The Astro-Imaging Thread

[Wjolcz]

Can you guys tell me something about the Astromaster 90AZ?

I mean, I'll probably buy it anyway, because it will cost me 1/3 of a price of a new one, but would like to know your opinions and what you can see with such telescope.

[Green Baron]

42 minutes ago, Veeltch said:

Can you guys tell me something about the Astromaster 90AZ?

I mean, I'll probably buy it anyway, because it will cost me 1/3 of a price of a new one, but would like to know your opinions and what you can see with such telescope.

I could only tell you about the disadvantages, but if it the price doesn't hurt or you're limited to <100 funds then do it.

Things that could reduce the experience:

- azimutal "mounting", must steer in two axises manually which can be frustrating, fotography only rudimentary possible

- plastik mounting probably either has built in "tolerance" or if screws are tightened can only be adjusted "digitally" (jerks), seems to have no fine adjusting knobs

- probably difficult to attach accessories, only suitable for eye-viewing

(- focal ratio 11, dark picture; 90mm aperture, small picture)

 

There has been a similar question with a tight budget just a few weeks ago here in this forum, there people advocated something like a used dobson ...

 

Edit: I see that for 10 funds more there is a version with an equatorial mount (metal ?) of the Astromaster 90. If you could get a grip on that one you'll avoid half of the disadvantages ....

Edited September 25, 2016 by Green Baron

[icedown]

You can't do long exposures on an azimuth mount either, as the image rotates as the mount slews.  If your in America, I'd check craigslist, or whatever your local equivilent is.  I picked up a like new 10" dobsonion with multiple accessories and eyepieces for $250.  While it's not good for photography, it serves my purposes.

[MaxPeck]

@Veeltch I guess my first question would be what you intend to use it for?  Are you using it for visual observation or do you intend to get into astrophotography?

[Wjolcz]

1 minute ago, MaxPeck said:

@Veeltch I guess my first question would be what you intend to use it for?  Are you using it for visual observation or do you intend to get into astrophotography?

I don't think it would be good for astrophotography, because it can't track stars, so mostly visual observation.

[_Augustus_]

9 hours ago, Robotengineer said:

I'm on CN, not active right now though. Mostly in the ATM sub, I'm crazy enough to grind my own mirror. 

 

6 hours ago, MaxPeck said:

I respect this.  You sir are hard core.

I want to grind my own mirror, I'm probably going to the ASGH mirror class in January and grinding an 8-10" mirror, and if my dad doesn't take me to that I'll grind my own 6" at home and then make a 10-12".

[MaxPeck]

@Veeltch Yes, this particular scope is good for visual observation.  Basically this will be ideal for looking at star clusters, larger deep space objects and the moon.  Planets other than the moon probably won't be a very exciting experience for you - most of them will look like a colorful blob.  You'll be able to see Jupiters moons and Saturns rings (and possibly the Cassini division if seeing is good).  Things will drift and wander in your field of view because of a lack of tracking, and probable flexure from the plastic mount head.  As for positives, it would be useable in the future for photography as it appears to have a decent steel dovetail permanently mounted to it, so if you got yourself an inexpensive mount with tracking capabilities (I recommend the iOption SmartEQ Pro for this), you would be able to do some rudimentary imaging.

That being said, there are two properties of a scope you should consider.  They are aperture and focal length.  The larger the focal length, the greater the potential magnification (depending on the eyepiece you use).  The larger the aperture, the more light it will collect, the clearer and more defined your images will be.  For most photography you want fat and short-ish, whereas for visual observing, longer will work.  You also want an equitorial mount, will will allow your scope to track with the sky, as opposed to an ALT/AZ mount, which will not (even the motorized "goto" mounts).  90mm is decent for the eyeball, but can be frustrating for photography, YMMV.  My first imaging score was a 114mm refractor and that was not all that fair at best.  

Hope that helps!

Edited September 25, 2016 by MaxPeck

[Matuchkin]

6 hours ago, Robotengineer said:

I'm on CN, not active right now though. Mostly in the ATM sub, I'm crazy enough to grind my own mirror. 

