parallaxerr

Well what a horrid night so far...Cloud, lots of cloud. Technical issues, more cloud and target (M33) now above max alt. D'oh. HOWEVER, I think I have carried out some valuable testing. Thanks to @Filroden's post re: ISO settings, I have experimented with mine... I took 30s subs on Capella (the only star that seem to be dodging the cloud) at ISO's from 100 up to 6400 and Hi1 (12800) and checked the histogram peaks. Only ISO100 showed clipping on the left side but from 200 up, there was no clipping. The lower the ISO, the darker and smoother the background sky with grain starting to show at ISO 800. As it happens, with my sensor, there is a dip in read noise level at ISO400, sooooooo ISO400 is where I'm currently at. At this level I also have 50% more dynamic range available and a higher saturation capacity, although I'm probably not reaching it with 30s subs. I think at ISO1600, despite marginally lower read noise, the reduction in dynamic range raised the noise floor and it was all getting amplified by the ADC. Scope is now on M42 clicking away, surprising amount of detail in the subs and a very nice dark background sky. Watch this space......

...Well I would if it wasn't for the 100% cloud cover that just rolled in

OK sounds like 3200 is OTT. Now, the reason I have dismissed 800 is because the read noise for my camera is higher at 800 than at 1600 (not by much). I thought the noise I was getting in my images was read noise but my darks are absolutely jet black so maybe the noise was something else? I'll try a few 30s subs to see how the histogram shapes up.

Here's a comparison. These were 10 second exposures but I'll be using 30s later, will that affect things, ie: will the histogram shift with a longer exposure?

Hi guys, I have a quick question hopefully one of you may be able to help with before I start imaging tonight... I have just set the ST120 up ready to go. Focused with the new Bahtinov mask (turns out the focus was a good way off last time) and also fitted the new Baader Semi-Apo filter. I took a few test shots after alignment and I'm pleased to say that the sky-glow/LP is practically non-existent with the filter and CA, whilst still there, is significantly reduced Based on my findings about sensor noise today, I took a few test shots at ISO 1600 and 3200 (supposedly 3200 has marginally lower read noise for my camera). At ISO 1600 the background sky is a nice deep blue but I noted the histogram spike is at about 1/4 from the left. At ISO 3200, the background sky is quite light grey/white but the histogram spike is nearly bang in the middle of the graph. So the question is - which ISO setting? Does the histogram position tell a story and am I going to have problems processing with a lighter background sky if I set 3200? TIA, Jon

What camera are you using Nige? It'd be interesting to see if your results correspond to the noise charts on sensor gen.info. EDIT: Scratch that, just turned signatures back on saw your camera listed! Looks like 1600 is the sweet spot for the 1200D and lower than 800 is a no go.

This little bit of research may explain why I struggled with noise so much in my attempt at M33. My Nikon D3200 & ST120 combo gives a resolution of 1.32"/pixel vs 2.04"/pixel with the ZS66. Perhaps I should go back to the little apo but the bahtinov mask and Baader semi-apo filter for the ST120 just landed on my desk...decisions, decisions. On the subject of noise, have any of you guys actively experimented with and defined meaningful results from trying different ISO settings? I read this article http://dslr-astrophotography.com/iso-dslr-astrophotography/ and went on to get my sensor noise info at http://sensorgen.info which suggests I may be better off at 3200 with my camera as the noise gets less at higher ISO! The only problem then is that the saturation capacity is reduced, but given the 30s exposures maybe it will work out OK?

Thanks for sending me on a half hour googling mission trying to understand pixel resolution Ian!!! A lot of information to absorb there and it seems the topic draws very different points of view. From what I can see, the fact that your sampling rates, with and without the reducer, are well bellow 2" which is probably the very best seeing we can expect, the only other factors to consider are target framing and SNR - the more oversampled the image is, the higher the SNR. I'd go with the reducer, but hey, I may be completetly wrong! At least, that's what I can extract from this article...https://starizona.com/acb/ccd/advtheorynyq.aspx

Thanks for the encouraging words Fil. The green seems to be a problem for me, in both scopes. When I use the colour tool in ST the image has a heavy green tint to it, same with my M31 attemp. I cap green to yellow and reduce green further with the sliders but then then struggle to balance red an blue. I just reprocess the same data in an attempt to remove more noise and address the ca, but I think the image is too soft now. I also didn't drizzle in DSS and only binned 50% which I think has proven to remove the noise a bit better.

Thanks Kat, I have readt that ST is good at removing CA but I don't know how. Will have to read up on it!

