Different exposure times vs same exposure times

[ollypenrice]

That's lovely with an excellent colour balance. In the linear image, what does the Trapezium look like? If it is lass saturated than this then you can blend it in using the Lodigruss tutorial in an earlier link.

Olly

[RichM63]

Given that the read noise of my DSLR is 5 electrons

How is this measured, please?

[ollypenrice]

I read somewhere an article, cannot remember who by now, that was talking about getting above read noise being the minimum exposure you want to aim for, and not necessarily the longest.  Is there a benefit under not so dark skies to go longer and use a high sub count overall.

You need to experiment in order to find out at what point you cease to be able to distinguish between faint capture and unwanted glow. As ever, I'd beware of theories based only on the numbers because a human being can often find little clues buried in the data, perhaps by looking at what's going on in the different colour channels. These clues my help you to prize out signal from LP.

Olly

[Singlin]

Great thread Vertigo!

This is something we all have questioned and to find the debate laid out in one thread is useful.

[vertigo262]

Thank you olly,

this is a closeup

it's blury though. I need to work on polar alignment. I'm using alignmaster, but I cant seem to get better then 2 or 3 minutes. Need to figure out what I am doing wrong.

hi Proto star, what do you mean by measured?

I am out right now taking about 100 subs and doing it right with calibration frames. I am taking about 100 subs, but I'm trying to figure out why the positioning is slowly moving out of frame after a couple of hours. is that from bad polar alignment?  or is this something I am doing wrong with guiding?

also, I did a lot of the stretching I think in DSS. I noticed a lot of people take the tif right after it's stacked and do it in photoshop. I don't know if it's better to do it in the fit file, then output to photoshop. 

When I stacked them I used someones settings in DSS from a youtube I found, but after stacked, just modified the curves in DSS.

I don't know what is better way of doing it but those seemed to be the  colors in DSS after trying to dial it in in DSS and cranking the saturation in DSS

seems like it would be smarter to do all that in the fit file, but not sure

[vertigo262]

any Idea what is doing this, I still have a lot of frame to go, but something isn't right.

also in stellarium, the telescope icon moved way off out of orion, but it's still pointed at orion on the mount

[D4N]

You mention guiding, if you are guiding there should be no drift at all.

Unless either guiding fails or your dithering is only set one way.

If your not guiding then just re-centre the image every now and then, the slow drift will induce natural dither.

Sent from my iPad using Tapatalk

[vertigo262]

I'm using a star sense. It seems to be tracking, but it seems off over time

[ollypenrice]

Sounds like polar alignment.

Olly

[sharkmelley]

How is this measured, please?

Measuring gain and read noise of a DSLR is not that straightforward.  It requires anaysis of various length exposures to plot a curve to calculate gain and then to subsequently calculate read noise. Christian Buil has documented a procedure at the bottom of this page: http://www.astrosurf.org/buil/20d/20dvs10d.htm

It is important to perform this only on the data from a single colour channel (i.e. R or B or G).

On the other hand you might be lucky and find the read noise for your camera here:  http://www.sensorgen.info/  but unfortunately it doesn't give the ISO that corresponds to the minimum read noise (since read noise tends to vary with ISO).

Mark

[vertigo262]

so here is from tonight. My first image with calibration frames.

As far as the PA or floating image, I need to figure out why no matter what i do, I can't get PA better then 2-3 minutes. unless it' a guiding issue, but Olly is probably right

This is what I did tonight.

30 x  30 seconds ISO 800

100 x  60 seconds ISO 800

10 x 120 seconds ISO 800

25 darks

50 Bias

50 Flats

I then threw everything in DSS, which I don't know if your supposed to put multiple exposures together

The center was blown out so I took the 30 second ones, made another one with that and blended the center like the youtube.

I've never used calibration frames, and it's like day and night. So much cleaner of an image.

I then adjusted most of the stretch in DSS and brought it in photoshop. so I've learned a lot. Just need to figure out why I can't get good PA with alignmaster

so here it is

[ollypenrice]

The mid to faint parts of that image are very good indeed. However, throwing multiple sub lengths into DSS is not the way.

This is an excellent tutorial on one of the most satisfying processing techniques in the book. http://www.astropix.com/HTML/J_DIGIT/LAYMASK.HTM  I remember, as a beginner, feeling on top of the world when I made this work!

