Guys, I have purchased a telescope and a camera (ASI120MC) and discussed this in other places but people don't seem to understand exactly what is my problem.
I tried (with no tracking and the proper mount) to point the telescope to a planet and no image could be seen in the preview (Sharpcap, Firecapture...)
They claim you need to track the object. However the telescope was pointed manually to where the object would appear. I also have doubts if 1) the sky could have influenced the results, we always get bad weather, 2) the telescope was a Maksutov 90, not good enough or 3) the settings I used for Sharpcap were correct (exposure, gain, etc.)
Let's assume for a second I failed to point to the exact spot where the object was. Better yet, let's say the telescope formed an image from the Moon, an obvious object with too much light in the sky.
However I didn't buy the camera hoping to capture one object, then adjust things for another.
What I wanted was to record continuously objects from the sky much like we do with surveillance cameras. So even if the Moon is not in my field of view or if I missed Saturn when pointing manually the telescope, a star (since everything is moving) would appear in the capture later for example...
But I was told this cannot be done, at least not if I wish to zoom the image so much.
For example, assume the Moon can be seen with so much details as this emoticon:
Then after a while, the Moon will not bee seen anymore and another object will appear in the front of your telescope. It will be the size of this: *
What I was hoping to achieve was to let the record continue without my constant interference to adjust things like the focus, exposure, etc. and to analyze the results later. People say there's no way to do that with Deep Sky objects or anything else from the night sky.
Now if I point the camera/telescope to an object from Earth I get all I want. It's pointing to a fixed spot and a bird or something might appear in the recording. Not only that but I don't need to do anything more than collect the video later.
Can someone explain briefly 1) why what I want is not possible and 2) what camera or setup can provide better results than this one with that goal in mind?
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If you pointed this telescope at an object on earth, you would have similar problems.
Your Maksutov 90 uses a 1250mm focal length (assuming this is already its 35mm equivalent). That is a very NARROW field of view. Plus, this is a "beginner/hobbyist" telescope with limited Light-gathering capability. And likely has limited optical resolving/focusing power. You combine those 3 things and you have an exposure that is dim & small & slightly blurry when shooting the night sky. And that's on a clear night, when you are located in a remote area (less ambient light from the earth).
To get a good image, you need enough exposure & big enough size. Distant starts are still little more than pinpoints, even at the greatest magnification on that telescope. Exposure is related to: Incoming Light level, Sensitivity/GAIN (ISO), Shutter/capture time, Aperture opening size. Since you have limits on the aperture (cannot get any wider than ALL-THE-WAY open on your cam & your telescope), your light level is limited by the dimness of the stars/sky ambient level, and your sensitivity is limited by sensor size & noise, your main tool for getting a strong exposure is shutter speed (capture time). But the problem with that is: THINGS MOVE.
If you pointed your telescope at a bunny located a mile away from you, sitting on the ground on a moonless, pitch-black night, you could MAYBE get an ok shot of it - until it moved. Then you would get a dim smear. Same with stars. Celestial bodies are in motion all the time, and they have even less overall incoming light (at least to the input of your telescope/camera), so unless your are TRACKING them, they will be dim & smeared.
If you want to get more of the night sky in the shot (wider Field-of-View), you probably shouldn't use telescopes, but instead use a standard camera with a lens giving you the FOV you desire. Even then, you will have issues with exposure & capture time, but because the overall incoming light level is more (because of the wider FOV), your exposure constraints are relaxed to the point of having capture times that won't show (much) motion blur.
Hope that helps,
For any kind of astrophotography you will need a proper equatorial mount. Period.
ISO settings should rarely be above 400 weather it is film or digital. Planetary objects
exposure times will be measured in seconds, anything else will be measured in minutes
or hours. So you will need a proper mount. Anything else you will find out to be disappointing.
Point your telescope at the North star and try different exposure times. That will give you a feel of what is required to photograph stars.
Let me ask you guys something:
If I happen to just insert the correct values and then use the red-dot finder from a telescope with no tracking to point right above an object... Let's say Mars.
Then seconds later Mars will go upwards because it is "moving".
Can I hit START CAPTURE from Sharpcap or whatever software I am using and in the exact moment the object can be seen in the telescope (if I was using an eyepiece I would see it), will a perfect image (perfect is a way of saying, I mean any image that can be different from a black screen of nothing) form?
Nevermind it will last less than 10 seconds.
What I fear most is the need to point the telescope 500 times with the red-dot finder until any result can be achieved. If any at all. I planned to buy a dobsonian (203 or 254/1200) but now I need to know something will work in next attempts.
I searched the internet and could not find such values for specific models and objects (only a few hints). Some people told me to use MS and not seconds. OK, I didn't try exactly in the best conditions and didn't use, say, 5, 10 seconds. I was afraid of using even 1, for fear my PC would freeze.
I am asking this to get some assurance that at least this kind of capture could work. I know we need tracking, EQ mount, I get it. But nobody tried what I am asking to at least see it works?
I'm sorry, it just won't work. Believe me MANY HAVE TRIED and got crappy results. The mount is EVERYTHING.
I've been doing astro-photography (off and on) for forty years, taught undergraduate general and
observational astronomy while I was in graduate school. What you are talking about has to be done
with an EQ mount.
If it is a shorter time period (1/125th sec instead of 1 sec, etc), your image will be dimmer (7 exposure's worth in the example). That is a LOT dimmer. So dim, that you you will have trouble differentiating the "star" from "the night sky", or from "noise".
Again, you could raise up the GAIN/Sensitivity to it's limit, but that also raises noise (quite a bit).
Why are you so reluctant to believe us (or others who it seems have told you basically the same thing)?
Of course people have tried it, but they have found that you can't cheat the laws of physics/optics. Nor make things up with wishful thinking.
