Hey all,
I’ve been playing around with autoguiding and I have an image I would like to share with the group.

NGC 7635 (The Bubble Nebula) this interesting looking object resides in the constellation Cassiopeia, a mind boggling 11,300 light-years from Earth, the bubble itself is roughly 10 light-years across. I captured this image last night under clear skies with average transparency, fair seeing and a little too much wind…

I still have A LOT to learn with long exposure imaging and processing, but it sure is fun trying to figure it out!

Image and Equipment Details

10” F/4.7 Reflector
60mm F/11 Meade Refractor (Wal-mart Special)
Atlas EQ-G
Meade DSIc
Phillips 900NC Web Cam (Un-modified)
PhD Guiding
EQMOD

10x120s & 25x120s= 70 minutes total combined
Post Processing with PSE 2.0 * PixInsight LE* Registax4

Nice shot! Looks like you're getting things running smoothly on that new EQ-G.

Thank you Dave. I'm still having some star trailing issues though. I'm not sure if it's the guiding software settings or periodic error in the mount, either way, I should be able to correct it. I just need to take the time to analyse and adjust...

That is a nice shot Larry!
Regardless star trailing it is very nicely processed

Things to consider (and hopefully rule out) about the star trailing:

1. How well was your polar alignment? It takes me almost an hour to drift align mine but its worth the time.
2. Flex. Is the guidescope really sturdy mounted?
3. PHDguiding should be doing a nice job with the default settings.
4. WIND. Windy conditions cause a lot of trailing. It doesn't matter how sturdy your mount is, any gust will move just a bit the scope and makes it wiggle back and forth until it settles again. That wiggling around causes football shaped stars.

My money is on 4. Wind = "bad imaging session"

Pablo

You are right about the wind Pablo, it was gusty at times during the session. One thing I've noticed about the mount, is that it prefers to be pointed east of the meridian when imaging (less football shaped stars) vs. west. What could be causing this? I believe the mount is well ballanced and polar aligned... I have not done any kind of pe correction yet, I guess I'll give it a go tonight.

Larry -

There could be just a little bit of backlash in the mount, and when you place the OTA west of the meridian, that would throw it off just a bit. Anyway - just something to think about. I don't think any mount has zero backlash as it comes from the factory. They all have to be tuned to some degree.

Hope you work it out.

Looks really good. Tracking is always the big problem and like Pablo, looks like a wind gust (possibly). A couple of other things you might try would be shorten exposures and see if this helps. If so adjust your mount while taking shorter exposures. If you want some work try some self guiding. Just make sure you get a really long straw to get a drink of your favorite beverage so you don't loose track
Mike H.

Thanks Mike. I'm going to start imaging from my backyard vs. driveway, I think I can get out of the wind some what.

That's right Larry. I used to save "All uncombined fits", and just switched to "Save all raw".
I cannot tell what the real difference is, except for the clipping of the histogram.

DeepSkyStacker gives a better image stacking raw than uncombined fits. I save in FITS and when open in FitsLiberator you can still move histogram sliders and get (what I think) the same image regardless if they were "uncombined fits" or raw.

So, who knows ?

Hello Pablo,
When capturing images with Envisage, do you take darks that equal the amount of subs and exposure time? In other words, if I want to image a target with 10x300s subs, should I take 10x300s darks or can I just take one 300s dark? I'm confused...

Hi Larry,

As far as my understanding goes, a nice set of darks consists of around 10-20 of them (the more the better). They should be taken with the same temperature and exposure time as your subs.

Remember the darks are nothing but the image of hot pixels that your CCD generate according to the exposure time and temperature. Some pixels might just show (or not) in one single dark but not in others. So, if you average say 10 or 20 of them then you get a better "average dark". Does it make sense?

Most stacking programs combine the set of darks to generate ONE "nice dark" that is used to cancel the hot pixels of EACH sub before stacking them.

In short, "more darks" = ONE better "nice dark".

You can see the number of darks has nothing to do with the number of subs. You are just generating a better dark to be used at EACH sub.

Needless to say the more subs the better picture, right?

Take a look at the link "How to create better images" in the DeepSkyStacker website: http://deepskystacker.free.fr/english/index.html

I'll stop typing before I get confused myself!!!
Best,
Pablo

Thanks Pablo,
I have another question or two... It looks like Envisage combines the darks into one master dark for each exposure length, right...? If that is the case and I change the save option to "Save all raw images" will it still combine the darks or will there be several raw darks? Does it make any difference?

Question #2:

What's the advantage of saving as raw? My camera is a one shot color and most of the time I want color in my images, so if I save as raw, is the color data lost or can I restore it, like in DSS?

