Why are my stars so big? NGC891

[ollypenrice]

Stars may need to be reduced when they are embedded in nebulosity. These are the tricky ones. Stars in pure background sky do not need to be reduced: it is better to replace them with stars not stretched so far in the first place.

Olly

[ollypenrice]

1 minute ago, Davey-T said:

Bit of a bodge really, bear in mind that some "experts" spend tens of hours processing their images and they've got excellent data to start with, as Olly said if you look for images of the same target on the net you may get disheartened at not being able to match them, I gave up trying years ago 

Dave 

It would be unusual for me, now, to spend more than ten hours on a single frame image, I think, but that does sound about right for a single frame image with five filters, Ha, OIII, L, R, G, B.

On the other hand, getting it down to ten hours has taken me ten years...

lly

[Star101]

1 minute ago, ollypenrice said:

It would be unusual for me, now, to spend more than ten hours on a single frame image, I think, but that does sound about right for a single frame image with five filters, Ha, OIII, L, R, G, B.

On the other hand, getting it down to ten hours has taken me ten years...

lly

10 hours? Isn't that a TEST SHOT for you Olly!!

Dave.

[Davey-T]

5 minutes ago, ollypenrice said:

It would be unusual for me, now, to spend more than ten hours on a single frame image, I think, but that does sound about right for a single frame image with five filters, Ha, OIII, L, R, G, B.

On the other hand, getting it down to ten hours has taken me ten years...

lly

I think it was Noel Carboni that published his Veil Nebula image that he spent days on, personally I get bored after 2 hours 😂

Dave 

Edited December 19, 2019 by Davey-T

[ollypenrice]

1 minute ago, Star101 said:

10 hours? Isn't that a TEST SHOT for you Olly!!

Dave.

I'm talking about just the processing. I can do a quick 'proof of concept' processing job in as little as five hours... 

The whole thing is madness. Madness, I tell you!!!

lly

[ollypenrice]

2 minutes ago, Davey-T said:

I think it was Noel Carboni that published his Veil Nebula image that he spent days on, personally I get bored after 2 hours 😂

Dave 

It takes me two hours, if I'm lucky, to get the background sky acceptable! You must be one of these juveniles with ADHD that we keep hearing about!!!

lly

[Chris49]

11 minutes ago, Davey-T said:

bear in mind that some "experts" spend tens of hours processing their images

There I was thinking 10 minutes processing was a bit excessive - I was hoping my subs were going to be nearly perfect straight out of the camera 😀.

Chris

[Davey-T]

4 minutes ago, ollypenrice said:

It takes me two hours, if I'm lucky, to get the background sky acceptable! You must be one of these juveniles with ADHD that we keep hearing about!!!

lly

Now you know why none of mine ever appear here 😂

Dave 

[kirkster501]

The better the data the less time needed to process it.  That’s what I’ve found.

[newbie alert]

To be honest you've done a pretty good job, imaging at 2000mm is always going to be a tough job,  so don't be too hard on yourself.. are you using a separate guide scope or oag?  As sct's are prone to mirror flop..not that I think that's your issue , they say to guide at half your image scale, it's going to have to be a very good mount to do that comfortably..

[Cosmic Geoff]

2 hours ago, ollypenrice said:

SCTs can deliver astonishing planetary detail in fast frame imaging but are notorious for giving large stars in deep sky work. I don't know why that is, but the evidence is overwhelming. 

That is exactly my experience.  All the star cluster images I took with my 8" SCT looked awful.  I think it's the seeing.  The star cluster images I have taken with my Startravel achromat (one quarter the focal length) look much prettier.

[fireballxl5]

for star size comparison here's my shot of the same target from back in 2008 using my 12" f9 MC (fl=2700mm), just 8x300s lum only. In not so great conditions🙃

This was at an image scale of 0.56arcsec/pix, long before I realised the importance (and inherent difficultly with my kit and sky conditions) of guide scale vs image scale lol. I've since backed-off to a more comfortable 1.7arcsec/pix😉

HTH, Andy

[michael8554]

I don't believe that analysing a stack tells the real story. 

Are your stars that large in the subs?

Also at 2032mm your RA and Dec guiding might luckily be equally "bad", giving spread - out and round stars. 

Michael 

[ollypenrice]

9 hours ago, michael8554 said:

 

Also at 2032mm your RA and Dec guiding might luckily be equally "bad", giving spread - out and round stars. 

Michael 

This is an important point. The 'round stars' test is really no test at all.

Olly

[Chris49]

11 hours ago, newbie alert said:

are you using a separate guide scope or oag?

The guide scope is the standard Celestron 50mm finder #51611 and holder that comes with the EdgeHD8 - not very satisfactory as it is not rigid.  Guide cam is SX Ultrastar.  I have since moved it to rings mounted on a Vixen dovetail on top of the main tube.  This is much more rigid and is symmetrical for balance so that should help.

11 hours ago, newbie alert said:

it's going to have to be a very good mount to do that comfortably..

Mount is iOptron GEM45 - just getting used to it, but I am impressed so far, way better than my A-VX which had so much Dec backlash that it never came back!

Thanks for the encouragement!

Chris

[Chris49]

10 hours ago, fireballxl5 said:

for star size comparison here's my shot of the same target from back in 2008

I'm not too far off then.

