simmo39

10 minutes ago, Cozzy said:

That's looking really good, what spacing are you at?

 

Tim

Thanks, Im at 65.5mm I think I need to go back a little to get it at its best. Im really impressed by the scope and the TS  FR/FF seems to work very well with my camera set up.

Hi, managed to get a single Ha frame in last night just to test spacing. The spacing is still a little off but here is a 240s sub tweaked a little i PI. Think Im going to like this scope!

1 hour ago, eshy76 said:

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!

1 hour ago, vlaiv said:

There is no definite value that one should use here. It comes down to when you can consider read noise contribution too small to matter. Best way to "visualize" this is by right angle triangle. If one side is much smaller than other - longer side approaches hypotenuse in length. Total noise is hypotenuse and read noise and LP shot noise are sides of right angle triangle. When LP shot noise "side" becomes much larger than read noise "side" - total noise (hypotenuse) comes close to that LP shot noise "side".

a = b implies c>b

O<<A implies H and A almost equal in length (O here being read noise almost has no impact on total noise - it's dominated by LP noise - here A)

Thank you, That does help. Now if you can calculate the clouds away  that would be brilliant!

56 minutes ago, eshy76 said:

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!

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.

BTW, I think the SQM value here is: 20.4–21.3

1 hour ago, cjdawson said:

Hiya.

I'm also getting myself up and running with LRGB imaging using a ZWO ASI1600MM-Pro.  Everything I understand on this subject is that there is no easy answer.  That said, I've got a plan.   Turns out there's an idea exposure time statistic in SGPro, I'm not expecting this to be the perfect answer by any means.

 

As an experiment, I'm going to take a sub next time I'm out, and will see what this statistic says.  From there, I'll adjust the sub time and see what happens.   I'm going to try this in L only to begin with, then if it works well, I'll try the same for each of the other filters on the same target.   If it all works out, then I'll should have some idea of the ratio of exposure times for each filter (yes, I know this is nieve, it's a start and not ment to be a complete answer)

 

From there on out, I'll have a starting point to tweak from.

Hope it works, I use APT and im not sure if it works the same.

1 hour ago, vlaiv said:

What is your light pollution like? Do you know average SQM reading for your site? (or maybe data from lightpollution.info)?

I think numbers that you mention are good for average to strong LP (120s). Only if you have very dark skies it makes sense to go longer.

from the numbers i would say my light pollution is average. I have no idea about SQM sorry.

47 minutes ago, vlaiv said:

That really depends on various factors. For any given setup and conditions, longer subs will always produce better results, but relationship is not straight forward. What happens is: going from 1m to 2m will have significant impact, going from 2m to 3m is going to have noticeable impact, going from 3m to 4m will be barely noticeable and going from 4m to 5m is not going to produce any perceivable difference.

Above numbers are arbitrary and serve just to show you that there is no linear dependence between sub length and improvement - at some point improvement starts to rapidly fall of until it reaches undetectable levels (meaning you can't tell difference to SNR by eye alone and it has virtually 0 impact on image quality as perceived by human eye).

Thing is - above numbers depend on many factors - focal length of scope, aperture, light pollution levels, etc ... With higher sampling resolution (less "/px) - increase sub length. With darker skies - increase sub length. With larger aperture increase sub length. When you have combination of those factors then it's not easy to say without calculations (like using lower sampling rate while moving to darker skies, or other combinations).

As for lum vs RGB, that one is even harder to tell. I usually do it equally for each filter - meaning same time for L, R, G and B. Some people do it like equal time for L as for R, G and B combined and they split R, G and B equally. In theory, given limited time budget there is optimum split - but it is way hard to calculate and one would need much more information than is available prior to imaging (like target brightness in each band - you don't know that until you image target).

One thing that can be useful is: don't be afraid of long exposures if your mount/scope system supports them (guiding and tracking are up to task). You can always get few short exposures at the end to blend in what ever you saturated in long exposures.

Hi Vlaiv. Thanks for the reply. I'm going to be using my new SW 72ED. Think it's about F5. Comparing to my SW 130 PDS is F4.8 I think. I have been using 240s subs for narrow band with that and was hoping that 240s for NB would be good for the 72ED.  As for the RGB I was thinking about 120s, do you think that would be a good starting point or should I see if I can push it to 240s?

9 minutes ago, eshy76 said:

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!

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.

Hi all, I know this has been asked before but i cant seem to find the answer. I have been doing Narrow band since I got the camera but with galaxy season coming I treated myself to some RGB and L filters. I have set my gain to 200 for the NB but will go back to unity gain for RGB. Im just wondering where to start my subs at, I thought about 120s for the RGB but what about the L? is that the same and also what kind of ratio am i looking at between RGB and L? 

Thanks in advance.

Hi all, I braved the winds last night just to see if I can get focus with my 72 ed using the TS ff/ reducer with my ASI 1600mm and im glad to say I did with no problems. I have still got to get the spacing right but from the single shot I took last night it all seems do able. There was plenty of Focus tube left! thank you all those who pointed me in the right direction. cant wait to use this scope in anger!

4 hours ago, Cozzy said:

That's good news, it's a great scope.

hope all goes well with the testing.

sorry can't help with the spacings, I will be using a Canon 450D and OVL FF, and just ordered a threaded 5mm spacer, so like you will testing when it arrives along with the great UK weather.

 

Tim

Thats it, no good weather for at least a month then! lol. good luck hope it all works out well.

