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Removing reducer for more detail on small galaxies

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#1 orionic

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Posted 13 June 2021 - 01:59 PM

Hi, I've had this on my to-do list for a while now but it occurred to me, others may have tried this - so if you have, I'm curious what your experience was. 

 

One of the standard recommendations for EAA is to use a focal reducer, for faster light acquisition, "closer to live" viewing experience and just plain better results (e.g. easier to stack and to plate solve).  As a result, I am pretty sure I have never done EAA without a reducer, or I may have briefly tried it but it was during early experiments when nothing was working anyway.

 

My EAA system has been working fine for a year now, using approx. f/7 (0.7x FR with 8" Edge HD).  I think I may have "done" all the big, medium, and possibly most of the medium-smallish galaxies visible from Colorado.  (I am excluding ellipticals in which I have no interest).  That leaves a few hundreds or thousands of small-to-tiny galaxies on my potential observing list, but many will be nothing more than a shape and one or two discernible blurry internal features.  I'd love to see more detail on them, so I thought removing the reducer might help.

 

But then again, it might not.  Perhaps the details I'm getting are limited by "seeing" or other things, rather than pixel size?  Or even if it helps with resolution, cutting light acquisition in half means weaker subs, hence pushing the limit on mount accuracy (currently I use 12 seconds, might be able to push it to 20 on the Evo mount?), or, may need to increase gain so much that I lose details in a different way.

 

Any thoughts on using f/10 for EAA on small stuff?  I'll probably try it in any case, but figured it wouldn't hurt to ask.



#2 Sky King

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Posted 13 June 2021 - 02:42 PM

+1 for removing the reducer! If you don't like what you get you can easily put the reducer back in the setup. Make a text file or some notes about the night, what you used and your impressions and save it in the same directory as the night's images. Then you can find the results and compare them later. Sometimes combinations that you wouldn't think would work very well really surprise you. Also EAA is pretty forgiving. 



#3 GazingOli

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Posted 13 June 2021 - 03:58 PM

I got a CPC800 and use it @ f/10 frequently, especially for small objects like planetary nebulae. Last summer I even did a closeup on the Pillars of Creation in M16. Just check out my gallery - link in my signature.

 

CS.Oli


Edited by GazingOli, 13 June 2021 - 04:02 PM.


#4 orionic

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Posted 13 June 2021 - 07:18 PM

Great!  I appreciate the encouragement, Sky King and GazingOli.  Oli, your Pillars of Creation looks really nice!

 

Making flats for the 294mc was (and is) kind of a big deal and I knew that once I twisted the camera, it would be time for a new flat.  Well, I have made that twist & committed to it now so there is no turning back step.gif (Yeah I realize that's not the perfect emoji for this - but I have only 5 minutes patience digging through the emojis before I give up.  There should be an emoji for "looking through the emoji list ad nauseum"...).  But vignetting might not be as bad without the reducer, we shall see...

 

Now measuring spacing in the basement, looking forward to giving this a shot!


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#5 orionic

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Posted 14 June 2021 - 12:00 PM

Okay, here's my initial report.  The first discovery, and this is really fantastic -- removing the 0.7x Celestron Edge focal reducer allows me to reach zenith, whoohoo!  (This is with Evolution mount and requires shoving the OTA maximally forward.) But, just barely, with a 1-2 mm gap.  This is after carefully swapping out spacers to reach about 104-105 134-135 mm spacing.  Ideal would be 103.35 133.35 mm, and I think I can do that with more fiddling (I have a variable spacer I haven't used), but I wanted to get this set up for a session last night.

 

The second discovery, and this is really great too, is that plate solving and stacking worked just fine without the reducer, i.e., I am now at f/10.  Far from the dim-and-grim experience I would had expected (especially prior to seeing Oli's image), there are lots of stars showing up and galaxies look good too.  I did only 3 targets (M109, M63, M51), and all three looked appealing after a few short minutes, i.e. it did not really take a marathon to get good views, despite collecting 1/2 as much light.

 

Third - how about visible detail?  Is it better?  Here I am not sure.  I have started comparing with some older images, but it is always apples & oranges due to different moon conditions, altitude from horizon, etc.  However, the one thing that seems pretty clear is that effectively, using f/10 means noise has finer grain.  Because of course, the noise target object is spread out over a larger sensor area.  So when comparing two images at the same viewing scale, the one that used f/10 has more fine-grained noised, which looks smoother.  So that is a real plus.  So effectively, this probably means I can get more "oomph" out of tiny galaxies.  Last night I did not have a specific plan, just wanted to try this out.  If I can come up with a meaningful set of comparison images I will post.

