does start to pick up some of the red emission nebula within the Running Man.
Inverted background sky and stars
The inverted background above gives a sense of the reduction in star luminance that is allowed through without a filter.
This image shows the background with the details of the readout from each of the pixel across the colour channels. Here you get a sense of the green seen in the ZWO filter is less the extra green coming through, moreover the lack of blue being allowed through. Without a filter the background sky is swamped with all colour channels.
With no filter the full effect can be seen above, much brighter background, nebula less colourful and less detailed.
With the Antlia filter above, the final single image I personally find is much more pleasing.
Finally with the ZWO filter you can see quite clearly the green effect.
So in summary I would say the ZWO filter is better than no filter except when imaging reflection nebula, however the best filter is the Antlia filter when paired with my one shot colour ZWO ASI2600MC camera.
Below is a random drawing of a scientist with a Tak laser beam.
So I opened the dome late this evening as it was not due to be clear. However an opening in the cloud meant I could test guiding again on the 12″, especially whilst it was light in the late Spring weeks.
The first job was as always to focus which brought me to a reading of 61944 at 19.83℃.
Focus run
Another small job was to sort the guider FoV out. I went ahead and used M92 to align the guider.
Aligning guider FoV using M92
The final FoV settings are here for completness.
FoV for guider
Set AS1600 to Gain and Offset 10 due to cluster being very bright and I needed to set a standard of 60 seconds minimum exposure. Gain 139 and Offset 21 gas saturated unless I selected 15 seconds, Gain 75 and Offset 12 saturated at 30 seconds so hence 10 and 10 which came in about 58k ADU.
I then performed a slew to a nearby star so I could centre the scope, there platsolve completed successfully and I updated TSX and the FoV for the 12″ with the new angle.
Platesolve
The first image of 60 seconds came down and was out of focus, I then realised changing the profile SGPro forgot the autofocus setting, so I had to stop the run, delete the images and set the original focus point then rerun.
M92 out of focusM92 in focus
Next I ran a few images but then to my horror I had the same guiding issue, where the star moves being dragged up and down in a periodic way. I slewed elsewhere and tried again and the problem did not occur. I was near M92 and just East of the Meridian and quite high up. Not sure why that is a problem.
Near the Meridian
I could not resolve, I waited a while then performed a meridian flip and low and behold the problem went away, again not sure why. I still have this terrible noise coming from the RA motor/gear area. I decided to bite the bullet and take off various caps on the scope listening and looking inside. I decided it was not after all, the through the mount cabling but coming from the RA gear itself, so I looked for the MEII guide for removing the worm block and then followed the instructions to take off the RA cover.
RA gearing and belt noise
This gave me instant feedback on what the issue was, the belts driving the axis were making a noise. On looking through forums on Bisque.com I found a few people with similar issues and needing to grease the belts, they were told Lubriplate was a good grease. This is an American grease so I will find a similar here and then apply, I will ask Bob first for his suggestion.
So the night wore on and the LRGB frames of M92 I thought I would take whilst testing guiding progressed. At one point the imaging stopped due to cloud. I just caught the dome before it closed to change the safety sensor due to cloud. When it cleared it never really cleared, with the sky temperature reading about -14℃.
Not very clear
Nearing the end of the imaging session, I had caught about 15 frames of each of the filters.
Good guiding and imaging
The guider was behaving mostly with he odd funny jolt. By 3:30 am the sky was lightening very quickly.
3:30am and bright
By this time I had stopped guiding and imaging. I closed the dome, slewed the scope to the flat panel and proceeded to take a set of LRGB flats for Gain 10 Offset 0 and also Gain 139 and Offset 21 as request from the previous nights imaging.
Another visit to Combe Gibbet tonight with another astronomer, GingerGeek. After forcing him to leave the comfort and safety of his own home, where his girlfriend gives him tea, coffee and beer and with the luxury of mains electricity, he joined me at the very dark, very blustery sight near 1,000 ft up in the North Downs of Berkshire, formally Hampshire, they moved the line!
We left at 7:30pm and after the 50min trip arrived at the long road up to and past the Gibbet, which is really not much of a road at all.
