The Astro-Imaging Thread

[_Augustus_]

32 minutes ago, Green Baron said:

If that earns it's designation of being an astrograph than some sort of field correction is built in ... 

 

No.

[munlander1]

If I used a 1in eyepiece I will not have to get a coma corrector?

[Green Baron]

8 hours ago, munlander1 said:

If I used a 1in eyepiece I will not have to get a coma corrector?

Visual is usually along the axis  you will not need a cc if you are content with the view. fotographic is in the field as well. If you have coma in the corners of the picture you will then need a cc.

If you use 100 or 120 degrees app. fov eyepieces then coma might get disturbing as well :-)

Edit: am on a reasonable pc now and can type ...

Visual observation occurs along the optical axis. You point at something, center it in the eyepiece and watch. So you will not notice any coma because that is out in the field of view, the farther away from the optical axis, the stronger. Only if you use those nice 80, 100 or 120° eyepieces you will probably notice the coma at the borders of the view.

Your instrument has an imaging field a little smaller than an aps-c chip, as written in the specs, which is absolutely ok. Most dslr cameras have a chip in that area, which is roughly half the size of a so-called full frame chip, the format the old analogue slrs had.

If you want to find out if you need a corrector: connect your aps-c -size camera and bring it in the primary focus (no correctors/filters/glass) using the correct adapters (see the manual, most probably a 2" or M48->T2 and then T2->bayonet of the cam or, if available, direct 2"/M48->bayonet), focus the center sharply and shoot. You will then have an image that is sharp in the center and shows more or less pronounced aberrations in the form of prolonged stars or stars in the form of "pears" towards the darker borders.

It is these deformations that you would want to correct with a coma corrector.

Hope that helped in your decision making :-)

 

Edited February 8, 2017 by Green Baron

[munlander1]

So my telescope cover has one hole in the left (or right, depends on how you look at it).  

Should the other side be cut out?

Edited February 9, 2017 by munlander1
Spell check did not save the day

[_Augustus_]

2 hours ago, munlander1 said:

So my telescope cover has one hole in the left (or right, depends on how you look at it).  

Should the other side be cut out?

No.

[munlander1]

11 minutes ago, _Augustus_ said:

No.

Why isn't it?

[MaxPeck]

That's to reduce the amount of light coming in for solar (or full lunar) exposures.  If you had both punched out, it would create a weird stereo effect. 

[munlander1]

1 hour ago, MaxPeck said:

That's to reduce the amount of light coming in for solar (or full lunar) exposures.  If you had both punched out, it would create a weird stereo effect. 

So then does the opposite budge that's not cut out do anything? And is this thread for general astronomy not just ap?

Edited February 10, 2017 by munlander1

[MaxPeck]

It's probably just a place to stick the cap from the opening. 

[munlander1]

Just now, MaxPeck said:

It's probably just a place to stick the cap from the opening. 

Thanks

[MaxPeck]

And I guess it's technically a thread for AP, but discussing visual astronomy too doesn't bother me if no one else objects. 

[munlander1]

1 minute ago, MaxPeck said:

And I guess it's technically a thread for AP, but discussing visual astronomy too doesn't bother me if no one else objects. 

Cool. I just did not wanna start another thread when we had this one.  

[Green Baron]

8 hours ago, munlander1 said:

Why isn't it?

Let's wait for your first exposures.

You have a nice newton reflector there for a very reasonable price, given the opening and focal ratio. Also i like the steel tube more than the modern plastic (carbon fibre) ones. It is better suited to bear a heavy camera.

An astrograph is not defined sharply. Usually one understands an instrument that has a corrected and flattened image for measuring distances and angles between objects, to get their exact positions in relation to fixed stars. This is what one needs when somethings new is involved, like discoveries of dwarf planets, asteroids, etc. Ideally the correcting equipment of such a thing is designed for the use of a special chip, to meet exactly its resolution (via pixel size) and plane. Usually one does not look through such a thing. It is mounted somewhere in a dome, accessible via cable and scans the sky all night long (yeah) :-)

 

[munlander1]

I am looking at getting a new camera because a lack of live view on my current one makes it unbearable. So is there something more valuable then a camera that I could get instead? I am looking at the rebel xsi that costs $600 but I gonna get a used in the $150-$200 range.

Edited February 10, 2017 by munlander1

[Green Baron]

If it's only for the life view, my D700 dslr doesn't have live view either.I take exposures and look at them in 1:1 magnification, that's more accurate than the viewfinder, which in my eyes is too dark to see clearly anyway.

Also, once your setup is balanced on the mount the camera will almost always be in the wrong position to look into the viewfinder, especially on a newton and you don't want to give up the balance. As said before, use a focusing aid, Luke. A Bahtinov-, Hartmann- or Carey Mask which you can cut out. Use it to get close, and then a few exposures to focus sharply. That is more accurate i think and you only focus once and then fix the focuser with a screw.

I practiced last summer with the cam on a simple tracking mount and telelens and found out that mirror pre-release is a must. Also i would strongly recommend a cable less timer for the cam, much more than life view ! Fingering at the release in the dark and cold may results in cursing.

