eshy76

31 minutes ago, Demonperformer said:

Here is a link to the video. This is good if you wish to explore the theory a bit further.

Thank you very much! Something to devour this weekend!

20 minutes ago, alan potts said:

I remember seeing something which in truth was Concorde for me, miles over my head, about short subs and CMOS sensors. I intend to get a couple of cameras this year, maybe a 071 and a 183 for my range of scopes. I am sure you would love to visit here where on a decently transparent you can see M33, but it does get almost overhead. Still what ever you have to do, it is still a lovely capture or should I say lots of captures.

Alan

I hear you! The basic theory is that each sub should be long enough to drown out the read noise and with the really low read noise of CMOS cameras, that level is reached quickly with light pollution. Of course you can shoot longer than that for practicality, but if subs are too long, stars and highlights could get saturated. That is it in words - there are mathematical formula that try to calculate the optimal sub lengths!

Those cameras have really good reputations - I was looking at a 071, but ended up getting a 294, which I'm still learning. I think the 071 has an APS-C sensor?

Thank you for the kind words!

51 minutes ago, Demonperformer said:

There is a video on here somewhere produced by rwg who goes into details of optimal sub length. I'l track it down for you when back on desktop.

As regards stacking, if you use sharpcap, you could stack them as you go along.

Thank you - that sounds interesting - I like the theory side of things as a guideline to what I do! And I did not know that about Sharpcap...I've got the pro version for the polar alignment routine, but have been using APT for capture. 

1 minute ago, alan potts said:

I like this it is a very nice image and I wonder how many more are there we can't see.

A question what advantage is there in doing so many short subs over fewer longer ones?

Alan

Thank you! That is the golden question which I intend to find the answer to - with my level of light pollution, 15 secs at unity gain on my camera is more than enough for my Lum subs to drown out the read noise...so I've always used that sub length...

...which was fine for 200-300 subs, but this project really brought home the impracticality of stacking 1000+ subs. I just was thinking in terms of total Lum integration time (250 mins) and set the number of subs that way.

As far as I am aware, SNR benefits of stacking enter the realm of diminishing returns from about 200 subs onwards...so going forwards I'm going to lower the gain from unity (139 on the ASI1600) to 76 or 0 to try and get sub length up to between 30s and 60s and hopefully more like 200-400 subs to stack.

I can then answer your question fully. The 15sec approach benefits in terms of guiding being less critical, and things like clouds or planes or satellites not meaning minutes of data being binned. The Lum integration WAS beautifully smooth at least!

35 minutes ago, Demonperformer said:

There are a lorra lorra galaxies in that image. Nice one.

Thanks! Yes - an incredible part of space. There is a tiny barred spiral in the bottom middle of the image which is just beautiful and perfectly formed...I'll focus on that one one day when I have some more focal length!

7 hours ago, tooth_dr said:

Very nice image. A lot of short subs to crunch. 

Thank you! Yes indeed...so while I got, erm, stacking benefits, the practical side was an issue in terms of the time it took to stack on a good PC. I also didn't dare drizzle the integration - that might have taken 2 days!

Hi everyone,

This is Markarian's Chain shot from my back garden over three nights in late March. Probably the deepest single image I've taken in terms of integration time...which leaves me feeling my processing is not quite doing the data justice...maybe I'll come back to it.

In any case, this is an incredible part of space; looking away from our galaxy reveals countless others!

The crazy number of Lum subs took a whole day for APP to chug through and so I've resolved to lower the gain from unity going forwards to get more manageable sub lengths than 15 seconds!

L: 1050 (!) x 15s R: 92 x 60s G: 165 x 30s B 165 x 30s....total integration time 8.2 hours.

Captured using APT, stacked in APP and processed in Pixinsight.

Thanks for looking!

Edit: Link to higher res version

Thanks for this comprehensive review - I've had the same scope for a year and I love it! The only fault I found was an inconveniently placed focus lock screw which means I need to turn the scope upside down if I need the dovetail to extend beyond the focuser for balance. Very minor issue.

Also would have been nice if the rotator was marked for degrees...available as a £200 extra!

Hi there...I managed to get a rudimentary shot of this amazing target when on holiday last year in Mauritius (southern hemisphere) where Antares is high in the sky with all the planets!

But in your case, a tool you can use is the Telescopius site (formerly dso-browser). Once you've clicked on a target, you can choose to see its transit for the current night or "monthly" at a certain time like 10pm - the latter shows the altitude of the target throughout the year - I think Rho peaked very low (15 degrees) around July to August in the Northern hemisphere.

I hope this helps!

Edit: I just checked the site - I searched for IC 4604 and at 11pm it peaks on June 28th at 15 degrees altitude looking from my location of Surrey. Good luck with it!

1 hour ago, simmo39 said:

So what is your average RGB sub lenght, L is 15s that seems v short. I think with number crunched by my poor head 60s for L but that could be off as I guessing the bits I dont understand ! lol.  At least I have got a starting point all I need now is the weather to cooperate.