Here's the result of Monday nights session with the ST120. M33, about 1hr of 30s subs, x50 darks, x50 flats and x50 dark flats. The light frames suffered heavy LP so I'm not suprised there's still significant gradient in the image. I'm still learning startools so maybe more could be removed? Due to the noisy light frames I used x2 drizzle in DSS, which left me with a 12068x8024 image @ 1.08GB I then binned to 25% in startools which did seem to smooth things out a bit. The CA is there, as expected, but I don't find it overly offensive. Hopefully the semi-apo filter will help address it and the skyglow when it arrives. Most of my attempts so far have been with about an hours worth of data, early in the evening. It's becoming clear I really need to capture more data, later at night, when the lights have gone off, target altitude allowing. Oh for a clear night on a weekend...and a new moon..and a target between 30° - 60°!!!!

Impressive NIge, thats's the result I'm after. Let's hope the Baader comes up with the goods with a bit of fringe killing added in.

That's one worth remembering alacant, thanks. If I can remove some of the orange/brown to start with, I'll be happy!

Oh well, impatience strikes again and I've ordered the 2" semi-apo. Below is an image showing the performance of the Baader neodymium on which the semi-apo is based, plus, based on my own experience with the 1.25" version, I should see less CA through the ST.

Thanks Nige, this is my goal. The orange/brown makes processing so much harder! I'm really tempted by the 2" Baader Semi-Apo now, the Neodymium on which it's based gets good reviews for LP reduction and the fringe killer should help with CA, as I've observed with my 1.25" version.

Thanks Alacant, posted as I was typing!

I've had a look back through the thread for posts about filters and came across two posts of interest. @SteveNickolls, you mention using the semi-apo, have you tried it yet? I rate the 1.25" one I have highly for visual and am considereing the 2" for AP. @Nigel G, you've had some success with an LP filter, can I ask which one you are using? Thanks both.

I think you hit the nail on the head here Fil, I really need to get my head into Startools some more!

Update on last nights trial with the ST120: I gathered about 150, 30s exposures of M33 along with 50 darks. Inspecting the light frames shows that the Baader MPCC works even better with the faster F5 ST than the F5.9 WO ZS66, I see no coma at all in the corners I had managed to get a reasonable focus but not perfect, so I've just ordered a mask. The CA isn't as bad as I thought. Obviously the fainter the target the better I suppose? It was heavy on Pleiades as noted above but that was just a rush job with bad settings. Being impatient as I am, I stacked the M33 frames up in DSS just to see what was there. No flats or darks flats yet, so there was some noticeable vignetting. I'll be doing them tonight and re-stacking. The main issue is horrendous light pollution though, such a horrible orange noise to the image. The longer nights may mean I can get out earlier but I'm thinking there's not much point starting until gone midnight when the streetlamps go out and the neighbours go to bed! But, work and the nursery run dictate that I can't be doing that really. Did I read somewhere recently that someone has had good results with an LP filter? I have a 1.25" Baader Semi-Apo which I found VERY effetive in the ST for visual. As I understand, it "overlays a slightly more aggressive version of the fringe killer coatings onto the neodymium substrate of the Moon & sky-glow filter" so I wonder if the 2" version may suit for AP?

I learnt this last night! After imaging M33 I slewed to Pleiades as it's one of my favourite visual targets, however, I left the exposure set at 30s. Result = Big bloaty blue (with the achro) stars!

Hi everyone, Good to see the thread's still going strong and getting far too technical for my brain! I've been out of it for a few weeks with illness and a back injury, boo. Anyway, decided to get back to it tonight, I was planning to add to my M31 data only to find out it's now too high in the sky for my mount. So...next target is M33. Also having toyed with the idea for a while I've decided to give the ST120 a shot, see if I can't capture a few more photons per sub! I couldn't use the St120 for M33, it wouldn't get it all in but no problem for the smaller targets. Really interested to see how the Baader MPCC works in a different scope - the ST120 is closer to the midpoint of the recommended focal ratio range so should be OK. My ST already has a Moonlite fitted so the camera is hanging well, but I struggled with focusing a bit, the CA made it a little more tricky so may invest in a bahtinov mask. Anyway, here it is in all it's glory. Results should be interesting although I'm not getting my hopes too high due to the moon glow and random left over fireworks!

Yep, exactly that Steve. At about 62°, the camera contacts the base of the mount. I may be able to address this by fitting a long dovetail bar and sliding the whole lot forward as it's quite tail heavy at the moment anyway.

Well I chickened out and put the WO ZS66 to work tonight, I will definitely try the ST120 on smaller targets but want to get a good M31 first. I got x106 30s subs on M31 tonight at ISO800, stacking now to see what comes out. Good news is I absolutely NAILED the alignment, M31 dead centre in all frames If the ISO 800 data is cleaner then I'll keep adding to it every clear night I can. The only problem I have now is the Alt limitation of the mount which puts a stop to M31 imaging at around 10pm.

Good shout Fil. There was me thinking"oh the focal ratios are about the same" but didn't think about focal length. Not gonna be much use on Andromeda me thinks...

Tonight's experiment. ST120 instead of WO ZS66. Nearly twice the aperture but plain old achromatic glass. Is it worth a shot? Either way I'm going to drop the ISO to 800, see if I can reduce some of the noise I had at 1600 during my first attempt.