My ccd's capture program uses a method called full width half maximum (fwhm) to calculate a value for a marked unsaturated star. In principle, if it is small and bright it is sharp, if it's large and dark it is out of focus. Maybe, if you get a cam that you can connect to your pc, you can find a program that does something like that ? I do not know though ... as said, bahtinov mask and test exposures ... it's so easy with the electronic stuff these days :-)

Visit astrobin and see which cameras people use for astrophotography. These are usually not the newest models ...

Edit: search "focusing dslr astrophotography". Everyone recommends the Bahtinov mask. Everyone. :-)

Edited February 10, 2017 by Green Baron

[munlander1]

2 hours ago, Green Baron said:

cam that you can connect to your pc

Yeah, I am using backyard eos which is really cool. Also, since I can't find the memory thing for my camera, backyard eos saves it.

 After poler alignment, would you recommend drift alignment? If so, what type. 

What Iso and exposure time should I use for the Orion Nebula? Venus?

Whats the better file format tiff or raw?

[Green Baron]

2 hours ago, munlander1 said:

What Iso and exposure time should I use for the Orion Nebula? Venus?

Whats the better file format tiff or raw?

Raw is the only format we ever use.

And ... exposure times are a strict Geheimnis i will never disclose :-)

Seriously, look at astrobin, everything is open there. That's the fun part.

[PakledHostage]

Some of you guys seem to really know what you are talking about so I thought I'd ask this question here:

Is there an ideal ratio of resolution (in arc-seconds per pixel) to Dawes limit?

I ask because both values are quoted on Astronomy Tools' Field of View calculator (https://astronomy.tools/calculators/field_of_view/) To me, it would seem that you'd want the scope's Dawes limit to be lower than the camera's resolution, but how much smaller? Is a 1:1 ratio sufficient, or should one try for significantly better than that?

Thanks!

Edited February 12, 2017 by PakledHostage
spelling

[Green Baron]

Very good question, i try a generalized answer.

Ideally the chip's pixel size should exactly meet the telescope's theoretical resolution, so, yes 1:1 would be ideal. But most amateur telescopes do not reach their theoretical limit. Some high-end APOs might, reflectors usually stay way below (or above :-)). That's why some people still spend five digit prices for an APO while there is two times the aperture for a tenth of the price. Sadly one of the best manufacturers of air spaced triplets (LZOS) decided last year to double it's prices for fresh lenses and skip the small ones up to 120mm totally.

Back on topic. If your camera offers a better resolution than the diffraction limit of your scope you have probably spent too much money on the cam because you will never use it really. We should have a set of different pixel sizes, because when using a compressor/reducer/barlow resolution changes :-) Also other considerations apply. Image field is probably the most important. Most chips are very small so that you only use the center of the image. It should have a size so that vignetting is not yet disturbing. And then the choice for us amateurs is limited. Even a simple ccd cam with a mass produced chip of the penultimate generation (example KAF8300), second choice batch, costs 1500-2500 funds new. High end full frame cameras easily go beyond 10.000, and still they have a mass produced chip you can accept or not.

Second there is the atmosphere. It limits the resolution to way below what even a 4" telescope can deliver. There are the one out of 100 days where you can see in the enlarged pixels that the telescope/camera really was the limiting factor. But then again we do not have the time for this every 100th day ...

In my case the combination telescope camera is a little below what the telescope alone (115/805mm apo) could theoretically deliver. The chip has a pixel size of 5.4 micrometers. So i am still a little better with the combo scope /cam than what the usual atmospheric conditions allow.

I decided for this one because of my limited budget (i waited for a price reduction of the old generation chips) and the chip size / imaging field (right size for most nebulae and clusters, a little too small for andromeda galaxy). Of course i checked the pixel size, but image field was more important to me.

Phew, hope that was an answer ... :-)

 

Edit: sorry, the focal length of the tube is 805mm, but i use a 0.79* reducer. Now you can do the math :-)

Edited February 12, 2017 by Green Baron

[PakledHostage]

Thanks, Green Barron! 

I was asking because I am thinking about buying a used telephoto lens for my DSLR camera and I noticed, while entering the specs into the Astronomy Tools' Field of View calculator, that my camera's resolution was close to the Dawes limit for the lens' optics and I wondered if it would affect image quality. I'll mostly be using it to shoot subjects that are within about 100 m of me so atmospheric effects usualy won't be an issue, but I might occasionally turn it skyward (such as during next summer's total eclipse in the USA), so your detailed explanation helps.

[munlander1]

I got some pictures that were not half bad, will post tomorrow.

[munlander1]

Forgot darks, taking them outside, in my garage after cooling my camera down and covering my telescope in blankets to keep out the light.

When done with darks, will post unedited images.

[Shpaget]

I don't remember if I shared this here already, but here it goes anyway...

I took it about a year and a half ago, using a Canon 100-400 mounted on a 70D.

[munlander1]

It seems dss has betrayed me. Is there a good way to manually stack images?

 

It's  doing something!

Edited February 13, 2017 by munlander1

[munlander1]

Would someone be willing to take a crack at my data sometime? Only have lights and darks.

Edited February 13, 2017 by munlander1