 

Cheers!  Next step is researching all over again (from recent threads in this forum), how to make a flat for this new setup (ugh) and then I suspect I will stay at f/10 for quite some time.  Last summer I EAA'd a few big nebulae and it was kind of cool but I'm more impressed by galaxies, in general.  And Pillars of Creation is on my to-do list now!


Edited by orionic, 14 June 2021 - 01:18 PM.

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#6 GazingOli

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Posted 14 June 2021 - 12:36 PM

Already curious for your Pillars of Creation!

 

I will do it again this year if I get the chance (weather/occupation). Different cameras, more experienced and the wedge... I hope this will have an effect on the quality that I can obtain.

 

CS.Oli


Edited by GazingOli, 14 June 2021 - 12:37 PM.


#7 Mark Lovik

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Posted 14 June 2021 - 05:40 PM

I just did a quick set of calculations for your system.

ASI294 without reduction - about 1 arc second per color group.  This means you are not seriously oversampling at F10.

 

With typical seeing of 2" -- this is not too shabby ... you should have good color balance for any visible feature.

 

If you are using SharpCap - you can variably tradeoff resolution and S/N with the enhancement options in the imaging live stacking.  At your native F/10 resolution and pixel resolution: these features may become very useful in your live imaging.



#8 Mark Lovik

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Posted 14 June 2021 - 05:48 PM

Ugh did the post THEN remember the missing part

 

You may want to make sure to save your raw images in the stack and play them back.  Use these images for playing with your stacking options (stack from folder).  This becomes a lot of data --- fast.  But you can use these to compare processing options, and may be good for comparing live stacking with and without the reducer.  You can quickly do what-if comparisons with your raw data.

 

I am interested in your results -- I have an 8" SCT, but have not yet used it for EAA.



#9 Clouzot

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Posted 15 June 2021 - 03:08 AM

Okay, here's my initial report.  The first discovery, and this is really fantastic -- removing the 0.7x Celestron Edge focal reducer allows me to reach zenith, whoohoo!  (This is with Evolution mount and requires shoving the OTA maximally forward.) But, just barely, with a 1-2 mm gap.  This is after carefully swapping out spacers to reach about 104-105 134-135 mm spacing.  Ideal would be 103.35 133.35 mm, and I think I can do that with more fiddling (I have a variable spacer I haven't used), but I wanted to get this set up for a session last night.

 

The second discovery, and this is really great too, is that plate solving and stacking worked just fine without the reducer, i.e., I am now at f/10.  Far from the dim-and-grim experience I would had expected (especially prior to seeing Oli's image), there are lots of stars showing up and galaxies look good too.  I did only 3 targets (M109, M63, M51), and all three looked appealing after a few short minutes, i.e. it did not really take a marathon to get good views, despite collecting 1/2 as much light.

 

Third - how about visible detail?  Is it better?  Here I am not sure.  I have started comparing with some older images, but it is always apples & oranges due to different moon conditions, altitude from horizon, etc.  However, the one thing that seems pretty clear is that effectively, using f/10 means noise has finer grain.  Because of course, the noise target object is spread out over a larger sensor area.  So when comparing two images at the same viewing scale, the one that used f/10 has more fine-grained noised, which looks smoother.  So that is a real plus.  So effectively, this probably means I can get more "oomph" out of tiny galaxies.  Last night I did not have a specific plan, just wanted to try this out.  If I can come up with a meaningful set of comparison images I will post.

 

Cheers!  Next step is researching all over again (from recent threads in this forum), how to make a flat for this new setup (ugh) and then I suspect I will stay at f/10 for quite some time.  Last summer I EAA'd a few big nebulae and it was kind of cool but I'm more impressed by galaxies, in general.  And Pillars of Creation is on my to-do list now!

As to your first point (is detail any better?), the answer is yes, definitely. Even with average seeing conditions, every sampling improvement brings something (up to a point, as was amply noted in this forum, and even mathematically demonstrated).

