symmetal

Stefan, the latest images are better and more consistant. Note the min and max FWHM are much closer. Yes, there does does look like a little top to bottom tilt so I should make a tilt test jig and get it fixed. I've found on all my setups that tilt occurs mainly in the camera and getting it right on the jig improved the results greatly and it puts your mind at rest too. 😊 Alan

Sorry Stuart, I missed the mount in the title. Also I believe the AZ GTI uses servo motors with shaft encoders and not stepper motors so my previous reply doesn't apply. Servo motors use DC motors so stall current is potentially an issue, but I assume the motor drive will have current limiting circuitry to avoid any stall issues. Anyway, even if not, 10 seconds at stall won't be a problem. 😊 Alan

You don't say what mount, but almost all will use stepper motors to drive the axis. Stepper motors operate in a stalled condition all the time with the current limited by the stepper driver circuit. Physically preventing the stepper motor from turning won't cause any issues so you will be fine. 🙂 Alan

It's worth getting a pair of rubber strap wrenches similar to these which should loosen them, as well as freeing any stuck adapters etc. 🙂 Alan

Stefan, I just noticed that SGP lets you centre crop the auto focus frames to stop edge stars affecting focus. Here's the help file description This should help your NB autofocusing to stop focusing near an edge which may help. If you do find that there is consistant tilt then it's easiest to make a test jig as shown here out of some odd pieces of wood and a cheap laser pen. You can put the camera, filter wheel and Zwo tilt adjuster on the jig and can remove the tilt adjuster to make any adjustments before refitting it and retesting. Much quicker and easier than having to do it outside with test images. Using the tilt rings you mentioned would likely be even quicker. Can be done indoors during the day too. 🙂 Alan

I was wondering why your final focuser positions were changing so much but I assume these were taken with and without the filter wheel etc. for test purposes. Your focuser backlash setting and step size look to be OK, but with your quality not always being 100% you could try another 10 units or so of backlash compensation and see if you get more 100%s. Having too high a backlash compensation setting is not a problem. Another thought is that your auto focus exposure settings are not optimum and they are being under or over exposed during autofocus. If you have noticeable vignetting then corner stars will be dimmer so possibly appear sharper than central stars which will be more exposed and throw off the autofocus results. Your O3 images tend to have the better results with the optimum focus more towards the centre compared to Ha. If you use the same exposure times for all your NB filters autofocus then the Ha autofocus images will be brighter than your O3 images with more central overexposed stars which are ignored, making the stars used for autofocus being more towards the edges. It may be worth trying lower autofocus exposure times for Ha and see if the results improve or are more consistant, hopefully focusing more towards the centre. I think it's worth leaving the tilt adjuster off and making up the FF spacing required. Any tilt indicated will be the same for all filters so it's best to just use L for tilt assessments as it's quicker than waiting for NB exposures. Just try and get the NB best focus to be more towards the centre which I think will remove the indicated tilt on the NB images. Alan

Hi Stefan, Your latest images are varied and all show rather large FWHM figures implying seeing isn't great. They have all got best focus in the top left corner. The filters themselves won't cause issues like this. For checking tilt issues it's best to use short L exposures of like 15 secs to minimize guiding issues affecting the results. Also use an average of like 8 or so images as consecutive images are unlikely to be identical. As far as focus is concerned what autofocus exposures are you using and are you binned 2x2 for autofocus to reduce exposure time. I use SGP like you and use 3s for L, 5s for RGB and 30s for NB. Is your autofocus graph a well defined U or V curve and the first segment, top right part of the curve and not more horizontal. When it does its validation exposure is the HFR reported at 100%. If you have insufficient focuser backlash compensation then the final focus position will not be optimum and tends to favour one of the edges or corners like to seem to have. A screen shot of your autofocus final curve plot would help here. Check the autofocus backlash compensation is set to IN direction so it always finishes focusing pushing the focuser up against gravity. Is the tilt adjuster set to no tilt with the small allen screws loose when the large screws are tightened up. It may be worth taking the tilt adjuster out and do some tests as I found with my initial cheap adjuster that it introduced tilt even when it was set for no tilt. Under SGP Auto focus options there is the option to use HFD, with ASTAP installed, rather than HFR which may be worth a try. As mentioned a screenshot of the final autofocus screen for each filter would be useful. Alan