I can't comment on alignmaster. I just use the classical drift method which is software free so the software can't get you...

Olly

[vertigo262]

it is kind of a rush, finally figuring things out, and things starting to come together. Except then you point your scope to something more difficult, like the veil nebula, and end up back in frustration.

As I am learning, You really can't do much without precise Polar Alignment. I am stuck at short exposures which Orion is fairly easy. But good to learn with.  Thats why I think my next challange will be orion with long exposures,  This will be a hurtle for me. But people swear by alignmaster saying it's quicker an more accurate then drift. So either I am doing something wrong, or they are wrong. So means back to research mode

[wxsatuser]

I tried Alignmaster, it got it there or there abouts but not spot on.

PHD drift align was the key for me, an immediate effect of any adjustment is easily seen.

I can easily do 20minutes but the sky never allows that here. 

[ollypenrice]

Drift is an absolute observation which cannot be wrong. It involves no calculations or models.

Olly

[vertigo262]

True, at least I can observe the alignment as it gets better. 

I keep redoing the alignmaster and I can't get good alignment, yet people swear by it. It's just drift alignment takes a lot of time to get right. Since I have to go out and set up each time, it's nice to be able to do it fast.

I don't use PHD because I have a starsense.

Although I should probably learn drift to get good at it just because. But it's nice to be able to set up fast.

How long does it usually take you to drift align?

[ianaiken]

I never had any luck with alignmaster either but within 20 mins of using phd2 I can be pretty well drift aligned.

[vertigo262]

Out practicing drift alignment although still couldn't get much better then 2 minutes.

although I am wondering if all these artifacts are normal for a canon 7D, or do I have Rubbish on my cmos sensor or something because this looks really bad. These blotchy things

[ollypenrice]

I would say the source for those is in front of the sensor. They are out of focus.

Olly

[tingting44]

Out practicing drift alignment although still couldn't get much better then 2 minutes.

although I am wondering if all these artifacts are normal for a canon 7D, or do I have Rubbish on my cmos sensor or something because this looks really bad. These blotchy things

they are not normal for any dslr let alone a 7d.... its sensor is dirty, flats will cure this, or you can get a sensor clean at any local camera shop, i take it any filters you have are clean if you use them... 

[ollypenrice]

Objects actually on the sensor give solid black marks of their own shape. For them to create a burred shadow like this they have to be separated from the sensor. There's a calculator somewhere that allows you to work out this distance, though I've never used it. I'd be really surprized if these were right on it.

Olly

[IanL]

They will be on the top of the stack of filters mounted in front of the sensor, about 1 or 2 mm in front of the actual sensor surface. A few dust shadows is pretty normal for any camera and can be eliminated using flats. Yours has definitely reached the point where cleaning is needed.

Try using the auto cleaning routine which will be in the on-camera menu. Give it a few tries and then take a flat frame to check (AV exposure of an evenly illuminated surface, preferably out of focus). If that doesn't do it you may need to clean with an airblower or a lens cloth. Read up before doing so as easy to do more harm than good.goods

[Rico]

I have a 7d and I saw one of these blurred artefacts on one of my images. I scrolled through the menu and did an immediate clean. Took a few activations for it to be 'shaken' off. Do you have auto cleaning switched off in your menu settings, because if you do, than that's probably why it got to that stage.

[IanL]

Probably worth bearing in mind that if you plan to use flats it is best to keep auto cleaning off, otherwise the cleaning routine will run each time the camera power cycles. If this happens the dust will move or disappear between taking lights and flats. You will then end up with dark spots and light spots as the flat tries to correct for dust that isn't where it was originally. I made this mistake and ruined an otherwise lovely M45 image. I keep cleaning off and run it manually two or three times at the start of each session.

[Rico]

Probably worth bearing in mind that if you plan to use flats it is best to keep auto cleaning off, otherwise the cleaning routine will run each time the camera power cycles. If this happens the dust will move or disappear between taking lights and flats. You will then end up with dark spots and light spots as the flat tries to correct for dust that isn't where it was originally. I made this mistake and ruined an otherwise lovely M45 image. I keep cleaning off and run it manually two or three times at the start of each session.

Good point Ian.