The EQ3 DSO Challenge

[bobro]

Thanks for the info Nige. The difference in detail is obvious between the 240 and 360s. Any improvement with the 600s subs isn't so obvious and I think there can be a noise problem combining a small number of subs with a larger number (in DSS anyway) when they are in different groups - noise will be determined largely by the group with the worst noise. (Hope I've got that correct .)

I wonder how a final image using only the 360s subs would compare to the one with both 240 and 360s subs?

Bob

Edited July 31, 2017 by bobro

[Stub Mandrel]

1 hour ago, bobro said:

Thanks for the info Nige. The difference in detail is obvious between the 240 and 360s. Any improvement with the 600s subs isn't so obvious and I think there can be a noise problem combining a small number of subs with a larger number (in DSS anyway) when they are in different groups - noise will be determined largely by the group with the worst noise. (Hope I've got that correct .)

I wonder how a final image using only the 360s subs would compare to the one with both 240 and 360s subs?

Bob

I think the problem is (and this is a guess) that say you have subs at two lengths. The shorter subs will show fewer stars so aside from losing any with badly deformed stars, DSS will throw out mostly short subs (which will have less noise but less detail) rather than choosing the best of each.

One thing I have done is register without stacking and checking (for the record this means putting an x in the box, not inspecting them all manually) just the high scoring subs in both (or all three!) groups then 'register checked images' which stacks 100% of checked frames. Don't forget to check flats, darks and bias!

Edited July 31, 2017 by Stub Mandrel

[Nigel G]

1 hour ago, bobro said:

Thanks for the info Nige. The difference in detail is obvious between the 240 and 360s. Any improvement with the 600s subs isn't so obvious and I think there can be a noise problem combining a small number of subs with a larger number (in DSS anyway) when they are in different groups - noise will be determined largely by the group with the worst noise. (Hope I've got that correct .)

I wonder how a final image using only the 360s subs would compare to the one with both 240 and 360s subs?

Bob

I used darks for each set of subs, I think ( but might be wrong) each set of darks will be used on the corresponding light frames.

I have processed with the 600s subs, its looks like there's a bit more emissions and dark dust, I will try the 360s alone, will be interesting to see the difference.

Tonight looks clear so this is calling out for a 135mm image. Fingers crossed.

Second image is + 2  600s light frames. I tried to process them the same.

Nige.

[Stub Mandrel]

Looks good Nige, I find the crescent a really frustrating target here - perhaps because of the big light dome in the NE here.

[bobro]

Thanks for the super fast reprocess Nige! 

I agree - the image that includes the 600s subs looks better. No sign of increased noise. My understanding of combining comes from a page on DSS theory (http://deepskystacker.free.fr/english/theory.htm) which states :

------------------------

Can I combine two (or more) resulting images?
Absolutely, the square root rule applies with a small twist.

When combining two images the SNR increases by 1.414 (square root of 2).
If both images have the same SNR then this is the same as doing a single stack. That does not mean that the combination is giving the same image, just that the SNR will be the same.

However if one stack contains more light frames than the other, the SNR of the two stacks will be different and the SNR of the combination will be lower than the SNR of a single stack containing all the light frames.

Thus by combining the result of a 10x1 minute stack with a single 1 minute frame the SNR is roughly the same as the one obtained by combining 2 light frames. This is due to the fact that when combining two images the noise is additive and the best image is damaged by the worse image in the process.

--------------------------

I must be missing the point (again!). 

Bob

[Stub Mandrel]

I read (past and present tense) that as meaning it's better to stack them all in one go with groups rather than stack separately and combine the resulting images.

Damnation, I can't predict tonight's weather at all, despite staring at the IR/Cloud satellite images until my eyes are popping out...

[Art Gecko]

I can't comment on the functions of DSS and stacking different length subs or the maths involved... It worked when I did it in Lynkeos, but then I do inspect all my subs manually and remove any of the files I don't want, plus I'm more concerned with getting the signal in the first place than removing the noise... but that's just me trying desperately to capture nebulae on an unmodded camera! Even so, I never let the computer decide on the quality... Visual inspection all the way!

I used to do a lot of music production and tend to think of my AP in similar ways.... rule 1: The better the signal you put in.. the better it's going to come out... Rule 2: It's easier to cut out unwanted signal than it is to boost signal you simply don't have enough of.

Those 10 minute subs made a real difference in my opinion, increasing the floor level of the darker areas has given more dynamic range and no doubt made stretching smoother and easier... Nice one @Nigel G, a really good experiment and I hope you're pleased with the results?!

I managed to get out and have my first go at M31 last night... haven't got round to processing it yet... Also tried it as a single panel rather than jumping straight into a mosaic... wanted to see exactly how much of it will fit in my FOV... will post the results later.

Art

[bobro]

A little green man going past last night in his flying saucer would have seen lots of scopes pointed up at him - a super night for AP! So time to revisit IC1396A, the Elephants's Trunk nebula, lit up by the nearby massive star. 18subs @ 480secs (approx 2.5 hours).

[Stub Mandrel]

3 hours ago, bobro said:

a super night for AP!

Not here, afraid I hit the bottle of laphroaig, which is starting to get a bit empty :-(

Astrophotography drove me to drink...

Edited August 1, 2017 by Stub Mandrel

[Art Gecko]

Well, here's my attempt from last night.... had quite a few subs with wobbly stars this time round... not sure why, putting it down to either wind factor (which didn't seem too bad while I was out there) or the increasing number of cats chasing around the garden! Only used 27 out of 100 subs!!