Hi Stefan, Running your latest images posted through CCDI they are much better. The min and max FWHM figures are much better than the previous two images implying the seeing was poor then. The top right of the 4 images below has the best FWHM, evenly distributed and has no tilt indicated. Note that the comparative difference between the best and worst FWHM is fairly consistant on all six images posted. 2.17 / 1.79 = 1.21 Best image below 6.73 / 5.18 = 1.3 Worst image from previous post The two previous 'tilted images' have been focused on the left hand side while the best images below are focused nearer the centre. The bottom two below are focused top right. If the two previous 'tilted' images were focused near the centre it's likely they wouldn't display any variations as tilt in ASTAP or CCDI. Overall, I now don't think you have a tilt issue, and the 'tilted' images are the result of rather poor seeing and the best focus being off to one side. I assume you're using autofocus. I've found that autofocus will often choose to focus nearer one edge rather than nearer the centre, possibly due to the distribution of stars picked for the autofocus routine. It would be nice if there was an option to have the autofocus 'weighted' towards the image centre. Also, your 56mm spacing is probably better than the default 55mm, as the filters, and the camera protect glass, will increase the required FF spacing distance by 1/3 their total thickness, so I should put it back to 56mm and see how things turn out when the seeing is better. 🙂 Alan

I ran your images through CCDI and it looks to be purely tilt left to right in the camera itself. The 55mm spacing looks good as the 3D view shows just a tilted flat plane. If the spacing is wrong the corners of the 3D image tilt upwards. As the 1600 is not a large sensor the spacing isn't that critical anyway. The tilt rings you linked to look like a lo-cost solution, and are quicker to adjust than the push/pull screw tilt adapters. A possible issue is that as they force the threads to be tilted when done up the threads are only tight on two opposite spots with the areas inbetween 'floating' between threads. As long as they are done up tight enough and you don't feel any rocking they should be OK. Worth a try I would think. 🙂 As the tilt is in the camera, this means you can't rotate the camera 90 degrees for framing without altering the tilt adjustment as well, which would be a pain. Good luck. 😊 0 deg 180 deg 3D view 0 deg Alan

The push/pull tilt adapters are awkward to adjust and they usually have the adjustment screws on the wrong side for easy access. If you can mount it further forward in the imaging train with spacers between it and the filter wheel to allow allen keys to access the screws without having to disassemble everything it certainly helps. The ZWO MkII adjuster looks like it's reversible which is a better design. The easiest to use tilt adjusters are the Gerd Neumann ones though they are espensive and M48 is the smallest, and with adapters to M42 may be too long to fit. Their adjustments are 'push' only mounted around the edge, as the 'pull' uses strong springs. Can't see any links to your subs though. 🤔 Alan

The dark current does have an effect at long exposures with higher temperatures but it isn't too bad. The ASI dark current graph is not very readable as the y scale is exponential. Plotting it as a linear graph it makes more sense Here's the above graph expanded to show the lower temperatures better. At 19.5C the dark current is about 0.03 e-/sec/pix so your 300 second exposures contain 300 x 0.03 = 9 electrons of dark current. The dark current noise is the square root of this which is 3 electrons. With the read noise at 1.4 electrons the total read + dark current noise is 3.3 electrons so the dark current noise dominates. At 0C the dark current is about 0.0024 e-/sec/pix so your 300 second exposures contain 300 x 0.0024 = 0.72 electrons of dark current. The dark current noise is 0.85 electrons. The total read + dark current noise is 1.63 electrons so the read noise is the main contributor. At -15C the dark current is about 0.0003 e-/sec/pix so your 300 second exposures contain 300 x 0.0003 = 0.09 electrons of dark current. The dark current noise is 0.3 electrons. The total read + dark current noise is 1.403 electrons so the read noise totally dominates and the dark current noise is insignificant. At 19.5C your read + dark noise is 2.4 x the read noise. At 0C your read + dark noise is 1.2 x the read noise. At -15C your read + dark noise is 1.002 x the read noise. So your dark noise at 19.5C isn't excessive John and even combined with the read noise is still much lower than a CCD camera read noise, though cooling to at least 0C is better. 😊 But, with the cooling off, the sensor temperature will likely rise significanly above ambient by around 10 to 15 degrees, so if your ambient is 19.5 then the chip may well be over 30C so dark noise is more significant then. At 30C the dark current noise is 6.7 electrons so dark+ read noise is 6.8 electrons which is in CCD read noise territory. Very nice image by the way. 🙂 Alan