Well, Got my 72 ED this week, Also went for TS FF/fr. I have had a quick set up pay in the house but as usual the weather not letting me test it. Also for those interested the  SW auto focuser fits no problem. I have set the spacing for the FR to 65mm for the first test, does that sound about right? Im using a ZWO 1600 mm pro and narrow band filters for this set up so wish me luck!

4 hours ago, knobby said:

Had another try last night , just over 69mm not good at edges ? might be the reducer ? it was never brilliant on the WO ZS71, might have to go another route

Oh well, I'll be looking at the TS one more expense! Hope you get sorted. 

25 minutes ago, knobby said:

Not too promising so far, tried 68.5 mm and 70.5 mm without success, trying 69 mm next clear night ( March ?)

Hi, well I hope you can get it sorted. weather is going to be a pain for the next few weeks. think ill be going for TS one if you dont have any luck.

All the best. Andy

On 23/01/2019 at 12:59, knobby said:

So, the lynx astro compression ring upgrade still throws the WO reducer out of true slightly. I had a M3 tap set so have 'modified' it and it holds perfectly square now .

Should add, the existing thumbscrews seem to be M4 but I only had M3

Just need some stars now.

Any news on this FF/ fr ? Just waiting to see which one to go for.

15 hours ago, knobby said:

One thing with this set up ... It's impossible to get the reducer /flattener to stay Square in the holder when tightening, I'm trying the lynx astro upgrade with compression ring to see if it's any better.

Thanks for the pointer.

15 hours ago, knobby said:

Okey dokey, just got in from social duties, scope arrived safely from @FLO swiftly stuck the William Optics ZS71 flattener in the scope to see if I had enough focus range ...

Well a picture paints a thousand words so here's one showing the flattener in position and the same again but exposed to show the 'very rough handheld Moon shot ' on the monitor.

No time or chance to check star shapes but hopefully tomorrow if clear.

looking good, hope the stars are ok, if so i will get the WO ts/r.

3 hours ago, knobby said:

Well, I'm joining club, just ordered a 72ED from FLO ...

I have a William Optics flat 6a, I'll give it a whirl.

Looking forward to hearing your thoughts on the William Optics flat 6a. Hope it works, it will save a few quid if it does!

18 hours ago, knobby said:

Is it me or is the TS flattener incredibly similar to the WO flat 6

https://www.darkframeoptics.com/product/william-optics-0x8-flattener-reducer-zs71

I see what you mean, and its a bit cheaper too. I think some good research is needed here.?. I wonder if anybody has used one with an ED72. Im looking into getting  one and the TS flattener was the one I was looking at adding to the ED72 but this WO looks like another option.

4 hours ago, Susaron said:

Simmo I did use a 33cm dovetail to proper balance the QHY168C, I also piggy back mounted the 30mm guide tube to help. Even with that is very very close to limit, see the picture

Hi Susaron. Thanks for the reply, I know its a bit bigger but my guide scope is going to be the SW Startravel  80! My guide scope will be bigger than the ED72! The reason being its what I have at hand. I think if I mount it forword of the C of G I may get some sort of balance. As for the reducer if i can fit my ZWO filter wheel  and still give me the required 65mm I may try one. Once again thanks for the advice.

2 hours ago, moise212 said:

At about 60mm backfocus I managed to get this

But I remember I read somewhere that the TS flattener fits better and performs better. Can't say where I read that, maybe even on this thread. Will look when I get back at the PC.

BTW, witch TS flattner do you recomend? there seem to be a few to chose from.

2 hours ago, Stub Mandrel said:

The ED66 makes a nice complement to the 130P-DS with its wider field but similar f-ratio making them similar to use.

Thanks for the pointer.

2 hours ago, moise212 said:

At about 60mm backfocus I managed to get this

But I remember I read somewhere that the TS flattener fits better and performs better. Can't say where I read that, maybe even on this thread. Will look when I get back at the PC.

Hi, nice image, saw that in the other forum. I would like to have the reducer to cut down on sub lenght as i will be using NB filters. as long as I can get focus with the SW reducer and my filter wheel I will go with the Sw one, if not i will have to again at just flattners.

. 

Hi all, Im thinking of getting a SW 72ED pro to complement my SW 130 pds. The one thing that is giving me food for thought is the back focus. Can anyone confirm that with the SW reducer/flattener that I would be able to get focus using my ZWO asi 1600mmc and filter wheel. If it is possible how much back focus do i have left and is it an issue?

Also what if it is possible what spacing is it the reducer from the sensor? is it 55mm.

Oh and another thing! lol. What length Of Dovetail bar do I need to help balance?

Thanks in advance.

Simmo

11 hours ago, Tommohawk said:

I fudged my earlier reply by forgetting the filter wheel, but I say again you shouldn't need to buy any extra parts. Unless maybe some packages are different to those I received. 

Camera is 6.5 then 20mm filter wheel, then 1mm male to male connector, then 11mm (female both ends) then 16.5 which has 48mm female thread at on end and connects direct to SW CC.

In the drawing below, taken from ZWOs website, its camera then EFW then parts 9, 7, 8 then then CC.

It's probably not the fewest part solution but it is the fewest £ solution!

1. M43-T2 adapter 

2. EOS-T2 adapter 

3. 2”Filter (optional)

4. 1.25” T-Mount 

5. 1.25” Filter (optional)

6. M42-1.25” Filter (optional)

7. T2 extender 11mm

8. M42-M48 extender 16.5mm

9. T2-T2 adapter

10. EFW mini

11. EOS adapter for EFW+1600

 

Yep that adds up. As you say a bit long winded but works out of the box!