9 replies to this topic

[ordnance11]

I'm planning to do my first DSO imaging session this week. Target is the Hercules cluster. My set up is the following:

 

Celestron 8 Edge HD

AVX mount

0.7X Celestron reducer
ZWO ASI 585 OSC cooled camera
Optolong L-ultimate filter
Celestron SSAG
ZWO EAF

Sharpcap 4.3 for image capturing

 

I'm looking to see if anyone can look over my notes if I missed anything.

 

Dark sky imaging notes

Initial focus visual with camera and telescope against distant target

Check target is not over exposed using image histogram tool

Select appropriate calculate focus score tool

Reset the graph to wipe the score history

 

Focusing- use multistage FWHM for globular clusters

Set camera gain to 100. Max of 198
Set exposure to 60 seconds initial
Go for 4 minutes?
 

30 sec? 

 

Imaging plan- how long to take exposures

 

Set for raw 12-16 or mono 12-16

 

30 x 4 min or 30 x 240 sec -2 hour exposure

60 x 1 min?- 2 hour exposure?

Take test image
No filters for this stage
Set exposure for 2 sec to speed up the process.

 

Check first sub exposure

 

Take dark frames- place cover on telescope
Set same exposure

Bias frames- same condition but shortest exposure

Flat frame- keep telescope in the same position in the morning. Stretch white cloth

Take 15 dark frames

 

Light frame capture?

Set for raw 12-16 or Mono 12-16
Set to low gain 100 for ASI 585
Adjust exposure for histogram to reach 30-80 percent

Bias frames- cap on
Set for shortest exposure time

Flat frame- same exposure frame

Place white T- shirt over telescope lens.
Point towards morning sky.
Take 15 frames.

I haven't done any of the calibration frames so I need to do those as well. Someone has suggested I use 1 minute light subs to prevent burning out the camera cable. Would it be enough? Or do I go down  to 30 seconds even?

 

Thoughts for improvement would be appreciated.

Regards

Edited by ordnance11, 18 June 2025 - 11:09 AM.

[EPinNC]

I would lean toward the shorter exposures (30s, or 60s maximum).  You don't want to saturate the core of the cluster.  There is also a good deal of variation in star colors across the whole cluster, so you don't want all your stars to be white.

 

It's pretty bright, and your big scope is going to capture a lot of photons quickly.  Short exposures can also help when using a long focal length because you would minimize tracking/guiding errors, keeping all your stars nice and crisp.  (What mount do you have?  Autoguiding or not?)

 

It's a fairly easy target to get something nice, because it's pretty bright, but that also means it's pretty easy to overexpose the core.  When you're processing, you should be able to retain individual stars all the way across the core if you haven't blown it out.  People frequently mention the "propeller" (a set of 3 dark areas resembling a 3-bladed propeller) as an indicator of image quality for M13, but I think there's more to it than that in making a really nice image.

 

In my opinion, the hardest part of imaging globular clusters is in the processing -- to bring out the faint, and truly vast, halo of faint stars that extend outward from the core.  Here's my version with 3 hours of integration.  It turned out well, but I've processed and re-processed this image many times to get it looking like that.  I've seen better.

[rj144]

Sorry... a bit confused.  You have a mono camera?  What does "Set for raw 12-16 or mono 12-16" mean?

 

Don't look at the histogram.  Just try not to overexpose many pixels for sub lengths.  I've never heard of burning out a cable... don't worry about that.  Are you talking 1 minute subs for the calibration?

 

15 frames should be ok, but try to take 20.  

 

Also, bias frames you can do any time you want.  And, if your camera is cooled, you can make a dark library whenever you want as well.

Edited by rj144, 18 June 2025 - 08:50 AM.

[jml79]

A few notes. Set to RAW16. You are using a 585 on an unreduced C8, no mention of the mount. Also you are using SharpCap. Set the gain to 252 to engage HCG and reduce the noise by a lot and shoot very short subs, think 8-15s. This will reduce the amount of errors from guiding and the mount tracking and take advantage of the way the ASI585 is setup. I know this is not traditional for AP but it will get you lots of usable subs. It's up to you if you want to keep all of the subs and stack later or not. Also, take your calibration frames before imaging. First you are way more awake and aware rather than the middle of the night but also SharpCap can use the darks and flats during live stacking. The nice thing about SharpCap is the immediate feedback. You can try 8s for a few minutes then reset and try 15s etc. You can see if the scope is not focused etc. Use this to your advantage and only run a long stack if it's looking good. You will be very oversampled. You will image much faster if you Bin2 the sensor. Even in Bin2 you will be a little oversampled. It will reduce the image from 8MP to 2MP but it will take much less time to get detail.