 

Following my usual "proof of the pudding" mantra, I ran a session that started at f/6.x then switched to f/10 on the same targets (tiny Hickson groups or Arp objects); there was a substantial improvement in definition. And, as you noted, the noise becomes more tolerable due to its fine-grained nature, and the fact that saturated stars don't "bloom" as much. The evident downsides were the extended integration time (almost 3 times, in theory and in practice), longer subexposures and tracking issues. The good side of things is you have an EdgeHD, so your OTA is well corrected across the field. Vignetting should be far easier to work with at f/10, and flats are more forgiving even if you're not capturing perfect ones.



#10 alphatripleplus

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Posted 15 June 2021 - 10:17 AM

For what it is worth, I have done some limited comparisons with my C8  and ASI290MM with regards to whether I can get more detail without a reducer at slower f/ratios:

 

I usually operate at about 750mm (f/3.7) with stacked f/6.3 reducers and a 2.9 micron mono ASI290MM, yielding a pixel scale of 0.8 arcsec/pixel. I have found that with one reducer, or no reducer (nominally f/10 ish), I do not get any more detail visible in EAA captures. However, if I use a different scope (C6) at a noticeably shorter focal length (e.g 585mm, C6 @ f/3.9), I do begin to lose a little detail. So given my typical seeing, 0.8 arcsec/pixel with a mono camera seems to show as much detail as I can extract in EAA captures.



#11 orionic

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Posted 16 June 2021 - 01:42 AM

Thanks everyone for your input & suggestions.  Mark - Yes, I am using SharpCap.  I should revisit the enhancement options; I did play with them a few times but somehow always ended up preferring the "unenhanced" display.  I suppose I just felt that I could always enhance later on if I wanted.  (This opens another can of worms, is EAA for the "viewing experience" or is it for "quick & dirty astrophotography" - I lean towards the latter camp.)

I don't have big plans for analyzing my stacks in detail (but thanks for the suggestion), but I will post comparisons of f/7 vs. f/10 if I can find comparable sessions.  I think for me, not understanding much of the technicalities, a simple visual comparison will be my focus.

 

Note, one thing I do not expect to do is take the reducer on and off during a session.  In theory it sounds good, but when time is limited, fumbling in the dark with my optic train is not my cup of tea!  Spacers also have to be swapped to restore correct back focus.  Also there's the issue of flats.  My experience over the past year is that a single flat can be used just about indefinitely, because I rarely make changes to my optic train.  Out of laziness/fear, I avoid fussing with flats unless absolutely necessary.  I usually have ~1.5 hours available for a session, and I don't want to eat up time making a flat for that specific session, if I can avoid it.

 

Anyhow, it's interesting -- I'm comparing images I took of M63 -- a 512-second stack at f/7 from last June, and two from the recent f/10 session (a 750-sec and a 1440-second).  (Note, this is not a properly controlled experiment so these are tentative conclusions.).  In the brighter regions (near the galaxy core), the f/7 has the best detail, even over the 1440 exposure at f/10.  But looking at the little puffs of flocculence in the periphery of the galaxy, the large pixels of noise in the f/7 image break up the outlines, so what should be oval shapes (visible in both of the f/10 images, but especially at 1440 sec), are broken up into pixelated contours in the f/7.  So I think that does suggest small galaxies will show better detail at f/10 with my system anyway.

 

ps - Realistically I should focus on the 750-second exposure.  (It's unlikely I'll have time for 1440 seconds in most sessions.). Definitely in some areas it looks better than the 512-second at f/7, BUT, again, there are too many uncontrolled variables.

 

pps - Sorry no images yet!  My computer is full.  Photoshop will not run until I free up storage space!


Edited by orionic, 16 June 2021 - 01:53 AM.


#12 alphatripleplus

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Posted 16 June 2021 - 07:54 AM

 

 

pps - Sorry no images yet!  My computer is full.  Photoshop will not run until I free up storage space!

 

 

Everyone please remember that if you post any images in the EAA forum, they should not be post-processed, per the Forum guidelines. Thanks.


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#13 orionic

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Posted 19 June 2021 - 05:42 PM

Okay, I put together comparisons between f/7 and f/10 sessions for M63, M109, M51.
 
Note - I used Photoshop merely to rotate, crop, scale, and annotate (I recognize it's an over-powered tool... I happen to have this software for my work so I use it for any sort of image manipulation.)
 
Here is the one for M63 (to illustrate what I described in my post #11, above).
What I did for the f/7 images (the three at left) was scale them up (by eye) to match the pixel size of the f/10 images.
 
