jimjam11

1. Depends on the ota in my experience. The zwo lrgb and ha filters are pretty much parfocal but the sii and oiii are not. In an ideal world focus per filter (or even better calculate offsets) using electronic focus. 2. Not very, if you are somewhere close it should be fine. 3. I get mixed results with a b mask, i tend to use the hfr measurements and mimic an auto focus run by starting one side of focus, moving in small increments to the other side and then trying to equal the best measurement moving it back. This is easy to do on the zs73 because the focuser has the wo logo so you can use that as a reference for your movements.

I have the skywatcher cc and it is excellent. I have used it in both a 150p and 200pds and it has never caused any trouble: The only thing to watch out for is the 0.9x reduction which can make the focuser protrude further into the light path (depending on back focus).

Another thought, do you have a different ota which can focus at shorter distances? I use my zs73 to get main and guide cameras parfocal. The zs73 can focus down to a few metres so this is easily done during the day. You can also capture flats to look for shadowing issues, although this varies from ota to ota...

An oag is a pain and causes all manner of issues which need to be resolved. However, if you get it working and your mount can handle the improved sampling you will get measurably smaller, rounder stars. It might be worth waiting for the moon because it is easy to find and you can then nail focus? Moving the prism in will not alter focus, you need to move the upper part of the stalk to change focus, or physically move the oag backwards/forwards with spacers. Also be very careful of shadowing the main sensor such as this:

I have a love-hate relationship with my ZWO OAG and this thread shows one of the issues I found with it: I setup my OAG on my ZS73 which can illuminate a full frame sensor and is only F5.9. When I used it on my 200PDS @ F4.5 the light cone must be steeper because the prism shadowed the main sensor. As a result I needed to move the prism even further out to the point where it isnt always well illuminated: However, despite this the result is measurably smaller, rounder stars with my 200PDS (compared to a 50mm guidescope). It makes no difference with my ZS73 because the image scale is much higher (0.86" /px vs 1.77" /px). The other difference I have noticed is variability in guiding based on seeing. It is not uncommon for my guiding to vary between 0.3" and 0.7" (like the above) on a resonable night. The 50mm guidescope just reports 0.5-0.6" RMS because it isnt sampled well enough to guide down at these levels...

Yes, I eliminated that as well early on. I have never had any issues with bias stability, I think this predominantly affected the earlier versions of the asi1600 which didn't have on-board ddr memory. I don't think the oag is playing much of a role in terms of f number suitability, I think it is down to the 200pds presenting a somewhat small corrected field especially compared to the zs73. I will retest when I next get clear sky and see if it is functional...

By subtracting the OAG flat from the Finder flat the OAG shadow is suddenly revealed: I have therefore pulled the OAG prism even further out until it no longer shows in this way and will hope it still works in the first instance. It needed approx 1.5mm more. If this doesnt work I guess I will be forced down the ST80 route...

The issue is not really visible in a single sub, it shows in a stack so the artefact must have a low SNR? Good point re amp glow, and especially relevant since I use gain 200 for NB and 76 for RGB so the calibration files are different. I therefore captured some new masters to rule them out; no difference. At this point I decided to dismantle the OAG and switch back to a finderscope to try and eliminate it (despite it not showing in NB stacks. Here are the results: Bottom left is the finderscope calibrated with darks and bias. No obvious band but the field is not very flat. Bottom right is the original OAG stack calibrated with the same darks and bias. The banding is obvious. Top right is the finderscope stack calibrated with darks, bias and the OAG flats. The horizontal band is very obvious. Top left is the OAG stack calibrated with darks, bias and the finderscope flats. The horizontal band is obvious as a negative correction. Top center is the finderscope stack calibrated with darks, bias and the finderscope flats. The field is essentially perfect (compared to the others). I have never seen anything like this with the OAG on my ZS73, so the light cone on the 200PDS must be significantly steeper (which makes sense, the ZS73 is F5.9, the 200PDS is 4.5 with coma corrector). This now gives me a problem; My main image scale is 0.87"/px and the finderscope is 4.39"/px which isnt accurate enough. PHD reports my guiding in the region of 0.5" RMS (sometimes less on good nights) with both setups, but my stars are slightly larger with the finderscope and eccentricity is measurably worse (typically 0.4 with the OAG, 0.55 with the finder!). PHD is doing a great job with the finder, but it isnt sufficiently sampled to genuinely guide down to this kind of accuracy. I think I therefore have a few choices: 1. Accept the guiding inaccuracy of the finderscope; the top centre stack has an eccentricity of 0.31 compared to 0.32 with the OAG so the errors must be pretty random in direction (I think this makes sense given the undersampling of the finder). 2. Pull the OAG even further out; it is already a long way out so I am not sure how much further I can get away with whilst still seeing anything. The ZS73 has such a large illuminated field it doesn matter, but the 200PDS does not. 3. Switch to my ST80 for guiding. This might be the best compromise (guider scale of 1.93/px) but it is another piece to go wrong and encourages flexure compared to the rigid fixing of the OAG.

Imaging from Home, bortle 5 sometimes closer to 4. darks etc are matched, but the fact it shows with no calibration frames makes me think it is a light leak or reflection of some kind. It was simple to blame the oag prism until I saw the NB stacks from last night. The linear nature of it makes me think that is still the most likely candidate...

