Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

RC8 softness... how to improve it or unavoidable?

  • Please log in to reply
27 replies to this topic

#1 davide.cattani

davide.cattani

    Lift Off

  • -----
  • topic starter
  • Posts: 5
  • Joined: 11 Jun 2021

Posted 11 June 2021 - 08:25 AM

Hi everybody, I'm struggling to get some good result with my GSO RC8 and I'm getting more and more discouraged. Regardless of my efforts, integrated images always look very soft to me... and I can't understand whether continue to invest on it or not. See the two images attached, Whirlpool and Needle Galaxy taken at 1090mm FL with a 0.67x reductor.

 

I think 3 factors count more: focus/collimation, mount/guiding and seeing.

 

As for focus, I use ZWO electronic focuser and I think there's not much to improve there: the depth of focus at f5.6 is not that critical, and I get a reasonable V curve when focusing. As for collimation, I entrust the managment to a professional (Artesky) to avoid messing up things, so it should be collimated. I say it should cause I didn't see appreciable improvements after collimation. I strugle to assess the collimation of the OTA, I find several tutorial on the web, all different between each other... very puzzled on this.

 

Mount, I upgraded to a Avalon Linear, that should be more then enough for my weights and FL. Still experimenting, but I think there's not much to improve here once it's well balanced (not 3D balanced though...).

Guiding, well... My OAG approach is probably not ok, but I'm not sure how to improve it. I'm using a 8x8mm ZWO OAG + ASI290mini. Finding guide stars is often a pain, for instance on Whirlpool, I have to raise the gain to the maximum and still hardly get 1 star. I'm wondering if upgrading to Celestron 12x12m OAG would worth the cost (in europe costs nearly 475$). I'm tempted also to go back to a 400/70 guide scope, wondering about flexures at 1090mm and at native FL 1600mm and if f5.6 will be enough to easly find stars with the asi290.

 

Seeing in the end... This is the most out of control. Proximity of the city and Bortle 5/6 doesn't help, but I really can't understand how assess seeing impact on my images. How to measure/estimate it and how much the problem is this so only a dark site far from the heat of a city could solve this point.

 

So I'm going by exclusion: 1st focus, 2nd mount and collimation - so far no improvements, 3rd guiding (other OAG? Guide scope?), 4th seeing/different site (?)...

 

I'm a little discouraged as I said in the beginning... Any advice from your experience to better focus my efforts and get satisfactory results would be grate! Many many thanks


Edited by davide.cattani, 11 June 2021 - 08:29 AM.


#2 davide.cattani

davide.cattani

    Lift Off

  • -----
  • topic starter
  • Posts: 5
  • Joined: 11 Jun 2021

Posted 11 June 2021 - 08:26 AM

M51 integrated and processed

Attached Thumbnails

  • M51_PRE_FINAL_larga low.jpg


#3 davide.cattani

davide.cattani

    Lift Off

  • -----
  • topic starter
  • Posts: 5
  • Joined: 11 Jun 2021

Posted 11 June 2021 - 08:27 AM

Needle integrated, not processed

Attached Thumbnails

  • Needle Integration low.jpg


#4 Peregrinatum

Peregrinatum

    Surveyor 1

  • *****
  • Posts: 1,753
  • Joined: 27 Dec 2018
  • Loc: South Central Valley, Ca

Posted 11 June 2021 - 09:16 AM

a couple of things come to mind...

 

-how often do you focus?  if the temp is changing quickly you need to refocus more often, but no later than 60' or 1 degree C... if you have temperature compensation on your focuser and combine this with an autofocus run every 60'/1C you can get some really crisp images... SGP can work with your focuser to do this while you image through the night... a solid autofucers and a imaging session program to run it are worth the money

 

-do you deconvolve your images? if you don't know what this is it is a "deblurring" of your image, and it helps greatly to bring some crispness back to your image... Pixinsight has this process, and I believe the free processing program Siril has it as well

 

-how much are you denoising your images? it looks like you use a lot of denoising... I only denoise the background in the nonlinear stage, then save a little denoising of the object for the end and as little as possible

 

-do you do any sharpening with your images?  you have to use a light touch, but a little sharpening at the end can bring some crispness back to the image

 

random thoughts:  you have excellent skies B 5/6 this would be a dream for me... seeing?  unless you live in a predominantly windy area you probably have average seeing like most of us do... collimation? perhaps, but if it was a big issue your stars would look like seagulls and yours look fine... big yes on the Celestron OAG with the ASI174mm mini, larger prism and chip

 

 

last comment is don't give up!  this hobby rewards the patient and the persistent, i think your images look good and with some changes in processing can look even better


Edited by Peregrinatum, 11 June 2021 - 09:18 AM.

