BrendanC

Hi all,

Right, so, I've been using darks, flats and flat darks successfully for quite a while with my EOS1000D camera. I use dark flats instead of bias. I have a darks library for 60s and 120s exposures, across all temperatures, at ISO800, which is what I always shoot at. All good.

However, now that I'm guiding, I can start pushing beyond 120s, to 180s, 240s, who knows, even further - I need to test my equipment and see what the limits are. So, as my exposure times start to increase, I'll need to build new dark libraries to match those exposure times. Library building already takes ages and is tedious, and it's going to take even longer and be even more tedious for longer exposures. For example, it'll take three times as long to build a library for 180s as it took for 60s. For 240s, four times as long. And so on.

So, this raises a thorny question: can I, as a workaround to having to create all these dark libraries, use bias frames to scale my darks instead? For example from, say, 120s to 180s? At least, in the short term, until/unless I can be arsed doing it properly?

I'm thoroughly expecting a massive discussion now, in which everyone has a different take on it - because that happens every time calibration frames are mentioned - but if you definitely 100% absolutely know the answer, rather than think you might possibly perhaps maybe have an inkling, then I'm all ears!

Thanks, Brendan

It's never easy is it...?

Thank you again @vlaiv, I'll get back to you when I've scratched my head some more about this!

On 21/12/2020 at 13:25, vlaiv said:

- complex rule - determine read noise for your camera (in electrons) and see if LP noise or thermal noise is higher for your conditions (will depend on ambient temperature and light pollution levels) and then make exposure such that read noise is 1/5 of higher of the two - thermal noise or light pollution noise (very similar to above given advice - except you should not do it in ADUs but rather electrons and you have to see if thermal noise (dark noise) is higher than LP noise).

 

Right, so, this is one of those 'backburner' projects that I've decided to have a think about.

Firstly, I shoot everything at ISO800 because according to this site, it's the best for an EOS1000D (it actually says it's ISO200 but that's ridiculous, and says to check out ISO800 too, so that's what I use): http://dslr-astrophotography.com/iso-values-canon-cameras/

I've found my camera at photonstophotos: https://www.photonstophotos.net/Charts/ReadNoise_e.htm#Canon EOS 1000D

Looks like read noise at ISO800 is 5.65 electrons with a bit of interpolation between the given values.

So, I have one part of this. Now, how do I determine the light pollution noise, or thermal noise? Any pointers?

Thanks!

I had two major discoveries while processing this:

1. Unsharp mask - I've been using the Google Photos pop slider, and liking it, and feeling guilty that I like it. So, I found out how to do this 'properly', and in Photoshop it's Filter/Unsharp mask. It's a wonderful tool for subtly sharpening things, despite its name.
2. Pushing sharpen, HDR, contrast and deconvolution much further in StarTools, but just keeping the image black and white. Then, in Photoshop, turning it into a Lab colour image, grabbing just the Lightness channel from it, and using that as a luminance layer. It really adds so much detail which you can overlay on top of the RGB exposures.

I quite like the effects, but I'm always wary of going OTT with a new toy, but this seems to have been well received so maybe I haven't. This time.

As has already (correctly!) been pointed out, opinions on the guide scope will differ, but the way I approached it was to get the hang of what I was doing without guiding first, which enabled exposures up to 60 seconds, then when I knew what I was doing, took the next big step and started guiding. There really is so much to learn, it's always better to split this up into manageable chunks. This is not least because you need to know what to do when things go wrong - and they will go wrong, believe me - which you can only do when you really know your system. 

Hi all,

Here's my M42 with the Running Man above it and some pretty twinkly stars above that. I have a feeling I might have overdone the post-processing so I'd like any constructive comments please.

Details are:

• Bortle 4 sky, moon 78% illumination
• 52x60s, 63x120s exposures at ISO800, guided
• Calibration: 25 flats, 25 dark flats, 50 darks
• Hardware: Sky-Watcher 130PDS scope (F5), Sky-Watcher NEQ6 mount, Canon EOS1000D astro-modded camera with Sky-Watcher 0.9x coma corrector, T7C guidescope (ASI120MC clone), Angel Eyes 50mm guidescope
• Software: polar alignment with SharpCap Pro, guiding with PHD2, capture with Astrophotography Tool (APT), stacking with Deep Sky Stacker (DSS), post-processing with StarTools and Photoshop CS2

Thanks!

1 hour ago, Tiny Clanger said:

I second that. Pretty sure that the dodgy ebay sellers are just burning that free download to a disk .

CS2 is a useful tool still, and it works fine on a pre-10 version of windows, but on windows 10 you get such tiny text and icons on the tool bars etc that the only solution I've managed to find (that doesn't involve some scary interfering with things I don't understand ) is to use the win10 magnifier on them , which is a pain. 

Heather

That's strange, I have it installed on a PC and 2 laptops, all Windows 10, different vendors, spec etc, and I don't have that problem. 

Photoshop CS2 is available free, and legally, at https://www.techspot.com/downloads/3689-adobe-photoshop-cs2.html

Admittedly it's a very old version, which suffers strange behaviour occasionally with dialogue boxes and windows, but I use it all the time in combination with StarTools.

Hi all,

I just had a persistent run-time error 6 with Platesolve 2.8 running via APT. The fix most people suggest - downloading the files and copying them over the current files - didn't work.

Two more things had to be done:

1. Re-installing the APM indexes. The APM folder was empty - no idea why. So, I downloaded that, ran the installer, and made sure Platesolve knew where it was. It still didn't work, but this absolutely had to be done first.
2. The actual fix was to re-specify the focal length of my scope in APT. For another weird reason, it had lost that setting.

Just documenting this in case other people have the same problem I did, which evidently was as a result of more than one issue.