How? Like, rotating a ball under a sheet of glass?

[_Augustus_]

1 minute ago, Matuchkin said:

How? Like, rotating a ball under a sheet of glass?

No, it's more complicated....

[Matuchkin]

Does anyone here have an Orion XT8 dobsonian reflector?

[_Augustus_]

Just now, Matuchkin said:

Does anyone here have an Orion XT8 dobsonian reflector?

A lot of people have 8" Dobs, but Zhumell's Z series Dobs are more popular than the XT series.

[Green Baron]

 

Did a few tests yesterday with my newly acquired used camera but had to give up because the wind was too strong. Just had a chance to realize i had a small drift, just a few pixels per minute. Probably not exactly balanced setup and/or battery dropped below 12V. More tests necessary.

Right now there is a steel blue sky, a foehn-wall is standing over the ridge and 60km/h gusts tear at the satellite dish. But the wind shall drop they say ...

@MaxPeck, do you use an autoguider ?

 

[MaxPeck]

12 minutes ago, Green Baron said:

 

Did a few tests yesterday with my newly acquired used camera but had to give up because the wind was too strong. Just had a chance to realize i had a small drift, just a few pixels per minute. Probably not exactly balanced setup and/or battery dropped below 12V. More tests necessary.

Right now there is a steel blue sky, a foehn-wall is standing over the ridge and 60km/h gusts tear at the satellite dish. But the wind shall drop they say ...

@MaxPeck, do you use an autoguider ?

 

Oh yes.  I use an Orion SSAG with a 50mm guidescope and PHD2.  I tried just doing the mechanical tracking thing, but I found that for the length of exposure you need for DSO shots, an autoguider is almost a necessity.  Plus, my view to the north is obstructed, so I have to do polar alignment by iteration instead of using a polar scope, so there's almost always a slight error that autoguiding can deal with.

[Matuchkin]

Just now, Green Baron said:

Probably not exactly balanced setup

And this is why dobs are superior. 

[MaxPeck]

If you want a really cool conversation piece, want to make a telescope out of cardboard and plywood and/or post photos of your comically enormous tube on Facebook, Dobs are for you. If you want to do astrophotography you need something that will work on an equatorial mount that weighs less than a VW bug. 

 

[Green Baron]

41 minutes ago, Matuchkin said:

And this is why dobs are superior. 

See MaxPeck, or to say it with a bit of correctness: if budget is low but you want a big opening and an easy ride into the countryside then a dobson is a nice thing. 10kg and 200mm (8") aperture. But i know of no-one who does photography with a dobson. That stuff is rather 35kg (plus battery) for 115mm aperture :-)

 

[Matuchkin]

9 hours ago, MaxPeck said:

If you want a really cool conversation piece, want to make a telescope out of cardboard and plywood and/or post photos of your comically enormous tube on Facebook, Dobs are for you. If you want to do astrophotography you need something that will work on an equatorial mount that weighs less than a VW bug. 

 

Dobs have high aperture, and are best for photographing DSOs. Most serious astronomers have dobs.

Edited September 26, 2016 by Matuchkin

[MaxPeck]

19 minutes ago, Matuchkin said:

Dobs have high aperture, and are best for photographing DSOs. Most serious astronomers have dobs.

I've been doing astrophotography for a couple of years now, I know no one who uses a dob for serious astronomic work. Dobs, by their very nature, are horrible for tracking anything. They are large and ungainly and while they give fair visual viewing experiences, due to their large aperatures, they are largely unsuitable for astrophotography without some kind of field rotator, and that just introduces a whole new series of problems in and of itself. Very often dobs are made from home brew plans, use materials of questionable quality, and are susceptible to a tremendous amount of chromatic aberration and visual distortion.  Because of the often comically large tube, air currents and thermal disturbances inside a dob itself can reduce seeing by an order of magnitude, hence the sudden "open truss" craze - which also brings the weight of the OTA down below that of a compact car.  