 

Learning with an SCT is much more challenging than using a wide field scope. Consider a 50-80mm refractor in the future. You are accidentally making a good choice by using SharpCap IMHO. It's more EAA software than AP software but with an SCT learning EAA first is not a bad idea. The instant feedback may help you identify and solve issue much quicker than a traditional AP approach.

 

This is your first time out with a challenging scope. Don't sweat the details yet. Shoot your cals but if all you get is on target and something of a focused image on the screen, that's a win. The 585 is pretty good without calibration etc and there is nothing wrong with just live stacking your first try. You can take longer subs and push the mount and guiding another night after you can reliably get on target and an image on the screen. You might consider posting in the EAA section for help at this stage. They are much more familiar with SharpCap and short sub imaging with SCT's and possibly your mount. Once you get these skills down then you will know better what challenges you will face transitioning to long exposure AP. In the AP forum you will get AP answers and they are not always the best for your rig.

[Tapio]

Adding what others wrote, you don't need Optolong L-ultimate filter imaging globular cluster. It's for emission nebula.

[jml79]

Adding what others wrote, you don't need Optolong L-ultimate filter imaging globular cluster. It's for emission nebula.

Missed that. You do need a UV/IR filter though. For now a cheap 1.25" SVBony from Amazon will work just fine and should screw on to the front of the camera.

[bobzeq25]

Some basic things you're missing.

You're going down a very hard road. That scope is long focal length, heavy, and slow. For starting out you want short, light, and fast. This happens often because people like looking through the scope, and think it's a good choice for imaging. For starting out it's not. This is what happens.

"I put together a video of my imaging rig along with some info on how I went from years of failure trying to image with a long focal length SCT, to achieving success on my first image."

https://www.youtube....h?v=MNQU1hdqz4M

"God bless each and every one of you that uses a SCT but if not for that little refractor, I would have put the whole kit (6SE) in the closet and walked away last year."

"After months of learning and overcoming challenges <with the SCT>, and finally buying a shorter FL APO refractor, I really really really wish I had listened to everyone on here and started learning the <traditional> imaging basics on THAT frac instead of on the SCT. Trust me"

"really really really" <smile>

You didn't state what mount you're using. That's more important than the scope or the camera.

"The recurring theme and BEST PIECE OF ADVICE BY FAR I received over and over again was a) the mount is king and everything else comes second b) start with a widefield refractor and build your skills and go from there. There is so much involved and building good practices and habits and foundational building from the start will save you a lot of wasted time and heartache."

Nor did you state what processing program, which is VERY important, processing is half the game. Just one reason of many. As stated above filters are good on emission nebulae. ONLY. The reason is that the light from emission nebulae comes in narrow peaks, ideally suited for a filter. Other targets emit across the entire spectrum. A filter costs you too much signal. What you use instead is gradient reduction in processing. You need a procssing program with a good gradient reduction tool. I recommend Astro Pixel Processor, many like Siril.

Exposure is MUCH less of a concern than the above, provided you really don't mess up. What counts is total imaging time, how many total photons you get. How you slice it into subs is a tweak. BUT. The histogram of your RAW data should be only a tiny amount separated from the left. That percentage is for stretched data on the back of a DSLR.

Example below, click on it to enlarge. Note the histogram. Note that the frame looks VERY dark. That's correct. If the unstretched light shows you much, you've messed up. You want to process unstretched data at te beginning, certain operations work best on unstretched data. The sequence is.

Stack the bias for a master bias. Calibrate the flats with bias. Stack for a master flat. Stack darks for a master dark. Calibrate the lights with the 3 masters. Stack. Do the operations best done on unstretched data. Stretch. Do the operations best done on stretched data.

Astro Pixel Processor walks you through the process.



This is a much more complicated and harder thing than many think. This book will help a lot.

https://www.amazon.c...d/dp/0999470949

Edited by bobzeq25, 18 June 2025 - 10:49 AM.

[ordnance11]

Sorry... a bit confused.  You have a mono camera?  What does "Set for raw 12-16 or mono 12-16" mean?

 

Don't look at the histogram.  Just try not to overexpose many pixels for sub lengths.  I've never heard of burning out a cable... don't worry about that.  Are you talking 1 minute subs for the calibration?