  • davide.cattani likes this

#5 fewayne

fewayne

    Apollo

  • *****
  • Posts: 1,343
  • Joined: 10 Sep 2017
  • Loc: Madison, WI, USA

Posted 11 June 2021 - 09:46 AM

Do individual subs suffer from the same problem?


  • davide.cattani likes this

#6 Midnight Dan

Midnight Dan

    Voyager 1

  • *****
  • Posts: 14,513
  • Joined: 23 Jan 2008
  • Loc: Hilton, NY, Yellow Zone (Bortle 4.5)

Posted 11 June 2021 - 10:02 AM

I'm a new RC10 owner so I don't have a lot of experience yet, but I have some thoughts.

 

If you're using the Astro-Physics CCDT67 reducer, it can be used at various backfocus distances which changes the reduction factor.  From what I've been told, using it at the specified 0.67x reduction is not recommended for sharp images.  Most people use it at 0.7x to 0.75x reduction and get better results.

 

You don't mention what camera or sensor size you're using. But M51 is not that large, so your image above is likely heavily cropped in. If you look at Astrobin and search for M51 with an RC8, you'll find many images that look very much like yours.  Point is, given the scope and the size of the target, your image is pretty good.  With some additional processing, you could probably pull a little more detail and sharpness out of it.  But I wouldn't consider this a substandard result.

 

Galaxies are small.  They need long focal lengths, steady skies, and lots of integration time to pull out great results.  Most of us image in skies that are around 2" seeing on a good night.  Proper sampling rate is considered 3x, so a pixel scale of around 0.7"/px is about optimum.  Can't tell for sure without knowing your pixel size, but I would guess you'd do better without the reducer.  Of course, you'll be at f/8 which will take much longer integration time to get the same results, but you should be able to record more detail if your skies cooperate.

 

-Dan 


  • dswtan and davide.cattani like this

#7 Madratter

Madratter

    Voyager 1

  • *****
  • Posts: 13,163
  • Joined: 14 Jan 2013

Posted 11 June 2021 - 10:52 AM

You are not far off.

 

In the M51 picture, your stars are clearly elongated. Several things could be causing that, but it needs to be solved. After that, it is mostly a matter of processing.

 

The stars in Needle look somewhat better. It is hard to judge that on the basis of what you have posted without proper processing.

 

What are your FWHM values like for the integrated but not otherwise processed stack?

 

EDIT:

 

ALSO and as mentioned above, the CCDT67 is much better used not at .67 but with lesser reduction on this scope. Is your 1090 measured or calculated? If it is calculated just using the .67, then you really need to measure it and find where you are really at (using platesolving). If it is measured, you have too much reduction for this scope. I run mine at about 1190 instead of the 1090 you say you are at.


Edited by Madratter, 11 June 2021 - 11:31 AM.

  • dswtan and davide.cattani like this

#8 the Elf

the Elf

    Soyuz

  • *****
  • Posts: 3,632
  • Joined: 06 Sep 2017
  • Loc: Germany

Posted 11 June 2021 - 11:21 AM

Here is what I get with my RC8 carbon on a SW EQ6-R with a DSLR. Image is aligned to yours. I'm at 1100mm focal length using a reducer as well (see signature). This is off axis guided. I did collimation myself indoors. I don't have automatic focus, this is manually focused using a self made Bahtinov mask.

Sorry, I don't have an image of the Needle galaxy.

 

M51_2020_Full_registered.jpg

 

I'm also not totally happy with the soft stars but your stars are still softer than this. There is deconvolution applied to the image. It is a mandatory step when using an RC8. If you post a link to your unprocessed stack I can give it a try.