Most serious astronomers either use apochromatic refractors if weight isn't an issue, reflector astrographs, or catadoptic scopes mounted to tracking equatorial mounts.  Anything with an aperature over 4" and a reasonably short focal length is going to give you good data on DSOs.  If you really want the superwide aperature "light bucket", a Schmidt-cassegrain cat is going to be far better than a dob any day of the week.

Dobsonian enthusiasts tend to be the counter culture Linux crowd of the astronomy world.  It's cool that you built your scope out of stuff from Home Depot,  but dobs are inexpensive amateur scopes for amateur astronomers.  I have all the respect in the world for someone who grinds their own mirror and builds their own tube, but let's not pretend for a minute that such a scope is a serious contender next to a scope with precision coated optics and a tracking mount. 

 

 

 

[Green Baron]

7 hours ago, Matuchkin said:

Dobs have high aperture, and are best for photographing DSOs. Most serious astronomers have dobs.

Might it be that you confuse dobsonian telescope with newtonian telescope ?

A "dob" is a newtonian telescope mounted on a simple rockerbox, a 2-axis device (yaw and pitch for us rocketeers :-)).

Newtonians are used for photography, but then are mounted on an equatorial mount.

Edited September 26, 2016 by Green Baron

[Matuchkin]

Just now, Green Baron said:

Might it be that you confuse dobsonian telescope with newtonian telescope ?

What the hell, you're right. I have a newtonian. Do I even know my definitions?

...

[MaxPeck]

Now see, if you go back and substitute "dob" with "Newtonian reflector", I agree with everything you've said. Dobs are functionally useless for photography, newts are much better. But it's all about the mount. 

[ProtoJeb21]

After 13 months of quietness, the Moon erupts in activity! Transient Lunar Phenomena, to be exact.

http://imgur.com/gallery/D2Hhj

@MaxPeck I have an idea for image stacking, but I'm not too sure it will work.

What I want to try and do is find a cool image I took of some object - M42, Jupiter, etc. Then I would make a few dozen copies of it. Next, I open up a program like RegiStax, take all the duplicated images of that one specific body, and stack them together. I think it will work similar to taking several dozen images of the same object and stacking them. What do you think?

[MaxPeck]

43 minutes ago, ProtoJeb21 said:

After 13 months of quietness, the Moon erupts in activity! Transient Lunar Phenomena, to be exact.

http://imgur.com/gallery/D2Hhj

@MaxPeck I have an idea for image stacking, but I'm not too sure it will work.

What I want to try and do is find a cool image I took of some object - M42, Jupiter, etc. Then I would make a few dozen copies of it. Next, I open up a program like RegiStax, take all the duplicated images of that one specific body, and stack them together. I think it will work similar to taking several dozen images of the same object and stacking them. What do you think?

I think everyone has this idea at some point.  Unfortunately, it doesn't work like that.  The whole point of stacking is not just to make dim things brighter, it's mainly to sharpen signal and eliminate noise.  The difference is that the signal will be constant, but the noise should change from frame to frame.  If you only use a single source of data, you're going to amplify both the signal and the noise equally and your SNR won't change, so your final product will be a product of not just all the good stuff, but any errors and artifacts in your image will also be amplified.  The benefit of stacking is that each image is slightly different, so the things that are different get filtered out while the things that are the same are kept.

Take for example, if you had (for simplicity) 10 images - what we refer to as "subs".  In each sub you'll have your target, like the moon image that you posted.  You're also going to have thermal irregularities, noise artifacts from your camera sensor, visual artifacts from aberrations in your optical system, etc.  So for each sub, your stacking software goes through pixel-by-pixel and compares the data value in each pixel.  If the current sub and the previous subs have the same (or close to the same) data values for a given pixel, the software either amplifies that data, or drops it.  There are other ways of eliminating noise as well, using dark frames, bias frames, dithering, etc.  but the whole purpose of stacking is to eliminate random noise and make good data stand out.  Hope that helps.

