Not enough stars - persuading DSS to stack anyway?

[AstroFrog]

Hi All.

A quick DSS question for you imagers out there - is there a way to get DSS to stack subs where there are not that many stars in the frame?  I've been attempting to get some frames close in to Alnitak, in a bid to get something of the Flame Nebula, but DSS keeps telling me there's only 1 frame suitable for stacking as there aren't enough stars.  I've tried using the slider to maximise the chances of it finding enough stars, but it's still not having any of it.

Any tips to get around this issue, and persuade DSS to stack the frames I have anyway, or is it back to the drawing board in order to get longer subs in order to find a few more stars?!

Thanks for any light you can shed on this for me!

Fred

[Owmuchonomy]

When you use the slider cut off how many stars is it saying are in the frame? Have you moved the slider right over to the left?

[AstroFrog]

Hi Chris,

I can't remember exactly, but it is very low - the first time I tried it said there were only 5!     I went through my frames again to see if I could pick some better ones, which got it up to 15+, and yes, that's with the slider right over to the left (2% I think it says).

I know my data isn't great, I was just hoping there might be an option that said "eh, give it a go DSS, give it your best shot and let's see what you can do!"

[happylimpet]

Ive been in this position and unless you can do soething to your light frames elsewhere so that DSS picks up more stars, you're stuffed.  I found thatr by dark subbing and flat fielding in IRIS, DSS then found more stars.  Im also considering doctoring the .info files to add some 'fake'stars interpolated between real ones......

Oh, dont forget to try the 'median filter' option.

[Owmuchonomy]

Hi Chris,

I can't remember exactly, but it is very low - the first time I tried it said there were only 5!     I went through my frames again to see if I could pick some better ones, which got it up to 15+, and yes, that's with the slider right over to the left (2% I think it says).

I know my data isn't great, I was just hoping there might be an option that said "eh, give it a go DSS, give it your best shot and let's see what you can do!"

Sorry Fred but I don't have an answer.  You may get more responses if you post one of your light subs for folk to see.  Good luck.  Also if you search for Doug German on Youtube he does a good DSS tutorial.

[Zakalwe]

I know my data isn't great,

IMHO there's not much to be gained by stacking poor quality data. The time spent messing about with it would be better spent on getting the basics correct. Once you have these nailed, everything that follows is so much easier. Bad quality data makes everything harder- stacking, processing, noise control.

Find the target.

Track it accurately.

Make sure that the focus is pin sharp.

Delete any out-of-focus subs or subs with trailing stars.

It's painful deleting hard-won data, but not as painful as trying to make a silk purse from a sow's ear.

[Christopher Davenport]

I have used image analyser to increase brightness and contrast on the raw files, this makes DSS find stars better if needed.

Edit:

Agree with above though, best leave bad subs out as they reduce the overall quality of the image.

[AstroFrog]

Hmmm, yeah, ok thanks all, I think you're probably right.  I think it's as I suspected - better data required.

I think actually I need a change of approach.  My imaging has been done with a Skywatcher 127 mak on an alt-az mount, which is obviously far from ideal.  I've been pushing for objects that are less bright, (e.g. compared to the glob clusters, M42 etc) to see how far you can go with this scope given limitations of sky and kit...but I think I may have found the limit!  Perhaps I'll focus on getting a load of good data on a couple of the brighter objects for now, until the finances allow me to upgrade the kit a wee bit!

Thanks for the DSS tutorial pointer as well Chris.  Having found and tried to follow a couple that really weren't helpful, I've been looking for a good one, so I'll check that one out.

HappyLimpet, thanks, I may give that a try in IRIS before I give up completely...

Thanks to all for taking the time to come back to me - it is much appreciated! 

[ollypenrice]

Focus might be part of the issue. It really needs to be bang on. Small stars disappear quickly when you're out of focus even slightly.

Do you have a Bahtinov mask?

Olly

[D4N]

Assuming your images are ok but just a bit dark for DSS to find the stars (can happen easily at high F ratio or with short exposure) then you could try increasing the brightness of the input files. In the RAW/FITS options bottom left there is a brightness scaling option. If your camera is shooting 14 bits you can stick a 4 in there, if it's shooting 12 bits you can use up to 16. The idea is that the camera didn't use the last 2 or 4 bits anyway so it doesn't hurt to move the data over to them to make it brighter.

I would rather do this than individually fudge every file as at least all files are treated the same this way.

You will however have to recreate your master calibration frames if you change this setting.

Obviously if your camera outputs true 16 bit images then ignore this

Sent from my iPad using Tapatalk

[dph1nm]

You need at least 8 stars (in common) to stack in DSS. One trick I have tried with ropey data is to use the DSS superpixel method of de-bayering the data. This reduces the resolution of the image (as it makes one pixel out of each RGGB 2x2 block) but I have found it helps to detect bloated or trailed stars.

NigelM

[AstroFrog]

Olly, you might be right - I'm fairly sure the focus was good to start off with, but looking through the data, I'm wondering if the later data might be less sharp as the temperature dropped.  I'm not using a mask at the moment, but I've actually got one on order after getting fed up of the 'hit and miss' nature of trying to do it by eye.  I think I'll also need to get in the habit of checking it throughout a session too, rather than assuming it's going to stay the same.

D4N, yes, that's the root of this problem I think, high f ratio + short exposure + someone who doesn't know what they're doing!  I'll have a look at the brightness scaling option - it's a canon 1100d so I think that it's 14 bit?  Might just give me a sporting chance with the data I've got!

Nigel, thanks for that, I wasn't sure at what point DSS stopped stacking.  I thought I had at least 8 stars, but like you say, they have to be common across the frames.  I'll have a look and see if I can figure out the debayering you mention.

Again, many thanks for your help!  That you would take your time to help someone who doesn't have a great deal of understanding or the best of kit for the job is very much appreciated, and is what makes this forum so special.

[Christopher Davenport]

Apologies I did not realise you where on a mak with the alt az mount.

This is the same place I started out in astronomy. See my story in my sig below.

I found my little mak came alive after adding a 0.63 focal reducer. Celestron make one for about £100.

If you can try to upgrade to a equatorial mount it will increase the sub length substantially.

If you going to spend money, first get the book making every photon count. Worth every penny.

Until then, you can chase bright objects in the north near to polaris, then you can get longer sub's before they trail.

Also if u use iso3200 and cappa stacking it will help.

You could also try your hand at planetary with a Barlow and a webcam.

James on this forum has squeezed a lot out of his mak, worth doing a bit of research.

[Christopher Davenport]

Oh also search the forum their is a good thread on the 127 mak

[Alien 13]

I think the problems are as already mentioned focus that is a bit off and a subs that are too dark balancing sub length with ISO settings to get the histogram in the 30% range is a good start although i can still get good results even when its close to 80%.

Olly did touch upon a very useful observation that small dim stars in the FOV dissapear quickly with incorrect focus i have used this to develop my focusing technique when using a DSLR, the liveview screen is used to look at the stars within the target area and then zoomed into one of the fainter ones now the ISO is adjusted till its only just visible at this point the focusser will act like a switch from off to on to off with the smallest of movement it might take a few goes with adjustments of ISO and focus but the end result is pretty much perfect focus with no guesswork involved.

It is also a good idea to make sure that your target star for focussing is around 1/3 the distance from the  center.

Alan