 

 


  • dswtan, Midnight Dan and davide.cattani like this

#9 BobT

BobT

    Mariner 2

  • -----
  • Posts: 224
  • Joined: 09 Feb 2012
  • Loc: Driftwood, TX

Posted 12 June 2021 - 07:52 AM

Great subject!  I'm into my fifth year of RC scopes and, like others, am not entirely happy with the "softness" issue.  Good focus is essential but there are two other factors to consider:  central obstruction (CO) and accurate collimation.  There is nothing we can do about CO, it is what it is.

 

Accurate collimation is difficult to achieve in an RC, particularly the GSO variants where there is no adjustment available for centering the secondary.  After spending a lot of time (and money) on various collimation schemes I have settled on a good laser, a Tak collimation scope (and some rigid adapters for them), followed by a star test.  This gets me into the "just good enough" category of collimation but is not as accurate as needed to achieve optimum results from the RC optics.  There was a good example of this in one of either Dr. Gaston Baudat's or John Hayes' internet presentations (I've lost the reference, sorry) where there were comparison astrophotos taken with a 10" (?) RC collimated with "standard" methods and then with wavefront analysis.  The difference was amazing.

 

There is a recent CN thread on the subject at https://www.cloudyni...nd-collimation/ that is interesting reading.


Edited by BobT, 12 June 2021 - 07:54 AM.

  • davide.cattani likes this

#10 the Elf

the Elf

    Soyuz

  • *****
  • Posts: 3,632
  • Joined: 06 Sep 2017
  • Loc: Germany

Posted 12 June 2021 - 03:18 PM

Is anyone following this who has shot M51 with a refractor in about the same price range and can present an image with pinpoint stars? I really wonder if I invest in a refractor in order to get rid if the soft stars. The big problem is I have about 2 clear nights per month, one of them with too much moon for dim galaxies. Given the little time weather allows I'd have to invest Eur 5k for a 155mm f/8 like this (and this is still slow compared to the RC 8 at f/5.7)

https://www.teleskop...zertifikat.html

Not to speak of a new mount to handle this monster. The open question is: what do the images look like? Would I get jaw dropping pinpoint stars and a level of detail I have never seen before or would it just be slightly better? Or do I have to move to Arizona in order to image galaxies with a low priced refractor where I have enough clear nights to collect enough data?

Or should I go for a classic Cassegrain with less central obstruction and a small image circle?


Edited by the Elf, 12 June 2021 - 03:21 PM.

  • dswtan and davide.cattani like this

#11 Midnight Dan

Midnight Dan

    Voyager 1

  • *****
  • Posts: 14,513
  • Joined: 23 Jan 2008
  • Loc: Hilton, NY, Yellow Zone (Bortle 4.5)

Posted 12 June 2021 - 03:42 PM

Is anyone following this who has shot M51 with a refractor in about the same price range and can present an image with pinpoint stars? I really wonder if I invest in a refractor in order to get rid if the soft stars. The big problem is I have about 2 clear nights per month, one of them with too much moon for dim galaxies. Given the little time weather allows I'd have to invest Eur 5k for a 155mm f/8 like this (and this is still slow compared to the RC 8 at f/5.7)

https://www.teleskop...zertifikat.html

Not to speak of a new mount to handle this monster. The open question is: what do the images look like? Would I get jaw dropping pinpoint stars and a level of detail I have never seen before or would it just be slightly better? Or do I have to move to Arizona in order to image galaxies with a low priced refractor where I have enough clear nights to collect enough data?

Or should I go for a classic Cassegrain with less central obstruction and a small image circle?

A refractor in the same price range will have a substantially smaller aperture and will not likely give you better stars at the same image scale as this image.

 

But ... star reduction is an area of processing that is as complex as noise reduction.  Search on the web and you'll find loads of pages, youtube videos, and many varied techniques.  The images you see with very small pinpoint stars, at large image scales, have gone though star reduction techniques to make the stars smaller and sharper.  The trick is to do it without introducing haloes or other artifacts around the stars.  Took me a long time and a lot of practice to get to where I can do it to some degree.  I know others who are far better than I am.