I am seeing a weird artefact in my stacks which I initially attributed to light leaks and reflections within the 200pds I am using. I also thought it might have been the OAG prism interfering, but having stacked some narrowband subs the artefacts are not present. I therefore stacked some blue subs with some/none/all calibration frames and below is the result: Top left is light frames only stacked in DSS followed by ABE. A bright horizontal band can be seen top and bottom along with a dark depression in the middle. Middle top is light, dark, bias stacked in DSS followed by ABE. The same defects are still obvious. Top right is a light, dark, flat, bias stack in DSS followed by ABE. The result is way better but the defects are still present. The bottom 2 are Ha and OIII stacks, both are perfect (if ABE is run they look the same). The light only stack proves it is originating in the lights, so I dont think it is a problem with the calibration frames per se. It might be possible to calibrate out with the right flat but this is difficult because the horizontal band is present top or bottom depending on which side of the meridian the OTA is pointing. Synthetic flats are producing excellent results, but the perfectionist in me wants to eliminate the problem at source. I have blackout material covering the primary mirror which can eliminate the central depression, but the horizontal bands keep appearing! This now has me confused because if it was the OAG prism it would also be present in the NB subs? My only other thought is that it is a light leak/reflection but the NB filters are attenuating it so much it is no longer detectable? Any thoughts?

I get the same but with plate solving such as Astap I just run solve and sync near my target. The second slew is normally very close (within 1000 pixels or so at similar focal length to you).

ASI1600MM Pro. Lum subs were 60s @ gain 0, RGB were 60s @ gain 76.

Just to confirm that is 120s @ a gain of 300? You will be massively over exposed with those settings, hence the lack of saturation? See this thread for optimal settings on the ASI1600: https://www.cloudynights.com/topic/573886-sub-exposure-tables-for-asi-1600-and-maybe-qhy163/

Are you able to use something faster like Nina for acquisition and then CCD Autopilot for analysis so you can significantly increase your efficiency?

I managed to test Nina a couple of nights ago and it performed very well. My SGPro routine yields about 48 subs per hour with interleaving, this increased to approx 51 without. Nina without interleaving yielded an average of 55 subs per hour, a significant improvement. The lowest value I got from Nina was during the meridian flip hour which appeared somewhat inefficient and wasted approx 4 mins. Autofocus after the flip took another couple of minutes so I ended up with 49 subs for that hour. SGPro is still the functionally richer option but its performance is so poor compared to Nina. Clear sky (especially moonless) is rare and wasting valuable time just waiting for stuff to happen in SGPro is becoming increasingly frustrating...

What exposures is this made up of?

If you are bortle 6/7 then your subs could be impractically short. This is a brilliant reference for the asi1600 in terms of exposure: https://www.cloudynights.com/topic/573886-sub-exposure-tables-for-asi-1600-and-maybe-qhy163/ Also watch out for your imaging efficiency with the asi1600, I have really had to work hard to avoid wasting tons of time. See this thread I started a few days back on the exact subject: With your skies it seems like a no brainer to use gain 0 for broadband imaging unless you want to stack potentially thousands of subs...

The banding is most obvious at gain 0 but it calibrates out fairly easily. I always use gain 0 for L, and gain 76 for RGB. That gives me 60s subs for both which is optimal in my sky (bortle 5/4). If I used gain 139 I would be down to <30s for rgb and <10s for l. If you use PI create a master bias and then run the super bias process to generate an excellent master bias frame. If after calibration there is any banding left use the canon banding removal script.

Out of interest, which lens cell screws are you referring to?

After 3 months of acquisition on M51 I think I have reach the point of diminishing returns @ 15hr of integration. In truth I only got a couple of hours in Jan, 0 hours in feb, a few more hours in March and the rest in April. Clear weather kept falling around the full moon so this has taken way longer than it should have! Seeing was generally average until my last run a few nights ago; The luminance stack has a FWHM of 2.68" but for most of the 14/04 night subs were < 2.2" with some dipping below 2". Just over 10h of Lum, and 90m per channel for RGB. Annotated version:

Glad you got it sorted, that looks like a huge improvement...

I think you are correct. I have noticed a significant difference in star sizes between the ZS73 and 200PDS:

I have been doing some more testing to see where I am losing time and what I can do about it. 5min subs significantly reduces wastage but at the cost of dynamic range (more stars become saturated because the histogram is pushed right). For my sky conditions (bortle 5) 60s is still slightly above optimal sub length but I am keen to reduce interframe wastage. FWHM is also lower with 60s subs compared to longer. My FWHM was not lower with 30s subs so 60s seems optimal for my setup. To do some testing I compared SGPro and Nina. I also tested using USB2 and USB3. Imaging laptop was my usual i7 with fast SSD. I programmed a sequence to capture 60x5s Lum frames to see what the wastage was like. The results surprised me a lot: I always knew nina was faster than SGPro but I didnt realise the difference was so large. SGPro wasted 3.3s per frame on USB2 and it only improved to 2.58s with USB3. Nina had a wastage of just 1s with USB2 and a staggering 0.07s with USB3! I then wanted to test interleaving of frames (the way I normally image) to see what the impact was there. I changed the test so that 15x each filter were captured. I knew swapping filter after each exposure was costly but this really highlighted it: Once again it looks like Nina is miles ahead of SGPro, but interleaving adds approx 2s per exposure in Nina and 3.5s in SGPro. I cannot fathom why a filter change is faster in Nina than SGPro! I definitely need to test Nina some more, I just dont have the confidence in it yet for unattended imaging overnight...

If you are using an asi1600 start here: https://www.cloudynights.com/topic/573886-sub-exposure-tables-for-asi-1600-and-maybe-qhy163/

Once I got autofocus working in sgpro I ran the routine, noted the position and then interated through the filters. I did this three times and then calculated the average offset before confirming with a b mask and b grabber. It would have been easier to just use a b mask but I used this process as a way of testing autofocus was configured correctly and would work reliably with all filters. I generally let sgp focus with whatever filter it chooses at the time (dependent on sequence) and the offsets seem to work very well.