Nicely made. 🙂 I used to use XLRs on my original distribution but since mounting everything on the scope I had to use something smaller and ended up with 2.1mm DC jacks, but they're no where near as robust. I assume you used pin 1 on the XLRs for your ground connection. 😉 Alan

I used to have similar issues with Platesolve 2 and SGP when solving rich starfields. Tinkering with PS2 setup parameters may help, but I switched to using ASTAP and it's much faster at solving, particularly when off by more than a degree or so where PS2 is slow, and it never seems to fail. SGP has ASTAP as a choice in platesolve setup. From ASTAP webpage download and install the program file and the H17 star database and you should be good to go. 🙂 Alan

John, I forgot to mention that while unity gain is generally best for LRGB, for narrowband, some imagers use 2 x unity gain, which is gain 200 on the ASI1600. This is because the read noise decreases with higher gain, though at the expense of reduced effective well depth and so dynamic range. The graphs in the ASI1600 manual give the figures. To check you are not black clipping pixels by having too low an offset, just take a 1 or 2 second dark and check the resulting stretched histogram curve in something like Fits Liberator to see what the minimum pixel values are. The image statistics in your capture program should also give the minimum pixel value too, which is a good indication of clipping. It's best to only use one offset for all the gain settings you use, rather than changing offset with gain as it saves you having to keep track of different sets of darks for different offsets. Having to have different sets of darks for different gains is complicated enough when calibrating. 😀 Alan

When starting out with a CMOS camera it's best to select the unity gain setting, where 1 photon landing on a sensor pixel increases the pixel output ADU by 1. This gives a good compromise on dynamic range, output signal and exposure time. On the ASI1600 this is at gain 139. The offset you used seems a bit low and would likely lead to black clipping of some pixels on your images which is never a good thing. Offset 50 is a good setting to start with. You'll need to use dark frames and not bias for calibration, as older CMOS cameras (like the ASI1600) suffer from amp-glow which increases with exposure. Also use flats exposures of at least a second along with corresponding flat darks as bias frames on some older CMOS cameras (including the ASI1600) are different to the bias component of longer exposures. CMOS cameras have significantly lower read noise than CCD so many shorter exposures are taken rather than fewer longer exposures, particularly with 12 bit cameras (like your ASI1600) as they have less dynamic range than the 16 bit CCD camera you're used to. Newer and unfortunately more costly CMOS cameras are 16 bit and have no amp glow, nor strange bias behaviour, so behave more like your CCD camera but with much lower read noise. Fot the ASI1600, L exposures are often around 60 sec, while RG and B exposures are 3 times longer at around 180 sec as they each pass only about 1/3 the light of the L filter. Narrowband exposures are as long as you can manage without seeing tracking errors, often 10 mins or longer if possible. If you have dark skies you can expose for longer. The aim is to expose until the noise from the image sky background is three or more times higher than the read noise, and in so, making the read noise contribution to the total image noise insignificant. If this point is reached there is no difference in S/N of stacking say 10 five minute exposures against 5 ten minute exposures. The shorter exposures are preferred as less highlights will be blown in the final stack at the expense of more storage space needed. With narrowband you won't be able to swamp the read noise by the sky background noise, unless you expose for several hours so just make them as long as you can get away with. You say your images look dark. Your total integration time is fairly low, so will look fairly noisy if stretched too much. I usually start out with 120 L subs of 60 secs (2 hours total) and 20 each of RG and B of 180 secs (1 hour each) and stack and process them to get a good indication of what the target looks like, and then add more subs as necessary to get the noise down. For fainter targets you may need to take 2, 3 or more times as many subs. For narrowband I start off with 10 of each filter of 600s subs, to see how it looks, and then add more as required. This exposing long enough to swamp the read noise works equally well with CCD cameras but due to a CCD's higher read noise the exposures would likely be longer than the 10 or 20 min exposures you usually use with CCD. Hope this helps. 🙂 Alan