 

15 frames should be ok, but try to take 20.  

 

Also, bias frames you can do any time you want.  And, if your camera is cooled, you can make a dark library whenever you want as well.

 I have a 585 color. I amended my answers.  I have an AVX equitorial mount. 

 

Adding what others wrote, you don't need Optolong L-ultimate filter imaging globular cluster. It's for emission nebula.

 

I was wondering about that. Thank you.

 

Missed that. You do need a UV/IR filter though. For now a cheap 1.25" SVBony from Amazon will work just fine and should screw on to the front of the camera.

I have a UV cut filter. thank you

 

Nor did you state what processing program, which is VERY important, processing is half the game. Just one reason of many. As stated above filters are good on emission nebulae. ONLY. The reason is that the light from emission nebulae comes in narrow peaks, ideally suited for a filter. Other targets emit across the entire spectrum. A filter costs you too much signal. What you use instead is gradient reduction in processing. You need a procssing program with a good gradient reduction tool. I recommend Astro Pixel Processor, many like Siril.

 

I plan to use pixingsight for this.

Edited by ordnance11, 18 June 2025 - 12:46 PM.

[ordnance11]

Some basic things you're missing.

You're going down a very hard road. That scope is long focal length, heavy, and slow. For starting out you want short, light, and fast. This happens often because people like looking through the scope, and think it's a good choice for imaging. For starting out it's not. This is what happens.

"I put together a video of my imaging rig along with some info on how I went from years of failure trying to image with a long focal length SCT, to achieving success on my first image."

https://www.youtube....h?v=MNQU1hdqz4M

"God bless each and every one of you that uses a SCT but if not for that little refractor, I would have put the whole kit (6SE) in the closet and walked away last year."

"After months of learning and overcoming challenges <with the SCT>, and finally buying a shorter FL APO refractor, I really really really wish I had listened to everyone on here and started learning the <traditional> imaging basics on THAT frac instead of on the SCT. Trust me"

"really really really" <smile>

You didn't state what mount you're using. That's more important than the scope or the camera.

"The recurring theme and BEST PIECE OF ADVICE BY FAR I received over and over again was a) the mount is king and everything else comes second b) start with a widefield refractor and build your skills and go from there. There is so much involved and building good practices and habits and foundational building from the start will save you a lot of wasted time and heartache."

Nor did you state what processing program, which is VERY important, processing is half the game. Just one reason of many. As stated above filters are good on emission nebulae. ONLY. The reason is that the light from emission nebulae comes in narrow peaks, ideally suited for a filter. Other targets emit across the entire spectrum. A filter costs you too much signal. What you use instead is gradient reduction in processing. You need a procssing program with a good gradient reduction tool. I recommend Astro Pixel Processor, many like Siril.

Exposure is MUCH less of a concern than the above, provided you really don't mess up. What counts is total imaging time, how many total photons you get. How you slice it into subs is a tweak. BUT. The histogram of your RAW data should be only a tiny amount separated from the left. That percentage is for stretched data on the back of a DSLR.

Example below, click on it to enlarge. Note the histogram. Note that the frame looks VERY dark. That's correct. If the unstretched light shows you much, you've messed up. You want to process unstretched data at te beginning, certain operations work best on unstretched data. The sequence is.

Stack the bias for a master bias. Calibrate the flats with bias. Stack for a master flat. Stack darks for a master dark. Calibrate the lights with the 3 masters. Stack. Do the operations best done on unstretched data. Stretch. Do the operations best done on stretched data.

Astro Pixel Processor walks you through the process.

an unstretched light.jpg

This is a much more complicated and harder thing than many think. This book will help a lot.

https://www.amazon.c...d/dp/0999470949

Thank you very much for the advice. Especially on the calibration subs. I was planning on using PixingSight. for pre processing.

Edited by ordnance11, 18 June 2025 - 11:44 AM.

[bobzeq25]

Thank you very much for the advice. Especially on the calibration subs. I was planning on using PixingSight. for pre processing.

PixInsight is a fine processing program. A bit complicated, but there are now some good scripts to simplify it.

The thing to know about PixInsight is that it doesn't magically process better. It has a vast array of highly adjustable tools that can do wonders if you know whch to use when and how.

This book walks you through the process of learning PI, step by step. It does not cover the latest scripts, though.

https://www.amazon.c...r/dp/3319976885