My L subs are 15 seconds, G and B are 30 seconds and R subs are 60 seconds. With my filters, my R needs longer to get the same ADU as G and B.

The formula I used was Jon Rista's:

Minimum ADU per sub = ((20 x read noise/gain in electrons)+(bias offset)) x 16 

So at unity gain, associated read noise of 1.8e- and using offset of 50 that would be:

((20x1.8/1)+50)x16 = 1376

The 16 multiplier is to gross up from the 12 bits of the ASI1600 to 16 bits which is displayed in SGP etc.

The 20 x read noise can also be 3 x read noise squared or 10 x read noise squared....there's some discussion about that.

It's a starting point as you say! My skies are light polluted which is why my subs are so short!

2 hours ago, simmo39 said:

Hi, thanks for the pointers.  I think my light pollution is classed Bortle 4 - 5. I will have a look at the CN site, once again thank you.

No probs - I forgot to mention I'm using an f5.9 scope - as vlaiv says there are lots of factors to take into consideration....

...but if there is one thing I am aiming for - it is a certain level of median background ADU per sub (you can see this in your capture software or Pixinsight). Based on my use of unity gain (139), default bias offset (50) and the associated read noise of the camera at unity (1.8 e-), the theoretical minimum median ADU I need per sub is about 1400 to swamp the read noise (by about 20x). You'll see this in the CN thread I mentioned.

That number of 1400 (for me) governs the length of the subs I mentioned....the R, G and B sub lengths get me in the 1500-1700 ADU range...the 15 second L sub actually delivers about 2500 ADU on average, more than I would want, but I can't really go much shorter than 15 seconds from a practicality perspective.

So I didn't just pluck those numbers out of thin air in case you were wondering! The theoretical stuff is a nice baseline for me to hang my hat on and know I am being efficient - not under or over exposing, although there is some leeway on the latter.

Ultimately, this low-noise camera allows you to take shorter subs and use stacking and sigma rejection benefits, but there's a trade off between being efficient and practicality (hard disk space as you'll need a lot of subs using my approach) and also your own preference on whether you would like deeper individual subs.

Good luck with it!

Hi there,

I have an ASI1600 - this will depend on your local light pollution. Where I live is Bortle 7-8 and I shoot 15 second L subs, 30 second G and B subs and 60 second R subs.  As far as I understand, your exposure time for each sub should be enough to swamp the read noise of the camera...and then focus on total integration time. So I get lots of subs, but am still imaging for 1-2 hours for each filter basically (more for L). 

If you have darker skies than me, those sub times will be longer for you.

I also keep things simple in terms of proportions of integration time - I shoot 50% L and 50% RGB.

I'm not sure if I can link to other sites here, but if you google "sub exposure time ASI1600", there's a great thread on CN that answers your exact question with tables of exposure time depending on different sky levels. It's an involved thread, so you can dive into all the maths or just stick to the tables in the first post.

I hope this helps!

4 hours ago, The Admiral said:

Looking back on reviews of the iOpton mounts, I note that the 25 has had a bit of stick. How have you found yours?

Ian

Hi Ian,

I really love mine, but it's my first and only mount, so bear that in mind!

It is reliable, tracks well, guides at about 1" RMS consistently, but when I've polar aligned well and reduced the payload, I've guided at 0.4". Nice polar scope, easy to set up, works well with ascom.

Above all it does this while being light - when I bought it I was carrying my stuff down stairs and then another 50 metres to set up and back again every time...I needed a truly portable mount and the CEM25P did that perfectly...a great combination of portability, decent performance and low price. My Astrobin pictures (see my signature) are nearly all using the mount, so you can see for yourself.

The only issue I had was some play in DEC which, as a newbie, I did not detect for a few months...I fixed it with a shim...but not ideal - QC might be an issue.

I'd definitely buy iOptron again although the beautiful CEM40 might not be enough of a payload upgrade for me!

Hope this helps!

On 08/03/2019 at 07:14, The Admiral said:

It's listed on the iOptron site now too.

https://www.ioptron.com/product-p/7400a.htm

I'm interested in what this mount can do, roll on some user reviews! I see that their 'iPolar' finder scope can align even without sight of the Pole star too, if I read it right.

Ian

Thanks for posting that info... permanent pec, payload capacity of 18kg, ipolar scope which does not need Polaris and through mount cabling of sorts. If I didn't have a CEM25P I'd be all over this!

2 hours ago, sloz1664 said:

No it has to run on both computers and they have to be pro versions. I run mine on W10 laptop to W7 astro PC.

Steve

 

Not sure if this is still the case - I am able to RDP into my mini PC (W10 Pro) with my desktop (W10 home), iPad and Android phone, the latter two using the free RDP apps.

For me just the scopeside PC needs to be W10 Pro for RDP to work.

Well that does look rather nice...colour me interested...built in polemaster, no dec cable, simple switches instead of clutch release thingies...price and payload will be key. I have the CEM25P which has been pretty good for me...but is payload limited for imaging...