 

So if it's just the stars you're interested, there are processing techniques that will help.  But if you're referring to the overall image softness, then the advice that has already been given above regarding collimation, focus, and seeing all come into play.  Deconvolution can definitely help here.

 

As for equipment, going to a smaller aperture is going in the wrong direction.  In fact, to get potential for more resolution, you need a scope with larger aperture and longer focal length.  I say "potential" because you're still going to be limited by seeing or poor collimation.  

 

Yes, a quality refractor will usually deliver better stars than a comparably sized reflector.  But when you get to the size aperture you need to image small targets, a quality refractor becomes extremely expensive, and heavy enough to need a very expensive mount to carry it.

 

Got $25K you want to drop on a 50 pound, 8" Apo?  ;-)

https://cfftelescope...ctor-200-230-mm

 

-Dan


Edited by Midnight Dan, 12 June 2021 - 03:43 PM.

  • davide.cattani likes this

#12 Madratter

Madratter

    Voyager 1

  • *****
  • Posts: 13,163
  • Joined: 14 Jan 2013

Posted 12 June 2021 - 03:54 PM

I guess the question is how soft is soft? What are your FWHM values for a sub. And at what image scale? With my lousy East Coast of USA seeing, I can consistently do 3 to 3.5 arc-seconds at .93 arc-seconds/pixel and occasionally better, and Sub 3 arc-seconds/pixel when at f/8 (.69 arc-seconds/pixel). I do know from experience that collimation is absolutely critical to good performance. And when approaching well collimated, even small changes can make a very big difference.


  • dswtan and davide.cattani like this

#13 Midnight Dan

Midnight Dan

    Voyager 1

  • *****
  • Posts: 14,513
  • Joined: 23 Jan 2008
  • Loc: Hilton, NY, Yellow Zone (Bortle 4.5)

Posted 12 June 2021 - 06:33 PM

Here/s a reference point for what you can do with a little larger aperture.  I just purchased an RC10, and am using a CCDT67 reducer with it.  This is 7 hours of integration to start with.  But I also spent a good 8 hours processing this image.  Many rounds of noise reduction, various star reduction techniques, deconvolution, etc.   Believe me, the stars looked nowhere near this good in the original stacked image.

 

get.jpg?insecure

 

-Dan


  • dswtan and davide.cattani like this

#14 bokemon

bokemon

    Viking 1

  • *****
  • Posts: 561
  • Joined: 28 Jul 2020
  • Loc: Silicon Valley, California

Posted 12 June 2021 - 07:45 PM

A good way to assess "seeing" is if you use sharpcap or some other program that can run your camera in video mode with > 10 frames per second.  Then you can just zoom in on an individual star and see how much it jumps around in real time.


  • davide.cattani likes this

#15 the Elf

the Elf

    Soyuz

  • *****
  • Posts: 3,632
  • Joined: 06 Sep 2017
  • Loc: Germany

Posted 13 June 2021 - 05:18 AM

Dan,

 

thank you very much for posting this and for telling the truth behind the image as far as processing is concerned. I aligned my version to yours and find it difficult to compare them because of the different processing. Your stars do have a clearly defined border while my stars have a bright central spot and a soft border. In my world stars look like tiny globular clusters. The price of the RC 10 carbon is about 2.5x the price of an RC 8 carbon at TS. It is hard to measure the quality difference between the images but my conclusion is that we have an example of diminishing returns here. Left: Dan's RC10, right Elf's RC8:

 

compare.jpg

 

This is my FWHM in all 88 subs taken in two nights:

SSS_88_best.png

 

This is in arcseconds, scale is 0.73 arcsec/pixel.

This is above average for my location. The lowest value I have ever seen in a color sub is 2.3 arcsecs. The 65mm refractor at 1.8 arcsec/pix give the exact same FWHM of 2.x in good nights so both scopes are seeing limited. Which is in accordance with theory. If I go for a bigger scope at all one fine day an 12.5 CDK is probably the best choice for me. With the spherical secondary collimation is not as critical and some optical errors that an RC has are corrected in a CDK. As the company I work for send letters offering a pay-off for anyone who decides to leave now investments in the 10k range are pure speculation.