Not a road, looking down the bottom part of the track
We drove carefully to the top, both parked our cars and set about setting up for the evening. GingerGeek had his Tak FSQ85 on his CEM 60 iOptron mount whilst I setup my usual array of large 4″ binoculars and at first the Mak 180 on the Paramount MyT. Later I would swap to the Esprit 120ED. To celebrate the outing, little geek had brought some beer 🙂
Beer O’Clock!
The Sun soon started to set, the day trippers hung around to watch the sunset and then were gone. I started looking at the Moon through the Altair Astro binoculars which is just a wonderful sight. A Camera cannot capture the experience of seeing the Moon with its Earthshine and in full as the FoV is 1 degree in the binoculars was a wonderful framing.
Moon through binos taken afocal with iPhone 6sPlus
Next up was Venus before it sets, it is amazing just how bright it still is given its phase. I found in the binoculars eventually as it was hidden behind that cloud band in the distance. I then tried to get it in the Mak180, when a few new things happened. Firstly I could not slew to it as I had yet to polar align, so I had to place the scope in the right place. At the focal length of the Mak180, some 2.7m it is difficult to find something faint behind cloud just by pointing. I eventually gave up. I then noticed my laptop power had diminished by nearly 30% over about 20-25 minutes, this was due to Firecapture just hogging the USB bus capturing 100’s fps. So I was going to start the night short on juice! So I turned Firecapture off whilst I went to find Mercury in the binoculars.
Venus through 4″ binos taken afocal with iPhone 6sPlus
Mercury was a challenge, so much so I could not find it, I put that down to that not well placed cloud band. Meanwhile I pointed the Mak180 at the Moon to have a look, but by the time I had finished I the laptop was done to 53% !!!! Not so good.
So it was time to chat to GingerGeek (GG) before I put the Esprit 120 on the mount. GG was having lots of problems setting up. Firstly he had swapped the rings on the mount for the Tak that day and was struggling to balance the scope. It was so bad the scope kept dropping nose first then camera first and then either way depending on its orientation. Eventually, after much cursing, actually a lot of cursing, GG settled for the imbalance and continued to setup, unfortunately not before he knocked his beer over in his boot of the car ? fortunately I did not laugh too much ??
I went back to my setup, placed the Esprit on the mount and then set about getting focus with Ezcap, the software that comes with the ZWO camera. I do find the software very straight forwards and does what it says on the tin as it were. I then slewed to one of the open clusters I wanted to image and realised it resided in the North and that the twilight was still very much apparent so not suitable for imaging. Instead I settled for M51 high up as to the West was the Moon.
M51 300s Image uncalibrated
It is a lovely image at 5 minutes, I could see instantly at least 4 other galaxies and the colour of the main Messier galaxy was very pleasing. I look forward to processing the resulting subs. I set the timer for 1 hour (12x300secs) and went back off for veggie soup, cheese rolls, brownies and coffee that GG had kindly brought along.
crop of uncalibrated 300s M51
GG at this point was having issues focusing for his 5 minutes shot of the Pelican in Ha, he had made some other changes to the software before heading up the hill too which was confounding him. After some more time he finally had focus and started to image. However whilst a few of the images were okay, the resulting imbalance and gusts of wind made it difficult to keep pin sharp images. It should also be notes at this point that GG and I were running from the same car battery, although GG was only running the camera from it, the mount was running from his Lithium battery.
Stunning Pelican Nebula – 300s Ha uncalibrated
At just before 1am the inverter connected to the spare car battery turned off due to loading and power. Everything stopped for both of us ??? however GGs mount kept running due to the Lithium battery. I closed down my setup and allowed GG to reset his camera and reconnect, he then went on to start imaging, however the resulting image had moved significantly and GG decided to give up. So we spend the next 40 minutes packing up. Whilst this was going on we looked at Saturn and Jupiter through the binoculars which was a wonderful sight. Now for the 50 minute drive home to unload the car just before dawn, although by 2am it was clearly getting lighter.