Thanks, Brendan

One of my upcoming targets. Great image.

Thanks all - lots to read up on and think about!

Thank you.

I'm shooting with a DSLR hooked up to my laptop, running APT (probs should have mentioned that).

Presumably the histogram I see on APT will be the equivalent of what I would see on the camera?

Also, for read noise, given that the https://www.photonstophotos.net/ site looks like a great resource but will take a while for me to digest, are there any tools other than Pixinsight that will give me an ADU reading for a photo? I've asked Ivo about APT and it doesn't do that, which is a tad disappointing.

I'm finally getting decent results now that I've got autoguiding working properly. I'm currently using 3m exposures but I've tested 4m and 5m and they're good too, which implies I can probably go even further.

However, this throws up yet another quandary! Which is: even if I can do these long exposures times, should I?

I'm aware that longer exposures will bring out the fainter details for example in nebulae, but could blow out the brighter stars. Also, they bring out the sky fog more, which I need to be wary of in a Bortle 4 zone.

I've looked but there don't seem to be any hard and fast rules (not that there tend to be anyway with astrophotography!). It would help if there was a formula or calculator of some kind? Or is it really another case of, try it and see what works?

Any advice?

Thanks, Brendan

Thank you again!

Brilliant. Just...  brilliant. 

Thank you again @vlaiv. I hadn't considered the impact on magnitude and therefore SNR and therefore imaging time. So, now I need to find out how to make those calculations to help me make my decisions -  any/all pointers welcome. 

So, I was just going through my list of targets, and landed upon spiral galaxy, IC 342.

The way I've formulated my list is by calculating when they'll be at the meridian at midnight, the idea being that generally, this gives me the most amount of time to image it, with the least amount of air mass, before and after the meridian flip.

I looked at it on Stellarium, got my plan together etc  etc. Then a thought suddenly struck me: IC 342 is circumpolar, so it can really be imaged ANY time of the year. In fact, you could argue that the WORST time to image it is when you need to do a meridian flip right in the middle of the shoot!

I can see that I would still benefit from having less air mass, and I probably won't change things around for the sake of losing 20 minutes doing a flip, but I was just wondering what thoughts other people had on this? Basically, as per the title, what's the best way to approach circumpolar objects? Perhaps they're just nice objects that you can pick and choose any time of the year?

Your thoughts/opinions/comments please!

Thanks, Brendan

She'll be fuming now, but that's ok because Jupiter is in the ascendancy and she's Capricorn, so she'll have an unexpected surprise tomorrow and will pull through with aplomb.

Wow, I must be getting good. I'm asking questions nobody has answers to now!

Interesting stuff, thanks everyone. I like the idea of aligning, switching off, then back on again and repeating, just to get a sense of what's going on.

Then, I'll spend a bit of time getting it as correct as possible. And then I REALLY like the idea of seeing how I go without it altogether.

It just never occurred to me over the past, what, six months since I started using the NEQ6 (my first EQ mount), that I didn't have to do it every time! Doh...

Tell you what though: I'm really good at it now...

I leave my mount in the garden, covered up with a fairly substantial tent which keeps it nice and dry. I bring the OTA indoors. 

Since doing this, I've been polar aligning before each and every session. I use Sharpcap so it's not so onerous, and it's generally not far out - but I suddenly thought: if I don't touch the mount between sessions, why does it go out of polar alignment at all? It's never far out, but it's also never as good as when I last did it.

Shouldn't I just be able to get it as close as possible, and then leave it?

Or is it really down to those minuscule differences between the mount when I left it, and the mount when I come back to it?

I mean, it's not awful having to polar align every time but I'd rather save a precious few minutes.

Hi all,

Title says it all!

I'd really like to start analysing my images more as they come through from my EOS1000D, particularly ADU values.

However, the Pixel Aid tool in APT only reports pixel values for a raw (CR2) image, in bitmap mode, and only for the selected area. Wherever I point it, the max is being given as 250-255, and whereas that does imply to me that my pixels are saturated, this isn't what I'd expect for an ADU value.

The alternative is to use FITS files, but when I try to import one created in DSS, it just doesn't appear in the list of images, so I can't take that route. This should also be able to tell me the value for the entire image or for individual pixels, but even if it did work, it's not ideal because I'd like to see the images direct from having been taken, in CR2 format, not converted into FITS.

Is there any workaround to this? Or is there a better way to interpret the bitmap mode values and convert them to real ADUs?

Thanks
Brendan

Thanks. It's kind of rough around the edges which it definitely wasn't a while ago. But, as you say, it's working ok so I'm going to leave it! 

So, after nearly crying the hot tears of a child to collimate this darned thing yet again, in the process I noticed that the silvering around the edge of the secondary seems to have been worn away in small patches.

I may have left the OTA out one night too many, although kept snug with all the cap covers on, hats over them, and inner and outer of a serious tent covering the whole thing.

I guess it's dew? In which case, can that, do this?

If so, given that I'm currently getting OK results with it, do I need to worry? I bring the OTA in now, so it shouldn't get worse. But I do know that whereas artefacts on the primary aren't too much of a concern, on the secondary, they are.

Any/all comments welcome other than 'Just give up Brendan, it's not worth it'.

Thanks. 

I'll certainly report back on the results.

I do actually think it's stupidly expensive for what it is, but I can also see that it's a kind of niche item with limited production runs.

I've been trying to use my 'concenter-like' method - in which it seems I sort of re-invented the Concenter without realising it existed at the time - and whereas it does kind of work, it's extremely fiddly. and not very reliable.

I just decided to treat myself to something that should sort me out. I've been going through all sorts of daft ideas in my head about trying to make one using printouts of concentric circles on clear acetate and sticking them into eyepieces etc, but decided, well, like the L'Oréal ads say, I'm worth it.