Novice Question

[KenMiller]

Here's a link to an image I took of NGC-7000:

http://www.astrobeginner.com/images/ngc7000.jpg

This was taken through a TOA-130 with an unmodified Canon Digital Rebel XTi. Three 30-second second frames combined as follows.

This image is the result of creating a dark by median combining 3 dark frames, then subtracting the dark from each of 3 lights, and then median combining the three dark-subtracted lights.

My question is "What are all those red pixels?" Is that IR to which the chip is sensitive buy my eyes are not? If so, is it representative of energy directly from stars, nebula emissions, both, or something else?

Also, is the diminishing frequency of pixels away from the center a result of vignetting or something else?

[neilfleming]

Basically just noise in the red channel, Ken. 30 second exposures is just too short for this object. You'd want to take at least 5-10 minute shots, depending on your sky's light pollution. I'd also suggest at least 20 subs in your stack. An unmodified XTi would probably be just a little weak in the red portion of the spectrum, especially Ha.

The fall-off in the edges is probably due to a lack of "flats". Your TOA-130 is a great, fast scope, and will be a little "hot" in the center of the images. Flats will divide into your shots, and smooth out the hot center and the inevitable dust motes that will start to show up in your imaging train.

...Neil

[KenMiller]

Thanks for the response Neil.

I'll be curious to see what I get with longer exposures. Everything I've done thusfar has been limited to 30 seconds because that's the longest exposure I could do remotely controlling the XTi. I just got the beta copy of Maxim DSLR that supports the XTi so I hope to remedy that shortly.

Why would there only be noise in the red channel and not the same amount of noise in Green and Blue channels?

I know how to shoot flats now so I'll make sure I do that next time.

[BT Technologies]

Ken, you might want to look into getting a Canon TC-80N3 remote timer. With this timer you will be able to program how many exposures you want to take as well as the duration. I purchased one for my Canon 20d over a year ago and it works great.

Originally Posted by KenMiller

Thanks for the response Neil.

I'll be curious to see what I get with longer exposures. Everything I've done thusfar has been limited to 30 seconds because that's the longest exposure I could do remotely controlling the XTi. I just got the beta copy of Maxim DSLR that supports the XTi so I hope to remedy that shortly.

Why would there only be noise in the red channel and not the same amount of noise in Green and Blue channels?

I know how to shoot flats now so I'll make sure I do that next time.

[KenMiller]

Thanks Brian. If MaxImDL doesn't pan out I may go that route. But I'll probably only keep plunking away with the XTi until the ST11000 arrives.

[BT Technologies]

I might be wrong Ken but if you do not have the Canon Timer I don't think that Maxim DSLR can control anything longer than a 30 second exposure without the timer attached.

Originally Posted by KenMiller

Thanks Brian. If MaxImDL doesn't pan out I may go that route. But I'll probably only keep plunking away with the XTi until the ST11000 arrives.

[neilfleming]

MaxIM should work great. BTW, my CCD Inspector run on your jpg showed that the focus was a little soft, and that the camera was not quite orthagonal to the image plane. One of the axes was quite flat, while the other was quite curved. This was not a usual result. Sumpthin else is not quite working right in your setup.

A larger run with more subs will help to analyze the issue.

...Neil

[KenMiller]

Neil, thank you so much for taking time to do that!

I'm not surprised about the focus. I took over 20 images doing trial and error and never could adjust it to exactly the degree of precision I wanted.

I presume a FeatherTouch focuser would help that?

I'm attaching a photo of the setup I used.

Aha!!!

In preparing to take the attached photo I believe I found the problem!

Tonight, unlike when I took the astrophotos, the camera locked in place on the adapter. I remember being surprised on the night I took those shots that it hadn't done that. I figured that was just a peculiarity of the Takahashi adapter.

That would probably account for the non-orthoginality.

[Spoonsize]

I would propose that anybody that can get these results right out of the gate can no longer refer to themselves as "Novice" or "Beginner"!!

(Man-Law??)

Very Nice work Ken...Do you give lessons??

Steve Durham

[neilfleming]

Originally Posted by KenMiller

Brain really did save me a lot of hassle - without his advice I'd be lugging a Bisque mount in and out every night taking a lot more time to set up and tear down and I'd have a scope that I couldn't put on that mount by myself. This stuff is a lot bigger and heavier than it looks on the manufacturer's sites!

What on earth would you have got!?

That EM-400 is great to set up. The PA scope is second to none.

...Neil