Guiding graph on Esprit 120 Paramount MyT last night, all over the place with wind
The SQM for the site last night was 20.91 although the Moon was very bright. The site is also very dusty, and my laptop was covered in the morning. Another incident was that I inadvertently unscrewed the cover from the guider as I transferred from one scope to another and the glass cover fell out, I now have dust and dirt on the sensor to clean. It is not a great design by ZWO for the ASI290MC as it really needs a locking grub screw to top that happening or a reverse thread.
Herbig-Haro object at the end of the long feature can just be seen.
GGs image has set us on a little project to image the HH 555 bipolar jet at the end of the major turn of gas in the Pelican Nebula. We will attempt over the next few nights to get an image from both the Esprit 120 ED Pro from the IMT3 dome in Ha and also from the OS 12″ to see what it looks like compared to the Tak FSQ85. Another good social distancing astronomy session ? goodnight.
Tonight I traveled 50mins and 26 miles to Combe Gibbet, a high point for us in the South of England called the North Downs in Berkshire, which at 940ft above sea level places it in the wind, so it is cold but affords a distance from many towns, so it is dark. The is indeed a Gibbet at the top if one should want to take a hanged man or woman and display them for all the surrounding villages to see ? very barbaric, but part of our history.
The car was packed with various astronomy gear, the Mak180 for Lunar and planets.
Mak180
The Esprit 120 ED Pro.
Esprit 120 ED Pro
And of course the Altair Astro 4″ binoculars with my Paramount MyT mount.
Altair 4″ Binos and Paramount MyT
I met a a social distance my friend Lawrence who was in a much better car than myself more suitable for the off-road terrain of getting to and just past the gibbet. Lawrence brought his trusty binoculars and his deck chair. Meanwhile I setup the 4″ Altair binoculars, the Mak180 OTA on the Paramount MyT and my Canon 6D on a tripod.
All set for tonightSun setting
As the Sun set from this location we spied Venus first and took a look through the binos.
Moon, Mercury and Venus
I captured some frames in the Mak180 with the ZWO 294MC camera. We then moved to the sliver of a Moon 2.8% illuminated and 1.6 days old. I placed the Mak180 on this for a few frames also. Lastly we moved the binos to Mercury, which is unbelievably small. Very faint in the twilight sky and surprisingly faint in the binos. I once again slewed the Mak180 and captured some 4GB files.
Lawrence looking at the Moon
We then went a hunting for comet C/2020 F8 SWAN but it did not appear in the star field where it should be despite being able to see Mag 8.9 star. The comet was purportedly magnitude 5.8 but this was not the case. I checked my ephemeris on both SkySafari and The SkyX and I wass definetly in the right part of the sky and confirmed the star paterns from my star hoping, but alas no comet.
Lawrence looking at Mercury
So although the wind was now dying down both Lawrence and I were cold so at just gone midnight we packed up and set off home. I must remover by coat tomorrow!
Addendum, I had read an article the following day that the comet may have broken up but I cannot yet confirm this.
I have placed new filter, Baader IR-Pass 685nm from Mark Radice on the ZWO tonight and aimed to go after Venus on the Mak180 and capture another phase of the planet. I recorded several sets of data although noticed the filter does produce a somewhat pinky image.
So the main approach here was to start testing the ZWO ASI1600mm on 5min images and decide which is the best Gain and Offset to use. As the object is a planetary nebula I have used my Astrodon 5nm OIII filter to bring out the faintest parts of the nebula. To be thorough, and this will take time, I plan on running the tests for all 7 filters I have.
I have done extensive reading on the topic of image analysis and hope to apply here what I have learnt. Given then camera is running 12 bit, I have a maximum pixel value of 4096 which represents saturation and then any further response is non-linear. Once I have completed 5min testing I will try for 10, 15 and 20 mins. I will then perform further testing by taking a sample set of 10 images to stack and see how that compares with similar total exposure times across the frames.
Amp glow is a particular problem with CMOS. Despite the ZWO site suggesting that amp glow is virtually removed in the Pro Cooled camera, it is clearly not, as can be seen in single 5 min subs. The good thing is a dark will remove it effectively. What I need to make sure is that the amount glow does not swamp the image so much that it overpowers the signal from the faint nebula.