27 x 60s at 1600 ISO with darks and flats.. more flats than light frames come to think of it! Anyway here she is.. my first ever Andromeda.

[Nigel G]

Thats a cracking Andromeda, plenty of detail and depth.

Shame about the bad subs, mind you theres plenty of time to add more on M31.  

Cheers.

Nige.

[Stub Mandrel]

8 minutes ago, Art Gecko said:

my first ever Andromeda.

Excellent!

Now find an image/chart with all the globular clusters in Andromeda marked on it, and you can have a fun time seeing how many you have caught.

[Nigel G]

1 hour ago, Stub Mandrel said:

Not here, afraid I hit the bottle of laphroaig, which is starting to get a bit empty :-(

Astrophotography drove me to drink...

I know what you mean, But I have a solution, setup before the first swig, if the sky looks clear, if not just hit it

It was so hot outside Sunday night I drank 4 cans of beer before I even realised  with no beer left I had to start a new bottle of whisky.

I did sleep well when I finally hit the sack

Nige.

[Stub Mandrel]

Been a bit grimmer here in the Midlands.

[Art Gecko]

21 minutes ago, Nigel G said:

Thats a cracking Andromeda, plenty of detail and depth.

Shame about the bad subs, mind you theres plenty of time to add more on M31.  

Cheers.

Nige.

Cheers Nige, I'll definitely be getting more data on this one, and certainly a good candidate for a mosaic! Was a tricky one to process without blowing out the core though!

[bobro]

3 hours ago, Stub Mandrel said:

afraid I hit the bottle of laphroaig

See any little green men?  (My favourite whisky!)

[jjosefsen]

I've been directed here from another thread I started with some questions related to EQ3 imaging.☺️

 

I've been testing the tracking of my mount, trying to find my retention rate at various exposures..

Had another test session last night. Tracking problems not solved..! ? still had more bad subs than good ones, about 40% retention rate.

Managed a pretty good polar alignment from the look of it. Mount balance looked good too, but still lost a lot of frames. The thing is, it looks like I lose the same amount of frames percent wise, when I'm doing 30 second exposures as I do at 60 second exposures..

I suspect it is the mount having uneven movement from sticky gears, does this sound plausible? All the trailing was in the same direction, RA axis by the look of it.

There were also small guests of occasional wind, don't think it had much effect..

Edited August 2, 2017 by jjosefsen
Spelling

[Stub Mandrel]

Try replacing the stock grease with white teflon grease, also adjust the backlash carefully and make sure the mount is set up a bit east-heavy.

[jjosefsen]

6 minutes ago, Stub Mandrel said:

Try replacing the stock grease with white teflon grease, also adjust the backlash carefully and make sure the mount is set up a bit east-heavy.

Sorry if I'm a little dim ☺️East heavy?

[Stub Mandrel]

When tracking the mount always goes from east to west, if you balance it then unbalance it a little so the eastern side is a bit heavier than the western side it makes sure there is always a small amount of force keeping the gears in mesh giving a smoother drive.

[Peco4321]

4 minutes ago, Stub Mandrel said:

When tracking the mount always goes from east to west, if you balance it then unbalance it a little so the eastern side is a bit heavier than the western side it makes sure there is always a small amount of force keeping the gears in mesh giving a smoother drive.

First time I've understood that properly, thanks. 

[jjosefsen]

13 minutes ago, Stub Mandrel said:

When tracking the mount always goes from east to west, if you balance it then unbalance it a little so the eastern side is a bit heavier than the western side it makes sure there is always a small amount of force keeping the gears in mesh giving a smoother drive.

That makes sense, thank you.

Loils like I will have to do the whole strip down thing after all.

[wimvb]

You probably also have a fair amount of periodic error in this mount. I would advise to first take care of backlash in ra and dec, before stripping the mount. As I wrote before, this mount is tricky, since it doesn't have bearings in the axes. Smooth running under load is entirely dependent on the right amount of lubrication and tension in the bolts. But if you go for it, there is a write up.

 

[Stub Mandrel]

Thanks for finding that Wim.

 

[wimvb]

I just wish I had read that before I started stripping mine. The aluminium ring on the ra axis (underneath the polarscope ring) is locked by three tiny grub screws. I found out about that the hard way. Nearly stripped the thread clean off this ring. Also had to redo the whole process, because the grease I used the first time was too thin. The thick chinese stuff is needed in this case.

The DEC axis is easier, and since DEC backlash usually is the main issue, I would start there.

Another problem I had with my mount is the alt adjustment. The bolt that holds the head on the fork, tightens when you increase altitude, making the altitude adjustment difficult. As I live at 60 deg North, this caused the altitude adjustment bolts to bend. I resolved this by loosening up the central bolt, and eventually adding a wedge to the 'lip' that the altitude adjustment bolts push against. (Hope this description makes sense. I will try to find some images that show what I mean.)

Found some:

the altitude 'hinge' bolt that tightens itself when increasing altitude

The aluminium ring in the RA axis behind the polarscope ring. Notice two of the three grub screw holes and the threading coming off.

Also a small indentation in the RA axis where a grub screw pushed against the thread (top of the image).

The 'lip' of the altitude adjustment mechanism. The adjustment bolt just bent underneath this, so I made a small wedge for the adjustment bolt to push against.

Edited August 2, 2017 by wimvb
added images