Here is the equation:
ExposureValue = log_base2 ((f-number squared)/exposure time)
Note that, while an ordinary indoor light is an EV of ~6, bright, sunny outdoors on snow is 16 and the dim MilkyWay galaxy alone at night is -11! And remember those figures are LOGARITHMIC, so the actual illumination difference is the EXPONENT of those numbers (6 vs. 2,821,109,907,456 vs. 0.00000000275). That is one F*****g huge range!
And here is a useful example of using that equation to capture evening/night images: http://blogs.oreilly.com/digitalmedia/2007/09/using-the-exposure-equation.html
The reason I am asking is because this guy claims to have captured these objects using a 254mm telescope without even the GO-TO feature:
And at least in the Wikipedia article it is saying things like:
Altazimuth mount: An equatorial telescope mount with clock drive was left out of the design. Equatorial mounts tend to be massive (less portable), expensive, complicated, and have the characteristics of putting the eyepiece of Newtonian telescopes in very hard to access positions. Altazimuth mounts cut the size, weight and cost of the total telescope and keep the eyepiece in a relatively easy to access position on the side of the telescope. The altazimuth mount design used in Dobsonian designs also add to simplicity and portability; there is no added mass or need to transport counter weights, drive components, or tripods/pedestals. Setting up for hard tube dobs simply involves placing the mount on the ground, and setting the tube on top of it. The weight of the Dobsonian style altazimuth mount is distributed over large simple bearing surfaces so the telescope can move smoothly under finger pressure with minimal backlash.
The altazimuth mount does have its own limitations. Un-driven altazimuth mounted telescopes need to be "nudged" every few minutes along both axes to compensate for the rotation of the Earth to keep an object in view (as opposed to one axis for un-driven equatorial mounts), an exercise that becomes more difficult with higher magnifications. The altazimuth mount does not allow for the use of conventional setting circles to help in aiming the telescope at the coordinates of known objects. They are known for being difficult to point at objects near the zenith, mainly because a large movement of the azimuth axis is needed to move the telescope pointing by even a small amount. Altazimuth mounts are also not well suited for astrophotography.
From what I could tell, even assembling an equatorial mount doesn't look like an easy task if you ask me.
I thought people were saying for me to track objects (or even better, use this kind of mount) because the object leaves your field of view very quickly and to capture deep space objects you need to "watch" everything that is there for more than seconds, and perhaps the same didn't apply for the Moon or planets.
But even if that's the case there's still time to see something at least using the telescope. The ASI120MC equals I believe an 8mm eyepiece. When using the 6mm (Wide Angle) eyepiece in the MAK90 you can see a star or Saturn for say 10 seconds. I know it's quick and to tell you one can only get crappy results at least I would need to see anything in the preview during the passage of such objects after I pointed using the red-dot finder.
Now if you are tellling me something along the lines of
THERE IS A DELAY AND SHARPCAP NEEDS YOU TO KEEP TRACKING ANY OBJECT FOR 15 SECONDS BEFORE IT SHOWS IN THE PREVIEW. BESIDES IF YOU ATTEMPT TO CAPTURE 10 OR LESS SECONDS YOU WON'T BE ABLE TO CREATE A PICTURE LATER USING IMAGE PROCESSING SOFTWARES
Then I could understand there's no way in this world this is going to work.
Still I have asked (waiting for a reply yet) the author of Sharpcap if the preview is working in real time, assuming the focus is OK and the values for the object are OK, too.
(You can always get a real-time preview if pointing the camera to an object from Earth during the day... I shouldn't even be asking this sort of question, but as you can see the answer can tell if one can capture anything from the night sky not tracking the object).
I'd say that guy claims a lot of things
First of all, This 'guy' is using a 254 mm f/4.7 scope. A modest aperture, FAST (note the f/4.7 spec) telescope to begin with.
Secondly, you are using a 90mm Maksutov scope with a f/10 or f/11 spec depending on the maker (there are many now)
AND then an eyepiece before the camera which may raise that value much higher which means WAY longer exposure
times to get the kind of magnification needed to say capture an image of Mars or Jupiter.
After visiting 'this guys' flickr site, I would say the only photo that could possibly be legit is the Orion Nebula shot (and
its not very good). The rest: not with that rig. Those are fakes.
Also, you are trying to compare what can be perceived by this camera with your eyes (HVS). The best cam that manufacturer makes is only 1.3 Megapixels monochrome using a 1/2" sensor, with only a 10bit ADC and 8bit output. What that cam has going for it is it's PC-control connector and the possibility of VERY short exposure times (32 microseconds, or 1/31250th sec). Our visual system is capable of 576 Megapixels total (40-60 in the fovea, & ~7-12 at any one instantaneous point of saccadian attention). It has ~24-26 stops of total dynamic range with an instantaneous range of ~14stops and capable of resolving ~16bit/colorchannel depth. And it is quickly ADAPTABLE, which is what is happening when you are looking through your scope's viewfinder.
About this guy (using a 254 telescope), I noticed he posted this observation in one of his pages. He is also using the ASI120MC camera:
SkyWatcher 10" Collapsible Dobsonian (No tracking)
Positive projection through 10mm eyepiece Sky-Watcher Plossl.
Hello, you write POSITIVE PROJECTION.. can u explain.. ? Do u mean you adopt your asi120mc camera with an adapter on the 10mm eyepiece? And exposure ?
Hi, thanks! The positive projection is like afocal method, the difference is that you have to attach the eyepiece, with a distance(5cm), in the camera without lens. The exposure varies greatly, and I regulate it in the capture software. Look my adaptation: http://i.imgur.com/MZsc9Em.jpg and http://i.imgur.com/w2TZZoz.jpg
Last edited by Perene; 22nd May 2014 at 07:03.