Increasing the gain and offset value from left to right you can see a marked increase in the amp glow. The image slices below are taken from the far right of each frame.
The offset figures in relation to the gain figures have been taken from my reading of various material. The median values are that of the background and the maximum values that of the stars. You can see on this 5min exposure that by the time I reached a gain of 300 one or more of the stars are saturated. In fact the brightest star in this slice is SAO 22510 which is mag 9.53.
Another way to visualise the saturation effect is looking at the raw unstretched image, whilst a star is visible in the image using gain 139 and 200, on close inspection within PI and looking at the values of the pixels of the star they are not saturated. However gain 300 is. The purpose of this is that an unstretched image is not the defect for telling if parts of the image are saturated as some texts describe, but one can see the increased brightening of the star by gain 300 to know it is a problem.
So whilst I have seen the clipping a a few stars at the highest gain I have tested, what about the planetary nebula itself? From the below stretched image one could assume that the brightest part of the nebula was fairly bright and heading towards saturation, but don’t be fooled! Also there is a noticeable increase in the background brightness as the gain increases.
Again as for the amp glow, the aim is to balance the ability to amplify the faintest parts of the nebula without swamping them with the background brightness.
Again here are the values of the settings for gain and offset against the central section of the image.
So how bright did the background get? The graph below shows a section of the background free from stars and charts the increase of brightness from a mean figure of 9 ADU with the gain set to 0 and a mean figure of 104 with the gain set to 300. So a large increase but but at least up until gain 200 not a problem, as we will see when we look at the faintest part of the nebula later.
This graph looks at the bright star SAO 22551 (HIP 8063) which is mag 6.66 and the brightest star in the image. Again as previously seen in the right hand slice of the image the star is saturated by gain 300. All figures are the maximum pixel values.
Now let’s focus on the nebula itself and go back to using the mean ADU figures. The picture below shows the section of the nebula I will use for analysis. In particular I focused in on the brightest lobe of the central portion of the planetary nebula and the faintest portion of the left arc.
So looking at the faintest nebula within the left arc we can see that it is not very bright at all and the brightest it gets at gain 300 and offset 65 has a mean figure of 96 ADU. Each and every image at the different gain setting and offset setting is seemingly just below that of the background, which in itself is interesting as the nebula seems to be fainter than the background. So more analysis was needed.
However I then went back and looked at a selection of areas of the background across the image to find that the original background selection to the bottom left of the image was brighter than other areas. Below you can see the image of gain 200 and offset 50, this time with 5 selection boxes. Preview 6 is the nebula as recorded before is mean 56 ADU. Preview 5, so the sky right next door to it has a mean figure of 55, so just below the nebula, hence it is only barely visible. Preview 1 is 54 ADU and Preview 4 is also 54 ADU. So there is brightening on that bottom left corner of the image, so had the nebula fallen at that spot then it would be swapped by the background.
There is only 1 ADU between the nebula and the background adjacent to it at gain 200 offset 50. If we looked at the same to regions in the image of gain 300 and offset 50 then you get a 2 ADU difference. The image with gain 300 and offset 65 gives a 3 ADU difference. So the results show that both gain and offset both help increase the contrast between the background sky and the faintest part of the nebula.
Various previews can be seen to analyse the background vs nebula brightness
The final image below shows the brightest part of the nebula. At gain 300 and offset 65 you see a mean value of 544 ADU which compares to 96 ADU for the faintest part of the nebula and an adjacent background of 93 ADU.
The final piece of information pertains to the camera/chip specification and performance. The graphs below are from the ZWO website and clearly show as expected the more you increase the gain the read noise is lowered but unfortunately so is the full well maximum (the amount of electrons you can store in a pixel) and the lower the dynamic range, which for deep sky objects is a required.