 

Also thank you for pointing out the pros and cons of a refractor. It is what I kind of expected from theory but it is good to read it from someone in a clear and short statement. My conclusion is that for the time being the RC8 is the sweet spot in my price range and substantial improvement requires an investment of about 4 times of what I have invested so far. Going to keep this in mind as a long term plan.


  • dswtan, Midnight Dan and davide.cattani like this

#16 the Elf

the Elf

    Soyuz

  • *****
  • Posts: 3,632
  • Joined: 06 Sep 2017
  • Loc: Germany

Posted 13 June 2021 - 05:29 AM

I guess the question is how soft is soft? What are your FWHM values for a sub. And at what image scale? With my lousy East Coast of USA seeing, I can consistently do 3 to 3.5 arc-seconds at .93 arc-seconds/pixel and occasionally better, and Sub 3 arc-seconds/pixel when at f/8 (.69 arc-seconds/pixel). I do know from experience that collimation is absolutely critical to good performance. And when approaching well collimated, even small changes can make a very big difference.

I did a collimation and mirror cleaning recently. This is the FWHM map I get. This is about my only means of judging the result of my collimation efforts.

 

FWHM.jpg

 

This is in Pixels, scale is 0.73. Eccentricity values are decent imho but lack symmetry:

ecc.jpg

 

Is this a good collimation? I have no clue.

 

The underlying image is the unprocessed stack of 72x5min of M63.

https://www.elf-of-l..._Sunflower.html

 

100% crop:

M63crop.jpg

 

Welcome back to soft star world :-)


  • dswtan and davide.cattani like this

#17 Readerp

Readerp

    Ranger 4

  • *****
  • Posts: 383
  • Joined: 22 Jun 2019
  • Loc: So Cal

Posted 13 June 2021 - 08:32 AM

Just to add here is my image taken with my 130mm refractor, in very good(?) seeing, Bortle 1.

Only 2.5 hours integration time, but probably good for a comparison.

I did do some deconvolution and sharpening. I could probably reprocess this today and do a little better.

 

https://www.astrobin.com/fq3yn2/C/

 

-Pete


Edited by Readerp, 13 June 2021 - 08:36 AM.

  • dswtan and davide.cattani like this

#18 Midnight Dan

Midnight Dan

    Voyager 1

  • *****
  • Posts: 14,513
  • Joined: 23 Jan 2008
  • Loc: Hilton, NY, Yellow Zone (Bortle 4.5)

Posted 13 June 2021 - 09:12 AM

 It is hard to measure the quality difference between the images but my conclusion is that we have an example of diminishing returns here. Left: Dan's RC10, right Elf's RC8.

Hi Elf:

 

I think you're being generous to say it's hard to measure the difference. :-)  When viewed at this scale yours is clearly better.  It has more detail and looks more natural.  My hat's off to you for getting that detail with an RC8! 

 

But you are correct that there's a point of diminishing returns, mostly due to the limits of seeing.  I went to the larger 10" for 2 reasons ... better resolution was not one of them, although again with 10" there's the potential for it.  The first reason was that I could use the CCDT67 reducer and get to my target pixel scale, while also getting to a reasonably fast f/5.5.  

 

The second reason was because when you get to the larger 10" truss model, the focuser and the mirror are separated.  Each has their own collimation screws and neither one affects the other.  This greatly reduces the collimation headaches associated with RC scopes.

 

I should also point out that this image was my first light image with the scope and I'm not fully optimized.  For one thing, I'm using a guide scope and I can see that I'm experiencing differential flex - my eccentricity is about 0.6.  I have an OAG on order to correct that.  Also, I've done collimation with a Glatter laser, but a star test shows the center of the donut a little off.  So I still need to do some collimation tweaking.

 

Based in that FWHM map, your collimation looks about as good as you can get.  And your FWHM is better than mine which may be due to seeing or collimation.  But your image gives me hope that when I get this setup nailed down, I can do a bit better.