I ask these questions because I would hate to waste a good set of subs, especially after standing in sub-freezing temps for hours! and I'm not too impressed with Envisage. I've read that most choose to use DSS, so I'm going to give it a shot as soon as I learn how to use it...

Thanks for your help!

Right. If you recall, when creating darks in Envisage it takes some 4 or 5 darks per exposure. Then it creates the master dark per exposure time. Of course this has to be done for each different temperature (within a threshold, maybe ±1 degree C).

Just some clarification about my last post. Combining darks to create a master dark is more important for reducing random noise than for averaging hot pixels. My bad. Sorry.

What I understand by "raw" among any digital capturing device is that the information of the picture is not altered or processed at all by the software used to capture it. What you save as raw contains the full information as captured by the device. Here comes the advantage: no info is lost, but I'm still not sure if when saving as fits you loose any info or simply the histogram and color balance are preset by Envisage.

So, in principle your raw pictures should contain the color information. I think it is a good idea to try during the day. Why not point your scope to some far away landmark and make some tests? Try saving both in raw and fits and compare the outcomes.
What I don't know about Envisage is if it will subtract the darks in raw mode (no way to know during day tests anyway), but if you stack somewhere else (DSS, Registax, K3CCDTools) that wouldn't be a problem.

What I like most about the other programs is the option of adding also bias and flats to correct further imperfections.

Best,
Pablo

Hi Pablo,
I just want to say how much I appreciate you helping me out…THANK YOU!

You mentioned using Flats and Bias to improve your image quality. That is one of the reasons why I rather use some thing other than Envisage. I understand their purpose and some what know how to create them, but never actually done it.
In your experience with DSS, what is the best way to save them in Envisage ie..tif, fits, jpeg ect.. And do you let Envisage combine them into a master as well? Another thing that is unclear to me is how often flats should be created. I would think that one should take flats at every session because the focus and camera could change positions; even temperature is a factor…right?

I’m getting close to running out of questions...for now.


Thanks!

Hi Larry,

I've been enjoying this discussion. It kind of makes me think why I go through that whole process

Here is what I do. I set Envisage to save all uncombined in raw.

a) Take my lights (the actual pictures), I spend basically all night doing this.

b) By dawn I take my flats covering the front part of the scope with a white t-shirt and pointing to the brightest part of the sky. Ask Envisage for the auto-exposure (it usually is less than a second, if not I wait a little longer till the sky is brighter. Of course you can take flats using some sort of light box). With exposures of less than a second you don't need darks for the flats. In my case I need to take flats for each filter. The idea is that the histogram peek needs to be between the first and second thirds. So I play a bit with the exposure to put it in the middle. Then I take around 50 flats per filter. This takes no more than 5 minutes. The temperature is not important. What the flats register are the "imperfections" of your "optical train". They usually look whitish with some vignetting and in our case with some doughnuts produced by dust in the ccd and/or filters. In case of a refractor they would look like a fluffy circle.
As with the darks, combining 10-20 or more is a good idea to get rid of the random noise.
And YES, you are absolutely right. Flats need to be taken each session. The reason: these imperfections are never the same, in particular, suppose your ccd catches another dust particle, or better yet it looses some dirt. ; your camera is not in the exact angle as the night before (that will produce slightly different vignetting), etc...

c) In case I don't have darks for my exp/temp set I capture them. Usually by this time of the day the camera is warmer than when I captured the lights. So I cover it with aluminum foil, cut the foil from the back and put it on top of a ziploc bag with water and ice until it reaches the desired temperature. Then I let it take as many darks as I want. that way I'm building a pretty comprehensive library of darks. I hope at some point I will not need to capture darks anymore.

d) Once I have my lights, darks, and flats, (all uncombined raw) I load them (along the bias) on DSS and start the stacking process. BTW, the bias were captured some weeks ago. The bias are just the shortest exposure the camera can take in the dark. These are useful to get rid of the inherent noise produced by the electricity required to make the camera work.

e) I save the image(s) as fits, open it (them) in Photoshop/FitsLiberator and do the processing.

I don't use anymore any combined image produced by Envisage. Using only uncombined gives you more control on which subs to stack and which discard. Both Envisage and DSS make "mistakes" evaluating the quality of some subs. These makes sense, the programs do not think, they just run some very smart algorithms to check for the quality of an image. Sharpness is an example, but sometimes the scope jumps during long exposures and you get ghost images which are very sharp but most definitely want to discard.

Greetings,
Pablo

Pablo, you are a gentleman and a scholar!

Now, if the clouds would just go away…

Cheers!