Again, I make no claim that this is a systematic or scientific analysis, I did this for my own curiosity and thought somebody here might take an interest. 
 
(In hindsight, the 744-second stack looks surprisingly weak to me - perhaps it was not well focused or something, but I included it for completeness.)
But the 512-second stack is an example of an f/7 exposure I was very pleased with at the time.  Comparing it to the 750-second stack at the right (f/10), I think the details around the galaxy core look a little better developed in the f/7, but the little flocculent puffs (white "clouds") near the bottom look clearer (smoother in outline) in the f/10.
 
F7 Vs F10   M63

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#14 Clouzot

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Posted 19 June 2021 - 05:50 PM

Well its seems you definitely have more detail in the f/10 stack (eg. the dark dust lane just left of M63's core) save in the fainter parts. f/10 is twice "slower" than f/7 ( (7/10)^2 = 0.5 ) so the only images we should theroetically compare are the 744s at F/7 and the 1440s at f/10, and imho the latter looks much better.



#15 orionic

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Posted 19 June 2021 - 06:57 PM

Here are the other two comparisons, using the same approach.  Again, lots of apples & oranges as the exposure times were not planned in advance; the f/7's all use flats and the f/10 does not; I did not check moon phase or altitude above horizon, etc.  But, it's interesting to watch the trends in noise & detail...

M51:

F7 Vs F10   M51
M109:
F7 Vs F10   M109

Edited by orionic, 19 June 2021 - 06:59 PM.

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#16 alphatripleplus

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Posted 19 June 2021 - 07:16 PM

In most of these f/7 vs f/10 comparisons, I'm not really noticing any consistent differences. Certainly, differences in altitude above the horizon and sky conditions/seeing from one night to another can have a much bigger impact than any potential resolution gain in going from f/7 to f/10.

 

If you have a chance to compare f/7 to f/10 on the same night for the same target, it would make it a more apples to apples comparison.


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#17 orionic

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Posted 21 June 2021 - 01:41 PM

Yes, the benefits are subtle, and not worth making any big sacrifices.  I feel the M51 comparison is pretty convincing, with a "crisper" look in the f/10, but again, that's just comparing two EAA sessions with lots of uncontrolled variables. 

 

I'll admit I'm equally excited that the shorter f/10 setup allows me to reach zenith (on the Evo mount) -- which itself can be a great way to improve quality.  Previously it was frustrating to know that all the best views were out of reach.  Now hoping to get a target up there soon...

 

Also this has allowed me to adjust spacing to the exact optimum(ish), 133.5 mm or so.

So I am hopeful that this, plus zenith views, and the slight improvement from f/10 optics, on a good night will give me more perceptible detail.

 

But it may mean doing only 2-4 targets per session instead of 4-8... there's always trade-offs...



#18 GazingOli

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Posted 21 June 2021 - 03:06 PM

ooohps... in a good night I am doing 20+ targets with my equipment...

 

CS.Oli


Edited by GazingOli, 21 June 2021 - 03:07 PM.

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#19 Cey42

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Posted 21 June 2021 - 06:01 PM

Thanks for posting this comparison. I have wondered if going to F10 would be worth it. For me, I don't see enough improvement in detail in order to take longer images at F10. I like the current speed I get at F6.3 and it probably works better given my typical seeing conditions.

 

It would be interesting to see a comparison with smaller galaxies



#20 AstroRBA

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Posted 22 June 2021 - 10:38 AM

Just my two cents worth: I've tried F10 out a few times with both main cameras; once with my 183 MM Pro specifically to target the M87 "spew"; it was reasonably smooth but highly oversampled (good guiding required!) - overall it's a bit of a pain because it's hard to get enough alignment stars and a good config etc., for the live stack.. some large surface area galaxies such as M101 were a challenge too;

 

Here's the M87 result:

 

Stack 51frames 279s WithDisplayStretch

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#21 unimatrix0

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Posted 22 June 2021 - 09:21 PM

Maybe less related, but people who image small galaxies and clusters or anything "small" , meaning it's not taking up a lot of the view, you can also remove the flattener (non-reducer).  Less glass between the sensor and your target, you may get sharper images.  I imaged (not EAA) without a flattener for 3 months and while the stars were flying towards the center  around the edges,  imaging the Whirlpool or the Pinwheel galaxy the crop in was plenty that the flattener made no difference as far as star shapes around the galaxies.  Less glass = less light dispersion; light directly focuses into your camera from the front lens. 