 

Bottom line though, with larger apertures and longer focal length, everything has to be spot on to get the best results.  Collimation, seeing, guiding, cooling, focus ... they all have to be near perfect to pull out the most detail.  And then of course, there's processing ... :-)

 

-Dan


  • dswtan and davide.cattani like this

#19 dswtan

dswtan

    Viking 1

  • *****
  • Posts: 626
  • Joined: 29 Oct 2006
  • Loc: Morgan Hill, CA

Posted 13 June 2021 - 11:41 AM

Going back to the OP, I do think it's key to manage expectations through apples-to-apples comparisons. Filtered search on Astrobin is helpful for that, but you need to take into account all the variables that have been mentioned (collimation, technique, processing), including things less (but still somewhat) in your control like seeing. OP's posted images do give an impression of some room for improvement, but not much.

 

Elf's and Madratter's results are gold standards to aspire to on the RC8, and I find Elf's images posted in this thread extraordinary. I think his collimation must be spot-on. This article is my go-to reference for collimation's importance: http://www.astrophoto.fr/collim.html. While planetary-focused, I think high resolution galaxy photography has a lot in common with planetary, in terms of resolution, despite the very different capture techniques. We are both trying to reach the performance limits of our optics. You need to nail that one. I am close now, but still not there.

 

I am also not being selective enough with seeing. Need to image (at least in L) always at high altitudes, and be more selective on my local conditions -- just like planetary.

 

And yes, I am lazy with deconvolution (I'm EZ suite all the way), because I just don't have the time or patience until retirement. That said, a *little* Topaz (fully disclosed) can be helpful if you are just posting for friends.

So as another RC8 reference point, this is what I'm getting in March this year from northern California:

0Fky82wKymJ__16536x0_ksngRFHd.jpg

 

Ref: https://astrob.in/55hub3/0/


Edited by dswtan, 13 June 2021 - 11:42 AM.

  • davide.cattani likes this

#20 the Elf

the Elf

    Soyuz

  • *****
  • Posts: 3,632
  • Joined: 06 Sep 2017
  • Loc: Germany

Posted 13 June 2021 - 03:37 PM

There seem to be two basic processing options for stars:

a) circular disks with a relatively large diameter but a sharp edge and absolute darkness outside of it.

b) tiny spots surrounded by some blur. While the center spot is smaller than the a) circles the blur is larger

I am on the b) route but honestly it just happens when I try to get the best out of the main target. It is probably a matter of taste. The big trouble is that any version looks different when the image is scaled down. There seems to be a flawless size and when zooming further in the stars start telling a story.

Thank you for your kind feedback to my image, dswtan!

Now going to set up my rig for Ha on the ring in Lyra. Yes, clear skies! Don't know how to deal with work tomorrow, though.


  • Mike in Rancho and davide.cattani like this

#21 bokemon

bokemon

    Viking 1

  • *****
  • Posts: 561
  • Joined: 28 Jul 2020
  • Loc: Silicon Valley, California

Posted 13 June 2021 - 03:45 PM

Hello folks, 

It would be more helpful to show images without processing such as deconvolution or star reduction if we are to assess the "sharpness" of a RC telescope.  A lot of these images posted here look like they have good "resolution", but there are mini-halos around the stars.  Almost like the point spread function (or whatever it's called) is not that sharp central peak of an airy disk pattern, but more like some kind of peak with broad shoulders and a long tail.  Is this blurring from seeing or mis-collimation?


  • BobT and davide.cattani like this

#22 bokemon

bokemon

    Viking 1

  • *****
  • Posts: 561
  • Joined: 28 Jul 2020
  • Loc: Silicon Valley, California

Posted 13 June 2021 - 10:00 PM

I was going to post an example picture, but the file size was too big, so let me just use 1000 words to describe it:

I have an images of M51, taken with a 10" F4 newt, that is sharper than the images posted here, and this is only as a stacked image under AstroPixelProcessor with no processing like sharpen, deconvolve, etc.  The smallest stars and diffraction spikes are about 2 pixels wide, and there is no halo around the stars.  This image was taken on a day of "good seeing".  During "average" or "bad" seeing the stars are significantly more bloated.