So from this first piece of testing what have we learnt? Whilst there seems to be a good sense for increasing the gain and offset to help with the SNR especially between the background and the faintest part of the nebula, the increase in amp glow, decrease in dynamic range and reduction in the well count are all factors. Stacking as we will see, will undoubtably help the situation without necessarily setting a high gain. You can see why people say use Unity Gain, so the setting where 1 electron on the sensor = 1 ADU potentially gives the best result from a tradeoff point of view.
I have spend over 4 hours today reading about the Gain and Offset settings for the ZWO ASI1600mm Pro Cooled mono CMOS camera I have on the back of the 12″ Officina Stellare 305 RiDK f/7/9 telescope.
ZWO ASI1600mm Pro Cooled on Officina Stellare 305 RiDK
In particular the posts by Jon Rista and the images with a similar setup from Glen Newell have led me to a handful of setting I will now try from my location and on M76, the Little Dumbbell planetary nebula that I had started to image recently. I must also comment that Kayron Mercieca also had some useful information pertaining to testing your camera and OTA imaging train for exposure times. See link here
Discussion on exposure times and setting – Cloud Nights
So I have already taken a set of images on the 8th October, 14 of them and they were at a Gain and Offset of 10 (I believe these settings are less than perfect) and an exposure of 1200s, so 20mins through an Astrodon OIII narrowband filter. My location is on a good night in the Orange Zone as per the charts borrowed from the forum discussions and when referring to broadband imaging. For narrow band as per my test here I am between the purple and blue zones.
Inspecting the original frames I took you can see slight amp glow from the right of the image, the background has a median of 10 ADU at 12bits. None of the stars are saturated or clipped. The brightest star is 1,854 ADU our of a dynamic range of 0-4,095 ADU. The faintest nebula I can see is 11 ADU so just above the background and the brightest part of the nebula is 77 ADU.
M76 – 1 x 20min OIII Gain 10 Offset 10 – ASI1600mm Pro Cooled
So I will attempt to take a set of images at the following settings across 4 exposure times of 300s, 600s, 900s and 1200s at or after astronomical night at 20:56 onwards if the clouds hold off.
Gain 0 – Offset 10
Gain 75 – Offset 12
Gain 139 – Offset 21
Gain 200 – Offset 50
Gain 300 – Offset 50
Gain 300 – Offset 65
So after several false starts of broken cloud disrupting my ability to keep the dome open, I managed to grab the first 6 frames of 300s as above. Here is an animated GIF of all the images in order of Gain lowest to highest. (Click the image to animate or right mouse click and download)
In my next blog I will look at the analysis of the first 6 frames whilst I take the other frames to compare.
An unexpected clear spell this evening, I was sitting out on the patio looking at the clouds clearing and so setup the dome to perform the Periodic Error Correction (PEC) analysis for the mount.
To perform this I needed to unplug the hand controller for the MEII, unplug the ST4 guider cable, turn off a bunch of settings within the autoguider software with The SKY X (TSX) and also turn off TPoint.
I then connected the ZWO ASI1600MM to TSX rather than SGPro. This was so that I could record the log needed for the PEC through the autoguider add on software which records in a format that the PEC software requires. The challenge again was that I could not get the ZWO camera to connect in TSX. I just kept getting error 200. Searching TSX forum I finally found the issue and downloaded the latest driver from ZWO but through the link from Software Bisque. To install I needed to log in as Admin.
So I started to record the star movement without performing any guiding. Once done I imported the log file Autoguider.010.log into the PEC portion of TSX.
I then performed a fit so that you could see the sinusoidal waves before I then fitted the correction to it. A quick look using PHD2 Drift Alignment to see what the drift now was, was very promising with a sinusoidal wave over 10-15 minutes.
Final fitted curveModified CurveSinusoidal Drift Alignment check
I then went off and tried to image unguided to see if it made a difference, it had, I recorded a 10min unguided image through the 12″ 2.5m focal length scope with no trailing of Altair.
10min Unguided Altair exposure
I then attempted to slew and take an image of the Elephant Trunk in Ha again, however I was foiled by not only the cloud moving in but also not being able to get past the message Guider Settling. I need to talk through with GingerGeek to see why that is. Meanwhile bedtime for Mr Shave-Wall.