[Green Baron]

I'm still waiting for reasonable weather. It's either cloudy, windy, too much dust and moisture in the air or the moon shines bright. Most of the time it's a weighted permutation of these factors ... :-/

My setup is a 115/805mm apochromatic refractor (LZOS) with 2,5" flattener (noname), a used 2mpx color-CCD Cam (Atik 420c) and all that mounted on a Losmandy G11, no goto, simply motorized. I also got a chinese branded (GSO) 200mm/f5 newton but that needs a lot of adjusting and the viewfinder is allways in a positon for vertebra-luxations. Maybe when i got the hang of it i'll use the reflector, because the setup with the apo only has a field of view of less than a moon-diameter (1/3 of a degree or so). That means Andromeda for example is too big to fit.

As i can't look through the telescope when the cam is mounted i focus with a self-cut Bahtinov-mask. First attempts on the moon and a nebula (i don't know which) where ok but blurry due to atmospheric disturbances. I also had a small drift in the setup during 2-3 minutes exposures, probably due to an imbalance on the moment.

A planetary/guiding cam is ordered, i plan to guide through an 80mm-refractor, piggyback on the apo, like MaxPeck with PHD2 if the camera and mount understand the software and vice versa. For this (and Morrowind while waiting for 1.2) i reactivated a Windows 7 partition and regretted that deeply. Error "messages" like "The side-by-side configuration is incorrect" remind of the bad old days when i had to use that "os".

 

The most colorful images are made with monochrome cameras using filters, L-RGB is one technique, using filters for the colour-channels and luminance (usually IR/UV-cut or so-called "contrast-boosters") for the brightness. Other filters include narrow-band H-/O-/S-filters for different elements. Colour is applied by software later. MaxPeck allready gave a nice description how the image frames are stacked. Some make more: dark images from the chip at the same conditions (temperature) for the subtraction of noise and hot-pixels, flat images of an even grey tone (like against a blanket) to account for an uneven field, vignetting or cold pixels, bias frames to account for the basic readout noise of a chip (what you get without anything exposed), ...

To scare away anybody who might think of trying astrophotography i searched and found this in a forum somewhere:

1. Acquire data frames,
2. Acquire dark frames,
3. Acquire bias frames if using scalable darks,
4. Acquire flat field frames,
5. Acquire dark frames for flat field frames of same exposure time as flat field frames,
6. Create master dark frames,
   1. For master dark frames of the same exposure time as the data frames, make a median combined master dark frame.
   2. If using scalable darks,
      1. First create a master bias frame by median combining the individual bias frames,
      2. Subtract the master bias from each dark frame,
      3. Median combine all dark frames to create a scalable dark frame.
7. Create master flat field frames,
   1. Median combine the matching dark frames to create a master flat field dark frame,
   2. Subtract the master flat field dark frame from each of the individual flat field frames,
   3. Median combine all calibrated flat field frames to create a master flat field frame.
8. Apply the master bias frames created in step 6bi above to each data frame if using scalable dark frames,
9. Apply master dark frames to each data frame,
10. Apply master flat field to each data frame,
11. Align all data frames,
12. Stack data frames by adding, averaging, median combining, etc.
13. Perform final image processing.

 

May i direct your attention to point 13: that probably is either a click with RMB and delete, or 2 hours of dragging saturation curves and applying sharpening filters. F5/F9 in our language. Ok. Message: equipment isn't everything.

And, of course, we still can just point our contraption at the sky and press the trigger. Or mouse button. Or whatever. It works, maybe not as perfect, but it works.

The rest of the week shall be cloudy and the moon is shining bright ...

1 day until 1.2 :-)

 

[Green Baron]

Yet another M31 ...

Full Frame DSLR, Telelens 200mm/f2.8 (image slightly cropped), 800ASA, 13 * 2min. Tracking mount. Deepskystacker, standard settings.

Will try again with 4min and dark frames and a little bit of image processing ...

 

Edited October 21, 2016 by Green Baron