  • davide.cattani likes this

#23 davide.cattani

davide.cattani

    Lift Off

  • -----
  • topic starter
  • Posts: 5
  • Joined: 11 Jun 2021

Posted 14 June 2021 - 08:08 AM

Hi everybody, first of all many thanks for your replies and help, all very important! I would have liked to reply you sooner, but parents visit (very appreciated) filled the weekend.

And was it maybe the first new moon and clear sky weekend after 2 months of clouds...?! Yes of course, but don't think about it...

 

I want to:
1) put my comments to your posts
2) publish underlying data, subs and integrated images
3) sum up the next steps based on yout suggestions, both for me and for those who are at my stage

 

I had to choose whether to reply to single posts, but fragmenting my reply, or put all my comments together in a more homogeneus post; I chose the second. Hope everying will be clear enough.

 

Point 1
- Refocusing frequency -> The trade-off is of course how much time you want to loose in refocusing, so I settled on 2h and no temperature trigger using 5s exposures at Unity gain (111 on ASI183) and f5.6 on my RC8 carbon. I don't see evident changes in FWHM before and after refocusing in this way, as you can see in "FWHM" attachement. That's why I think it should be sufficient. The Elf manual focusing also could suggests refocusing may not be that crucial, given its great results.
Anyway, there's actually no reason not to trigger on temperature if one have an electronic focuser, so I will put it back in my refocusing procedure.

 

- Decovolution -> M51 is deconvolved with SPF calculated on actual image stars. I stop encreasing deconvolution strenght when I see stars start to "melt" with each other, creating sort of filaments between them. It's the same for you?

All of you have underlined that deconvolution IS mandatory on RC, so no more doubts on keeping this processing step.

 

- Denoising -> I found Jon Rista method to be the best I tried so far, especially in terms of repeatibility and effectiveness. I see softening coming in expecially when denoising after stretching, but I think careful (patient...) tuning of TGV and MMT parameters can avoid it, so I'll try to improve on this.

 

- Sharpening -> Weak on this point... I see visible effects only when using Unsharp Mask (Pixinsight), but it degrades overall image quality quite soon. So I don't have clever methods on this point actually.

 

- Seeing, FWHM and Eccentricity -> as Bokemon said, high frequency imaging could help to assess it. I don't actually know other ways to do it, other than beeing confident that all my set-up is perfectly configured and therefore seeing is measured by FWHM.

 

Actually I can't understand if my FWHT is seeing limited or not, but I guess not. See FWHM Attachment: avarage is 4.2", but my points are very scattered, far too much. Why, in your opinion? These are 2 minutes subs at unity gain (111 on the ASI183 native driver) from 1 night out of 2 of LUM on the M51.
Nothing to do with The Elf FWHM plot, much more regular and lower valued.

 

Eccentricity suffers the same problem, see the graph attached. Again, avarage quite reasonable on 0.45, but scattering all around. I guess my signal is too weak and I need to encrease exposure and/or gain. Maybe this will stabilize these values.

 

The Integrated LUM (attached, see point 2) of M51 has 4.2" FWHM as measured by Subframe Selector and Ecc 0.22 (so low?!), quite in accordance with FWHMEccentricity tool which gives 4.39" and 0.21. I often get very different results from this two tools, but that's another story and not the case here.

 

For completeness I attach also SNR and Stars from the same set of subs.

 

- Collimation -> I understood from many of you that perfect collimation is much more important than I hoped. I didn't choose a Newtonian (even though I saw the best pictures at 8" from these scopes) to avoid recollimating it every time I mount/dismount/travel, but this point come back to be crucial also with RC. I found that atmospheric instability (seeing) degrade defocused star patterns so much that evaluations based on these moving and vibrant patterns becomes (to me) very very unprecise; extending exp times of course stabilizes the pattern, but rings become less visible and the whole processo gets much longer... But I'm probably wrong, given you all do it. Does an artificial star maybe solve this problem completely?

 

As for FWHM and Eccentricity Contour plots, and referring to The Elf plots, I struggle to get them with the RC8 since my subs have too few stars, as PixInsight tells me... I probably need to abandon 60s and 120s exposures and go for 180s or 240s, maybe increasing gain also from Unity gain which I currently use.