Thanks for checking the results vlaiv. 😊 I know the 1600 Zwo gain values for multiples of unity aren't exact but they were the values used by Zwo initially so I kept them here. As 60 Zwo gain units is 6dB which is 2x voltage gain, then half unity gain would be 139-60=79 rather than the 76 Zwo quoted. Likewise 2 x unity gain should be 199 rather than 200. I can redo it with with the actual Zwo gain settings needed but as you say they are close enough. 🙂 4 x unity is 12dB gain so gain Zwo gain 250 should really be 139+120 = 259. For the more modern cameras only one or two gain settings are actually used so the full table is not necessary but as Zwo provided the data I thought I'd use it anyway. For CCD cameras there would be only one table entry so the graphs are then redundant and you'd have a small spreadsheet. 😀 For the 1600 a 2 second dark is probably better to use than the bias ADU, as it's more representative of the 'bias' in longer exposures. They do differ by about 4 ADU, while for later cameras there is no significant difference between bias and shorter darks. Alan

Here's my sky swamping exposure excel chart for the ASI 1600 which I'd posted before, but now modified to include vlaiv's criticisms of the original formula which was posted on the CN forum. It uses the bias ADU value instead of manipulating the offset with camera bit depth and the change in ADU per offset increment. The 10xRN^2 CN formula actually swamps the read noise by √10 (as vlaiv pointed out) which is x3.16, so I've included it as a swamping figure in the chart. These are using offset 50 and my bias reading. If you want to download the excel chart to enter your own values, (in the editable green cells) and also use the handy exposure calculator from a sample ADU and time here it is. 🙂 ASI1600, Sky Background ADU v2 Prot.xlsx If @vlaiv would be kind enough to check it's valid, here's the unprotected excel chart so he can check the formulas. ☺️ ASI1600, Sky Background ADU v2.xlsx The graphs actually reflect the values entered in the table so is a double check you've read them right from the ZWO camera data graphs. The spreadsheet will work for any camera if you have the read noise and the e-/ADU gain values available. The ZWO offset and gain values entered are for information only and aren't used in the calculations. If vlaiv gives it the OK I can make a spreadsheet for any camera that anyone is interested in. 🙂 Alan

Mounting the cover backwards would allow extra thick filters to be used I suppose, which the previous owner may have had, or else they needed an extra mm to get the backfocus right. 😁 Glad it's all OK. 🙂 Alan

It's a different design from mine which is about 4 years old where the backplate extends the full width. I measured the thickness and it reads 19.95 to 19.98mm on my calipers. Mine looks the same as the photos on FLO's site so yours maybe an earlier version which is now updated. 🤔 FLO's picture Alan

Excellent for the relatively short integration. Your Redcat is certainly shining now. 😊 Alan

