StuartT

thanks. But just to be clear, you are suggesting I will need to stack for an image like that? How would I make several light frames given that this is a 16m exposure (so the stars would be in very different positions for each one)?

Or do you mean just take a bunch of darks and stack them with this one light? 

Just now, Cornelius Varley said:

The "red dots" are mostly likely to be hot pixels on the sensor. Dark frame subtraction should deal with them.

thanks. Actually, they are barely there on the JPG version I attached here, for some reason. They are really prominent on the RAW file.

Last night I took this image (16min, ISO 100, f/11) with a 17mm lens on a Canon EOS750D.

Can anyone tell me what the red dots are? They are clearly not stars.

38 minutes ago, skyhog said:

And one more thing. I couldn't get the meade ascom driver to work under win10 with my lx200gps. In the end I downloaded a generic driver after getting a tip from this forum... It's the meade generic driver written by Colin Dawson. It worked a treat. Might be worth trying that. 

YES!! That was it! I installed the Dawson driver and it now works 😃

Thank you!

Ok, so I installed ASCOM platform 6.5 SP1 and the Meade Autostar driver (#494, #495, #497) and have set up my scope in Stellarium using the correct COM port for my USB.

Stellarium is telling me the scope is connected (see screenshots), but it won't slew anywhere. Any suggestions?

10 hours ago, AstroExploring said:

What setting did you change out of interest? Glad you got it working 

I was using JPG images, so I re-shot some RAW images last night and it worked a lot better

Maybe JPGs are too noisy?

Ok thanks everyone. I seem to have it working ok now. I got an aligned image at least 😊 This is Regulus

I think my biggest problem is that my tracking won't allow me exposures longer than 10 sec, so I am prob not catching enough photons

10 hours ago, michael8554 said:

Sorry to say Stuart, DSS will reject stars that aren't fairly round, stacking won't make them round

 If you look at the stretched sub, there are only about 40 stars, so "300 stars" will probably comprise hot pixels, not stars.

If you open DSS there's their own comprehensive user guide (bottom of the side window), did you read that first ?

Michael

ok, so I have the threshold for star detection set too low. Thanks.

I didn't read the user guide 😬 but I shall now!

So I tried again with a higher detection threshold (in case it was just finding hot pixels). But still it can't seem to align them.

7 minutes ago, The Lazy Astronomer said:

What does a stretched single sub look like? Can you visually see a very large number of stars? Maybe DSS is mistaking hot pixels or some other kind of noise for stars and that's messing with the registration and alignment.

When you say 'stretched' do you mean using the levels control in Photoshop to move the midtones point? If so, here's how one of them looks

2 hours ago, AstroExploring said:

What setting do you have selected in the Results and Alignment tabs in the Stacking Parameters window? I made a video tutorial for DSS about 12 months ago which you hopefully find useful https://www.astroexploring.com/deepskystacker you might be able to skip to about the 7 minute mark to confirm your settings for stacking. 
 

HTH

Results tab : Standard and Alignment tab : automatic

Thanks. I'll check out your tutorial 🙂

I am dipping my toe into the water of image stacking. Last night I took a relatively small number of images (20) of a star field with a bright star in the centre. I have not taken any flats or whatever, I just wanted to try DSS in a very basic way to see if I could align the images and stack them.

The 20 images were shot at the prime focus of an SCT with a Canon EOS750D (10sec exposure at ISO800). An example is the first image below. The tracking is slightly off, so the stars drift slowly in position through the set. But I am assuming the registration process takes care of that. 

Problem is, the first go at stacking makes them into trails (so doesn't seem to be registering them properly (second pic below). I set the threshold quite low and it was detecting plenty of stars (about 300)

Can anyone help me with the basic settings I should be using? I was basically following this article by way of a tutorial.

6 hours ago, davew said:

I first became aware that all is not well in ISO land in the summer of 2017. I read this article from a guy named Ian Norman -

https://www.lonelyspeck.com/how-to-find-the-best-iso-for-astrophotography-dynamic-range-and-noise/  

I tried a few experiments of my own and it turned out he was telling the truth ! It must be stressed that this ISO result depends to some extent on the camera and if it's ISO variant or invariant. It also only applies where the read noise is the dominant source of noise. In other words, if light pollution is the dominant noise then you won't see the read noise but you will at a dark site. If you need persuading that the read noise reduces when you raise the ISO then have a look here - 

https://www.photonstophotos.net/Charts/RN_e.htm

That's the good news 😎 The bad news is that dynamic range also decreases - 

https://www.photonstophotos.net/Charts/PDR.htm

I've also tried that experiment and in real life Dynamic range reduction isn't as bad as some might suggest especially when stacking. 