Edited by unimatrix0, 22 June 2021 - 09:22 PM.


#22 dcweaver

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Posted 27 June 2021 - 07:32 PM

Great image of M87!  The knot in the jet is really easy to see.

 

Given how well it turned out, I would not necessarily say it was highly oversampled.  After all, the jet is little more than 2 arcseconds wide.  Nyquist criterion of two pixels (each direction) is a bare bones minimum.  You really need more to get a representation that isn't blocky looking.  How much more depends on what the atmosphere is doing.  If seeing is bad, you may end up just sampling more of the atmospheric blur.  Looks like seeing was at least good enough to resolve 2 arcseconds, perhaps more.

 

A lot of people reduce exposure time on this target to reduce the surrounding galaxy glow and bring out the jet.  The lower limit is a foot race between the time needed to capture photons from the jet, and the time needed to capture nearby stars for the stacker to work.

 

Here's a link to my attempt at the Jet with a tiny 4 inch Mak.  I couldn't do it without a reducer, but that was driven more by lousy tracking.  There are lots of good posts in the thread from people with outstanding results.

 

https://www.cloudyni...he-grey-smudge/

 

 

 


Edited by dcweaver, 28 June 2021 - 01:34 AM.

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#23 orionic

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Posted 30 June 2021 - 12:41 PM

Today I was excited to find that a tiny, interesting galaxy (NGC6195) was near zenith, so it was a chance to try out my f/10 system which offers "breathing room" in the mount to reach zenith.  First, I tried 15-second exposures.  Wow, the stars were reaaally long.  I guess that's a zenith thing?  I had literally, never EAA'd at zenith before, so I'm new to this. 

 

So I cut it back to 10 seconds.  Now stars looked like "double images".  Really??  But I let it stack anyway, and I guess the stacking averaged things out.  Next problem was that the galaxy ended up off-center (it was centered after I plate-solved, but after I did a new dark for the new settings, it had migrated in the view).  There was a laaarge black edge rectangle working its way across the field, right towards my galaxy.  The kind of rectangle we always see due to rotation, but bigger than usual, and as per Murphy's Law, located exclusively on the side closest to my galaxy.  So anyway, I only got shortly past 860 seconds before it got swallowed up.  As you can see despite the lack of moon, it must be a pretty faint galaxy (I've never done this one before) because couldn't bring out the arms without a lot of noise.

NGC6195 6 30 21 Stack 86frames 860s crop

 

Session continued with gorgeous views of M106 and NGC4490.  I scrutinized them in detail, and similar to galaxies discussed earlier, it isn't a "slam dunk" for any decision, it's something of an ink blot test deciding whether f/10 or f/7 is better.  I think one of these days I will probably do an actual controlled comparison, it just takes determination, and I doubt it will be all that conclusive.  Virtually all the detail in galaxies consists of fluffy chunky stuff that is well over a pixel across, meaning that the size of pixels does not make or break the image. 

 

ps - I've been upping the gain as a way to "deal with" the reduced light collection at f/10.  Generally with f/7 I used around 300, now I'm using 380.  Gain can be used to "compensate" in this way.  Ultimately when we do a comparison, it's debatable what settings should be used.  Should it be same gain, same exposure, f/7 for x frames and f/10 for 2x frames?  Or should it be different gain (because who wants to sit around twice as long), and then, how long for f/10 is appropriate?  More apples & oranges...


Edited by orionic, 30 June 2021 - 12:45 PM.


#24 dcweaver

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Posted 30 June 2021 - 10:00 PM

Nice work!

 

A long focal length combined with an alt-az mount at zenith is a tough set of conditions.  At zenith, alt-az mounts have a hard time tracking, so you get lots of jitter that can smear exposures.  You also get the worst field rotation which isn't helpful either.  Shorter exposures might be a benefit in this situation to combat the jitter and rotation.  See how low you can go before the pesky field rotation borders start to become problematic.  Higher gain in the 350-400 range will also keep exposure time down.  The ASI294 has a fairly flat read noise curve out there.  Looks like a good place to be.



#25 alphatripleplus

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Posted 01 July 2021 - 06:40 AM

Yeah, as DW points  out once you increase the gain to the point where read noise is minimized, there is unlikely to be any value in increasing the gain any further. That looks like it is about 350 for the ASI294.




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