Clear skies at full moon on 14th June, so imaged it for something to do. Altitude was only 11.5 degrees so it was skimming the treetops. To maximize chance of a sharper image I tried using a solar continuum filter along with a FLT98 scope, 2x Powermate and ASI178MM. Wanted to keep the exposure low at 5mS, and with the filter cutting out a lot of light had to use a high camera gain of 360. 6 panel mosaic with each panel 2mins at 40fps and stacked only the best 10% in Autostakkert as the quality graph was pretty bad so only ended up with best 480 frames stacked per channel. I did try stacking 15% but it was noticeably softer. I also took 10mS exposures at lower gain but again it gave less sharp results than 5mS. Stacks processed in Imppg and PS. I had to resample to 50% in PS to get a reasonable looking result so it would have been better not to use the Powermate I suspect. Click for full size, (3000 x 3000 pixels) Alan

Thanks tomato. That's what I thought would be most convenient. I'll do the same. 🙂 Alan

Thanks @iapa, but the Starizona Filter holder is for cameras with 6.5mm back focus like your ASI294. The ASI2600 and ASI071 have 17.5mm back focus so the filter adapter can be no more than 7.5mm thick to maintain the RASA-8 backfocus distance. 🙂 Thanks @tomato for the details of the adapter you made. It must be handy, having access to machine tools. 😀 Looks like I'll be getting the Artesky adapter though there's no hurry as I'll be trying it wideband for a while I imagine. What do you do when wideband imaging? Do you put the RASA glass back in and use your adapter with no filter or use a 'clear' filter in your adapter? To save having no extra glass in the way was my reasoning for having say a 6.8mm spacer between the 2600 and RASA, rather than the standard 7.5mm spacer with the clear RASA glass in place. Alan

After seeing all the great images produced by this scope with short exposures, more suited for cloudy UK, I've decided to splash out on one, along with an ASI2600MC. 😊 The 2600 may be some time in being in stock so I could try it with my ASI071MC in the meantime as they have the same 17.5mm 'backfocus' distance. I'm wondering how people use it with 2" filters like the IDAS NBZ. The 25mm scope backfocus is with the clear glass kept in the rear of the RASA8. Would it not be better to remove the clear glass as it's only there to allow the Celestron light pollution filter to be used instead, without affecting the back focus. It seems 2" filters like the NBZ can be fitted there instead if the 2" mounting ring is removed but that seems a bit fiddley. If there is no filter or clear glass present I would expect the back focus to be more like 24.3mm so a spacer of around 6.8mm is needed with 17.5mm 'backfocus' cameras rather than the usual quoted 7.5mm spacer. 🤔 Artesky make a filter holder which with the ASI2600 gives the stated 25mm backfocus, presumably with the RASA8 clear glass filter removed. If not using the NBZ filter a 2" UV-IR cut could be inserted here instead rather than putting the RASA 8 clear glass back in to maintain 25mm backfocus. What do people here do when imaging RGB concerning backfocus, and do they leave the clear glass in the RASA-8? Is there any alternative to using the Artesky filter holder? The Starizona holders sold by FLO don't work with 17.5mm 'backfocus' cameras. 365Astronomy sell the Artesky or perhaps buy it direct from Italy, or from TS. Thanks for your help. 🙂 Alan

Dwarf spheroidal galaxies are similar to globular clusters in that they are both roughly spherical in shape and consist of old stars which are gravitationally bound. The dwarf galaxies have stars which are more spread out, and the galaxies are around 1000 to 2000 light years in diameter while globular clusters are around 100 light years across. The light output from dwarf galaxies is spread over a larger area so appear much dimmer, and they are also significantly further away than the globular clusters we can see. Globular clusters we can see exist within the Milky Way galaxy, while the dwarf galaxies are outside the Milky Way. As they are separate from the Milky Way they are classed as galaxies in their own right. Most dwarf galaxies that we see are orbiting the Milky Way or the Andromeda Galaxy. As they contain far fewer stars and are much smaller than their 'parent' galaxies they are classed as dwarf galaxies. Hopefully they are not 'offended' by this classification. 😊 By contrast, open clusters consist of new stars which formed from the same mass of source material but are not gravitationally bound, so have drifted apart throughout their lives forming irregular shapes of stars. Open clusters tend to exist in the outer spiral arms of the Milky Way. Alan