Astrophotography with a Dobsonian mount

[rogerinnh]

<p>It is my understanding that it is not feasible to do astrophotography on a telescope that has a Dobsonian mount, the reason being that although you can motorize the two axis and control them in such a way (using a computer controller, for example) that they keep the telescope properly pointed at the target object for the duration of the exposure, the fact that it is a simple Dobsonian mount (as opposed to an equatorial mount) means that the image of the target at the focal plane does not stay properly oriented. In fact, the image rotates over time, so that you&nbsp;get a rotationally blurred image.<br /><br />It seems to me that there are at least two approaches that could be used to overcoming that problem. One would be to set it up so that it takes multiple short-duration images and then combines them into one summed image, with the rotation between images taken into account when the summing is performed. The other would be to motorize the image sensor itself so that it rotates appropriately to compensate for the rotation of the image.<br /><br />Have either of these approaches been tried?<br /><br />Roger Garrett</p>

 

[Testing]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>It is my understanding that it is not feasible to do astrophotography on a telescope that has a Dobsonian mount, the reason being that although you can motorize the two axis and control them in such a way (using a computer controller, for example) that they keep the telescope properly pointed at the target object for the duration of the exposure, the fact that it is a simple Dobsonian mount (as opposed to an equatorial mount) means that the image of the target at the focal plane does not stay properly oriented. In fact, the image rotates over time, so that you&nbsp;get a rotationally blurred image.It seems to me that there are at least two approaches that could be used to overcoming that problem. One would be to set it up so that it takes multiple short-duration images and then combines them into one summed image, with the rotation between images taken into account when the summing is performed. The other would be to motorize the image sensor itself so that it rotates appropriately to compensate for the rotation of the image.Have either of these approaches been tried?Roger Garrett <br />Posted by rogerinnh</DIV></p><p>Yes, both are a viable option with the software approach being the the better unles you are taking exposure beyond 30 seconds. I start seeing trails at 20.<br /></p> <div class="Discussion_UserSignature"> </div>

 

[TahaSiddiqui]

I recently took photos of the M42 for the first time . I used an Orion XT8 on the standard dobsonian mount it came with. I also used a regular digi-cam, not a DSLR or those astro cams. My exposures were all under 10 seconds and 800 ISO i believe?? Even at 2.5+ seconds I started gettin star trails. Is there something i'm doing wrong?? I was using the afocal method using my 32mm eyepiece which provides um.. 37.5X. Here are some photos, does anyone have any advice to get better photos?? Nvm its not uploading for some reason !! lol

 

[Mee_n_Mac]

Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I recently took photos of the M42 for the first time . I used an Orion XT8 on the standard dobsonian mount it came with. I also used a regular digi-cam, not a DSLR or those astro cams. My exposures were all under 10 seconds and 800 ISO i believe?? Even at 2.5+ seconds I started gettin star trails. Is there something i'm doing wrong?? I was using the afocal method using my 32mm eyepiece which provides um.. 37.5X. Here are some photos, does anyone have any advice to get better photos?? Nvm its not uploading for some reason !! lol <br />Posted by <strong>TahaSiddiqui</strong></DIV><br /><br />My first impression is that I'd expect star trails looking through a non-tracking scope after fairly short times.&nbsp; There was a table I ran across that showed the longest times vs focal length for a 35 mm SLR that you could use w/o getting star trails.&nbsp; Looking through your scope is of course a very long focal length (in photography circles) so your 2.5 secs doesn't seem wrong to me. <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>

 

[adrenalynn]

<p>Nope - you're not doing anything wrong.&nbsp; The greater the magnification, the faster motion is perceived.</p><p>Stacking images is fine, but even if you take a trillion 2 second images, you still don't have enough photons to do much with.</p><p>The best way to address photography with a big dob is to get rid of the mount and put it on an EQ mount like the Atlas.</p> <div class="Discussion_UserSignature"> <p>.</p><p><font size="3">bipartisan</font>  (<span style="color:blue" class="pointer"><span class="pron"><font face="Lucida Sans Unicode" size="2">bī-pär'tĭ-zən, -sən</font></span></span>) [Adj.]  Maintaining the ability to blame republications when your stimulus plan proves to be a devastating failure.</p><p><strong><font color="#ff0000"><font color="#ff0000">IMPE</font><font color="#c0c0c0">ACH</font> <font color="#0000ff"><font color="#c0c0c0">O</font>BAMA</font>!</font></strong></p> </div>

 

[RogerInHawaii]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Nope - you're not doing anything wrong.&nbsp; The greater the magnification, the faster motion is perceived.Stacking images is fine, but even if you take a trillion 2 second images, you still don't have enough photons to do much with.The best way to address photography with a big dob is to get rid of the mount and put it on an EQ mount like the Atlas. <br />Posted by adrenalynn</DIV></p><p>Let's see: Two trillion seconds is about 6,341,958 year's worth of photons. That ought to be enough for a fairly good image. But seriously, the issue with taking multiple imags and summing them to generate a final image comes down to how accurate the image sensor is. If, for example,&nbsp;you get on average a&nbsp;thousand photons in each sensor pixel, but the sensor needs ten thousand photons in order to register as a pixel value of "one" then you&nbsp;end up with&nbsp;a value of "zero" for each such pixel, and no matter how many times you add another zero, it's still zero. On the other hand, if the sensor can register a hundred photons as being a value of "one" then you could indeed sum up pixel values over multiple images and get a reasonable total image. Of course, if your sensor actually counts individual photons there's no problem and you can indeed sum up the multiple images.<br /><br />But,that aside, why not take just one long exposure (on the DOB mounted scope) and ROTATE the sensor array so that it properlyy tracks the target image? Seems like it would be a lot less expensive to add a sensor-rotator than to remount the entire scope on an equatorial mount</p><p>&nbsp;</p><p>&nbsp;</p><p><br /><br />&nbsp;</p> <div class="Discussion_UserSignature"> </div>

 

[crazyeddie]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>....Seems like it would be a lot less expensive to add a sensor-rotator than to remount the entire scope on an equatorial mount&nbsp;&nbsp;&nbsp; <br /> Posted by RogerInHawaii</DIV></p><p>You could also forego the equatorial mount and just build or buy an equatorial platform, and put your dobsonian on that. &nbsp;That will give you to to 75 minutes of tracking time. &nbsp;There are many plans for building them yourself on the internet, and they can be bought ready-made for a couple of hundred dollars (used).&nbsp;</p> <div class="Discussion_UserSignature"> </div>

 

[Mee_n_Mac]

Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>But,that aside, why not take just one long exposure (on the DOB mounted scope) and ROTATE the sensor array so that it properlyy tracks the target image? Seems like it would be a lot less expensive to add a sensor-rotator than to remount the entire scope on an equatorial mount&nbsp;&nbsp;&nbsp; <br />Posted by <strong>RogerInHawaii</strong></DIV><br /><br />How smart would the controller have to be ? I'll guess that the rotational rate is tied is a non-linear way to the Alt/Az angles.&nbsp; And now you have 3 error sources, the alt tracking, az tracking and rotation cancellation.&nbsp; Still there's no reason why it couldn't work.&nbsp; If you had an alt/az mount it might be less $$s to add.&nbsp; Might be a fun project to do. <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>