Thanks for posting your image.

Chris

[vlaiv]

4 minutes ago, Chris49 said:

The guide scope is the standard Celestron 50mm finder #51611 and holder that comes with the EdgeHD8 - not very satisfactory as it is not rigid.  Guide cam is SX Ultrastar.  I have since moved it to rings mounted on a Vixen dovetail on top of the main tube.  This is much more rigid and is symmetrical for balance so that should help.

That setup has nowhere near needed precision to guide your setup for close in shots.

First "upgrade" would be switching to OAG.

4 minutes ago, Chris49 said:

Mount is iOptron GEM45 - just getting used to it, but I am impressed so far, way better than my A-VX which had so much Dec backlash that it never came back!

This mount should provide you with enough precision for high resolution work, provided you don't overload it an or do something else that could hamper its performance.

[Chris49]

2 minutes ago, vlaiv said:

That setup has nowhere near needed precision to guide your setup for close in shots.

First "upgrade" would be switching to OAG.

You are right of course, but I think it is a case of "small steps" to make progress without getting discouraged.  I am a bit wary of the OAG because people say they are hard to use.  I hope to get there eventually!

Thanks for the advice.

Chris

[Chris49]

10 hours ago, michael8554 said:

I don't believe that analysing a stack tells the real story. 

Are your stars that large in the subs?

You are right, they are not.  So I was just lucky they came out roughly round!

Back to improving the guiding!

Thanks

Chris

[carastro]

Chris, I have just given your image a quick run through the star reduction technique demonstrated in my video above.

This is the result.

By the way further to above discussion, I can sometimes spend a whole day post processing an image trying to get the best out of it, especially if it's done on a dual rig or multiple session. 

Carole 

Edited December 20, 2019 by carastro

[newbie alert]

6 hours ago, Chris49 said:

You are right of course, but I think it is a case of "small steps" to make progress without getting discouraged.  I am a bit wary of the OAG because people say they are hard to use.  I hope to get there eventually!

Thanks for the advice.

Chris

Hard to setup but no harder to use than what you are doing at the moment 

Cem45... not too sure what the payload is and you may well be under but it's not just weight but focal length that you're asking it to achieve.. look at your image scale

[vlaiv]

3 minutes ago, newbie alert said:

Hard to setup but no harder to use than what you are doing at the moment 

Cem45... not too sure what the payload is and you may well be under but it's not just weight but focal length that you're asking it to achieve.. look at your image scale

Please don't continue to keep that way of thinking alive - focal length has nothing to do with performance of the mount.

Put two identical scopes (in terns of size and weight) on the same mount - one with 200mm FL and other with 4000mm FL (I know, such scopes don't exist, but let's imagine some crazy CN 212 that can be both hyperbolic newtonian and folded maksutov type scope in the same package). How on earth would mount know the difference between focal lengths of the scopes and why would it behave differently because of that?

Again, it's not focal length that is the issue - it is sampling rate (which in part depends on focal length - hence origin of the myth, but you can control it via other means - like using focal reducers, binning, using different camera with different pixel size, etc ...).

I use OAG and never understood all the "it's difficult to use" hype... Never had issues with not being able to find guide star and with guiding itself.

[michael8554]

8 hours ago, Chris49 said:

You are right, they are not.  So I was just lucky they came out roughly round!

Back to improving the guiding!

If your stars are smaller and round in the subs, then your guiding is "adequate", so isn't it your processing that is to blame for stars that are too big?

Michael

[cfpendock]

Keep going and eventually you will get there!  I frequently image at 2800mm focal length and although it is true that SCTs do produce rather larger "wooly" stars, this can be overcome with careful setting up and processing.  I have found the following helps:

Use of an off axis guider - almost essential for long FL work, and you should be able to guide at around 0.25 secs.  But in order to achieve this, attention to balance is critical - I usually balance slightly off so the mount is always pushing slightly - I find this helps with overcoming backlash.

With my C11, any significant wind ruins everything.

As has been pointed out, round stars are no indication of good guiding.

When focussing the SCT, I always make sure that I am focussing so that the for the final adjustment the mirror is being pushed upwards ( I do use an auto-focus system), but I find focussing at F10 anyway is not too difficult compared to short focal lengths. 

To keep the stars from bloating, I always re-layer a less stretched version into the final image as Olly suggests above.

It seems to me that your subs are quite short.  Do you have a lot of light pollution?  I normally have average rural light pollution, but I do use subs of around 5 minutes.

For your information, my sampling rate is just over 1"/px using my Atik 4000 and binning 2x2. I stack in DSS and process in Photoshop. My version taken at 2800mm FL is in my album: https://stargazerslounge.com/gallery/image/33396-ngc-891png/.

Hope this helps.

Chris.   (No, seriously!)

 

[Chris49]

9 hours ago, carastro said:

Chris, I have just given your image a quick run through the star reduction technique demonstrated in my video above.

This is the result.

I did watch your video last night and it looks very useful.  I'm such a novice with processing that I will need to follow it through step-by-step with an example of my own to understand it.

I have CS6 - yours may be a different version but I guess that won't matter.  Your star reduction certainly shows an improvement on my picture.

Thanks

Chris