So, should we raise the ISO to the max ( In some cameras to almost half a million ISO ) and have no noise ? No ! There comes a point where there is no Dynamic range worth talking about. I would suggest in most ISO variant cameras that you push up to 1600 or 3200 and stop there.

Alyn Wallace gives a good explanation on the latest cameras with invariance and the older cameras with variance -

https://www.youtube.com/watch?v=d8QV00mkJW4

The video gets interesting around the 4.30 mark and shows what happens in most cameras.

So why can so many people prove beyond all reasonable doubt that raising the ISO totally wipes out the image with noise ? Simple. If you follow the exposure triangle you'll know that for one stop more of ISO you need one stop less of exposure. So if I take an image at 20 seconds at ISO 100 that equates to 10 seconds at ISO 200, 5 seconds at 400, 2.5 seconds at 800 etc. No one EVER mentions the fact that the light is being reduced in half every time the exposure is reduced ! So 2.5 seconds at 800 is 12.5% the amount of light that was gathered at 20 seconds ISO 100. Who would have thought the image would have been noisier ?

It turns out that some cameras have twin gain circuits where a second stage kicks in. In my camera, the z6, and the Canon R5, this occurs around ISO 800 so at that setting the read noise drops significantly.

Right, lunch calls and my fingers are falling off 😀

Dave.

this sounds like exactly what he was saying. Thanks!

9 hours ago, DAVE AMENDALL said:

Hi , Certainly with film the higher the ISO the more grain in the image therefore you are right. Also true of digital. Sounds like he made an Oops! in his video presentation. .........Dave

I don't think he did. He was explaining that you have to balance ISO and noise. Dynamic range decreases with ISO, but noise increases. So you want the sweet spot between the two. He pointed to a rather nice site called photons to photos which allows you to see this noise vs ISO profile for any camera. Here is mine (screenshot)

8 hours ago, shropshire lad said:

Can you put a link to the video please.

 

I was watching a nice youtube video about imaging M31 and stacking with a DSLR. The guy said something which surprised me. He said that noise decreases with increasing ISO. This is the opposite to what I always understood - that the higher the ISO, the more grainy your image.

Can anyone explain this for me?

1 minute ago, Nigella Bryant said:

I lowered the contrast, adjusted the clarity and saturation. 

Smashing. Thanks. I still need to figure out the right exposure I reckon. 

But focusing is def still my biggest issue.

16 minutes ago, Stuart1971 said:

And why was it so funny, you mentioned just the one filter, which is extremely dangerous...?

apologies. I thought you were joking. yes indeed I had solar film on the front of the scope. I'd be surprised if you could get any kind of image without it (it would completely destroy the sensor)

2 minutes ago, Nigella Bryant said:

Yep you definitely have it. I just tweaked your image a bit. Hope you don't mind.

Ooh! thank you! What did you do?

1 hour ago, David Smith said:

Having used an 1100D to image the Sun, focusing is very tricky. The most reliable method I found was to connect the DSLR to my laptop so I could focus using the live image on the laptop screen. If there was a decent sized spot / group (spots that are visible at the moment are relatively tiny compared to the size of the disk as John mentioned above) then I was able to use live view on the LCD. John mentions an ND filter as the image you posted is possibly a bit over-exposed, so get the exposure right down, my raw files looked very, very dark straight off-camera. You don't mention exposure time but it was rare for me to use anything slower than 1/2000 second with my 120mm frac.

Only problem is I have a 750D and it can't be connected wirelessly to a laptop with EOS Utility for some reason. And I really don't want to have to buy the world's longest USB cable! (If you know a solution, please shout!). Also, I don't have an electric focuser on the scope, so I don't think I could do that.

That image was 1/60s at ISO 200

aha! I think I got it!

1 minute ago, David Smith said:

They look a little out of focus. What camera did you use?

 

yes, it's really hard to focus. I am using an EOS 750D. I magnified the live view mode to try and get the edge as sharp as I could, but it still looks off to me

I can't seem to see any of the sunspots supposed to be there at the moment. 😕

These were taken at prime focus of my 20cm SCT with a Baader white light filter on the front and a Baader continuum filter at the back. Why are they so featureless?

Thanks for this explanation vlaiv! I really appreciate it. So far I have been mostly taking images of the moon (where obviously this is not a problem). But I decided I'd see what happened if I pointed the scope at some stars instead.

I am considering buying an EQ mount so I can then consider guiding (if I am clever enough to figure out how to do it!)

This evening I took a couple of star field exposures at the prime focus of my 20cm SCT. It's only on an alt-az mount with tracking, but I was surprised I was seeing trails at fairly short exposures (30s and 60s). Does this mean my tracking is rubbish?

and tonight

slightly happier with this one. I used my 20cm SCT at prime focus (100 ISO, 1/50 sec)