 

Last, a friend on Facebook made me understand CCDInspector is absolutely necessary. There are other free tools around, but from my trials none of them is actually as straight forward as it is. Since we are saying that collimation must be as perfect as possibile it could be that this tools is actually mandatory...

 

- Focal Reducer -> As Dan reminds me, I'll abandon 0.67x reduction factor, going for 70mm backspacing (from flange) and 0.72x.

When I tried to image without Focal Reducer I found the same softness in my image, so the FR itself doesn't not play a big role in the overall softness.

 

- Image scale -> both M51 and Needle are shot at 0.45"/px. Do you think such strong oversampling contribute a lot to the final image softness? The answer is probably yes, but I'm still hesitant to accept it, since oversampling should add (useless but not harmful) information... And enlarging pixel size is apparetly no that easy with commercial CMOS sensors, coming all of them with pretty small pixels. Anyway, future 2x2 binning will go to 4.8um pixel size and 0.9"/px (with focal reducer), which is about 1/3 of avarage 3" seeing, so should be fine.

 

- OAG -> Large majority of you agree on OAG instead of guide scope. I'm currently guiding at 0.55"/px on my small OAG (8x8mm), so 5.5 times an avarage seeing of 3" and near the 0.45"/px of the main camera. All these image scales are too low. I was temped by a 70/400 guide scope, which should do the job if rigidly secured to OTA and run at 1,5"/px with the ASI290. It would have some advange, but consensus goes for the ASI174 and Celestron 12x12mm OAG (or similar), which is another investment by the way...

 

- Reference Point -> The Elf, your images set the best practices to follow, at least in this thread but I think in general. I guess you're quite "spot on", showing the improvements to do both on RC8 and RC10. It looks like your color layer is much softer than the LUM, because of those colored halos around stars and probabily all over the image. Don't you think? Sharpening color layer would probably boost perceived details even more.

 

Point 2
Subs are here, together with master lights: https://drive.google..._ZM?usp=sharing. I put also darks and reference frames I used, just in case.

Upload is in progress, it will finish in about 4 hours.
Many thanks in advance to all of you willing to give them a look! (In case you use darks, "_A_" are for LUM and "_B_" are for COL subs).

 

Point 3
Coming in the next post...

 

ps: how to paste images between lines of text, here on Cloudynights?

Attached Thumbnails

  • FWHM.jpg
  • Eccentricity.jpg
  • SNR.jpg
  • Stars.jpg

Edited by davide.cattani, 14 June 2021 - 09:55 AM.


#24 Midnight Dan

Midnight Dan

    Voyager 1

  • *****
  • Posts: 14,513
  • Joined: 23 Jan 2008
  • Loc: Hilton, NY, Yellow Zone (Bortle 4.5)

Posted 14 June 2021 - 09:07 AM

Decovolution -> M51 is deconvolved with SPF calculated on actual image stars. I stop encreasing deconvolution strenght when I see stars start to "melt" with each other, creating sort of filaments between them. It's the same for you?

Yes.  I'm familiar with the filament artifact you mention.  I refer to it as the "spider web" :-)  But, if you use proper masking of the background, it should make it easier to avoid that.

 

 I found that atmospheric instability (seeing) degrade defocused star patterns so much that evaluations based on these moving and vibrant patterns becomes (to me) very very unprecise ...

 

This may be a key to your image softness.  If the seeing is so bad that collimation is difficult or impossible on a video-rate image, then it will definitely reduce your ability to get sharp images.  And it may also be responsible for your variability in FWHM and eccentricity.

 

-Dan


  • davide.cattani likes this

#25 Peregrinatum

Peregrinatum

    Surveyor 1

  • *****
  • Posts: 1,753
  • Joined: 27 Dec 2018
  • Loc: South Central Valley, Ca

Posted 14 June 2021 - 11:34 AM

the deconvolution "spider web" can be reduced/eliminated by increasing the global bright setting


Edited by Peregrinatum, 14 June 2021 - 11:36 AM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics