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Focusing on DSO with DSLR

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#1 ClySue

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Posted 01 June 2020 - 07:47 PM

When I am imaging the moon I have no trouble focusing, etc. When I switch to DSO, M13 for example, I cannot see the object in my camera viewfinder or "live view" screen. I, of course, have to change the settings on the camera (higher ISO, reduced shutter speed, wide-open aperture) to record the DSO image. All I see on the live-view is a screen filled with colorful, dancing pixels--no stars to focus on. The set up I'm using is: Explore Scientific 80HD Triplet f6, Nikon D7500, ISO 1600 (or higher), shutter speed of 20-30 seconds, Celestron AVX mount. I can find the object with no problem through an eyepiece. Thanks for any suggestions.

 

Clyde Witt

 

 



#2 Kendahl

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Posted 01 June 2020 - 07:51 PM

  1. Focus on a bright star.
  2. Lock down the focus.
  3. Go to your DSO.

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#3 kathyastro

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Posted 01 June 2020 - 07:51 PM

Focus on a bright star, before you slew to the target.  If you use a Bahtinov mask, your focus will be very precise.



#4 scadvice

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Posted 01 June 2020 - 07:54 PM

As kendhl and Kathy said above get a Bahtinov mask. They work really well.

 

https://www.youtube....h?v=k0FIluj9ndQ


Edited by scadvice, 01 June 2020 - 07:54 PM.


#5 kel123

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Posted 01 June 2020 - 07:59 PM

When I am imaging the moon I have no trouble focusing, etc. When I switch to DSO, M13 for example, I cannot see the object in my camera viewfinder or "live view" screen. I, of course, have to change the settings on the camera (higher ISO, reduced shutter speed, wide-open aperture) to record the DSO image. All I see on the live-view is a screen filled with colorful, dancing pixels--no stars to focus on. The set up I'm using is: Explore Scientific 80HD Triplet f6, Nikon D7500, ISO 1600 (or higher), shutter speed of 20-30 seconds, Celestron AVX mount. I can find the object with no problem through an eyepiece. Thanks for any suggestions.

 

Clyde Witt

Wow! You have been giving yourself one heck of a job.

You don't focus on the moon, then go to a DSO and start adjusting focus. You instead focus on star, not necessarily in the field of view of the DSO. Focus on any bright star and you will be focused enough to capture any DSO you go to without refocusing. You are not going to see any DSO with a short focus.  And try to get a bahtinov mask, it will help you a lot.

I don't want to talk about plate solving or it will be information overload.

 

As I am typing this, i just saw that there are three replies already. Bet they are saying the same thing.



#6 bobzeq25

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Posted 01 June 2020 - 08:29 PM

The other thing that can work is to focus to maximize the visibility of dim stars.  I find that works about as well as a Bahtinov, and dim stars are more available.



#7 ClySue

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Posted 03 June 2020 - 01:41 PM

Thanks for all the tips. I think I must not have explained my problem well enough. I do use a Bahtinov mask and I do not move from focusing on the moon to the assumption that a DSO would also be in focus. I can clearly see the DSO in the eyepiece. When I swap out the eyepiece for the DSLR, the DSO, after adjusting time and ISO, cannot be seen for focusing. I've been able to get images of the DSO via trial and error--thank goodness for digital photography. So, I guess my question is: Why can't I see the DSO through the camera viewfinder or the live-view? Again, thanks for the help.

 

Clyde



#8 kathyastro

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Posted 03 June 2020 - 03:07 PM

Camera optical viewfinders and LCD screens were never designed for showing DSOs.  There is likely not enough contrast to show them.

 

When I was using a DSLR (a Canon 350D with no live view), my aiming and focusing routine was:

1. With the mount aligned and an eyepiece installed, slew to the target.

2. Centre the target.

3. Sync the mount to the target.

4. Slew away to a bright star.

5. Install camera.

6. Focus camera with Bahtinov mask (using test exposures, since there was no live view).

7. Slew back to the target.

 

The final slew to the target (#7) is blind, since I did not want to ruin the camera's focus.  However, the Sync (#3) after the first slew adds the target to the handset's sky map as another alignment point.  Its sky map is exact at that point, so barring bad backlash, the target will be centred after returning to it.

 

I was using a Skywatcher HEQ5, but as far as I know, all goto systems allow a Sync, though each manufacturer has their own name for it.



#9 TelescopeGreg

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Posted 03 June 2020 - 03:27 PM

Thanks for all the tips. I think I must not have explained my problem well enough. I do use a Bahtinov mask and I do not move from focusing on the moon to the assumption that a DSO would also be in focus. I can clearly see the DSO in the eyepiece. When I swap out the eyepiece for the DSLR, the DSO, after adjusting time and ISO, cannot be seen for focusing. I've been able to get images of the DSO via trial and error--thank goodness for digital photography. So, I guess my question is: Why can't I see the DSO through the camera viewfinder or the live-view? Again, thanks for the help.

 

Clyde

The eyepiece and DSLR will focus at different points, often very different points.  So simply swapping out the eyepiece for the camera will likely be so out of focus that nothing will show. 

 

Most DSO objects are really really really really dim.  You will never see most of them in Live View, as the camera (mine, at least) will drop the exposure to something like 1/30 sec, which is far far too short.  If you max out the camera's ISO (12,800 on my Nikon), you should be able to see the brighter stars in Live View, enough at least to focus.  Note that the Bahtinov mask is intended for use with the camera, not the eyepiece.

 

With the Moon out right now, I'd use that as the initial object to focus on.  Do it in the terrestrial manner, getting it as sharp as possible in the viewfinder or with live view.  You'll need to shorten the exposure a lot, of course, and probably reduce the ISO.  Then move the scope over to a bright star and with both the camera ISO and the live view exposure at maximum), see if you can see a star in live view.  If so, use the zoom function on live view on maximum, and focus it a bit more (going for the smallest star you can get).  Then try the Bahtinov mask to see if you can make out the spikes.  If the star is bright enough, they should be there, otherwise you'll just have to use the minimum star size method.

 

At that point you can start imaging.  Do it at first with the camera still at max ISO, putting the camera exposure back up to, say, 30 seconds (it's the most mine will do on its own).  It will be grainy; that's ok.  You're trying to get some sort of an image of a DSO object.  That will prove that you can aim and track the object, and give you an idea of what sort of exposure (combination of ISO and shutter time) you need.  Aim for one of the brighter objects; M13 would be good, though it's kind of close to the Moon right now.  Maybe in a week it will be out of the way enough for imaging.  Whatever you do, don't try to go after a nebula right now.  Too much Moon, too dim an object, and unless your DSLR is modified, it's likely that the built-in filters will remove all the nebula's light anyway.

 

Greg

 

Pre-posting EDIT:  Kathy types faster than I do...  But I think we're both suggesting about the same thing.  Bottom line, you should not expect to see a dim object in Live View.


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#10 ClySue

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Posted 03 June 2020 - 05:03 PM

Okay, I think I've got it this time. If we ever have clear sky, again, in Northeast Ohio I'll give any and all of these trips a try.



#11 kisstek

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Posted 03 June 2020 - 05:43 PM

My experience has been that the Moon and DSOs are far enough apart (distance wise) that focusing on the Moon will not be sharp for the DSOs. Before I got plate solving to work, I would focus on the Moon, then a planet (with a noticeable difference in focus), and then a bright star. With plate solving, I just go straight to Arcturus or Vega and focus there.

 

As a former terrestrial photographer, I always lumped everything over a quarter mile or so away as being at "infinity".  The quarter million miles to the Moon is not the same as a couple hundred million miles to the planets which is not the same as a couple hundred light years to the nearby astronomical objects.



#12 Alex McConahay

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Posted 03 June 2020 - 06:30 PM

I think it safe to assume that you do not want to use a computer to help with the focusing. I can see that. It can be a lot of extra gear (and power) just for that one feature. 

 

So, make yourself a parfocal eyepiece holder

 

http://alexastro.com...cuserindex.html

 

It's cheap and easy. Basically, it is an eyepeice holder that mounts directly to the same t-mount that holds your DSLR camera. Put it in place. Find and focus as you would any eyepiece (although it is a bit more difficult because there is no diagonal and thus a bit awkward). Remove the device and put your camera on that same t-adapter. The camera is now aimed and in focus. 

 

It works because you have adjusted the eyepiece in it to have the same focal position as the sensor in your camera. And, your camera is spaced precisely the same as the eyepiece holder because the t-mount demands this. 

 

Now, it is only as good as it is used. That is, you must foces the camera precisely, and then replace the camera with the eyepiece holder and focus and lock the eyepiece precisely in place. But, you only need to do that once, and you can use the same device night after night. And eyeball focusing is not as precise as motor driven computer focusing. And even the slight changes in swapping the eyepeice holder for the camera introduces some change.......but it is fast, easy, and very accurate. And you can use it with other methods like a Bhat mask. 

 

Give it a try.

 

Alex



#13 TelescopeGreg

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Posted 03 June 2020 - 09:55 PM

My experience has been that the Moon and DSOs are far enough apart (distance wise) that focusing on the Moon will not be sharp for the DSOs. Before I got plate solving to work, I would focus on the Moon, then a planet (with a noticeable difference in focus), and then a bright star. With plate solving, I just go straight to Arcturus or Vega and focus there.

 

As a former terrestrial photographer, I always lumped everything over a quarter mile or so away as being at "infinity".  The quarter million miles to the Moon is not the same as a couple hundred million miles to the planets which is not the same as a couple hundred light years to the nearby astronomical objects.

Interesting; I'd not noticed that.

 

One of the reasons I suggested the OP start with the Moon, then a bright star with live view, then with the Bahtinov mask, was as a way to move through progressively more difficult, as well as more accurate, methods.  We sometimes focus {ahem} too soon on the "best" way to do something, without providing guidance on how to gain the experience to get there.



#14 scadvice

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Posted 03 June 2020 - 10:37 PM

I've imaged a few DSO that you cannot see at all without stacking a bunch of shots and stretching to see them. So lately I taken to finding a bright star close as possible to the object and using the Bahtinov mask then Sync on the star position to be sure I'm right on and moving to the object.  This has been the best method for the DSLR focusing for me. Still you have got to refocus as the temp drops during the night. I've gotten lazy a few times and paid the price on the image.

 

I've become very aware of the focus changing depending were you are in the sky too. i.e. the focus is different near the horizon than it is at the meridian so I take care to assure focus is good close to my subject.



#15 Ryou

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Posted 03 June 2020 - 11:05 PM

I'm going to beat the proverbial dead horse and repeat what others have said. Do NOT focus on your DSO (unless it has a bright star in it) and instead focus on a star near it.

 

Also be aware that you may not always see the DSO in your image on a DSLR even after a proper exposure. I've done several sessions imaging the Elephant Trunk Nebula and no matter what I do, I don't see the nebula in a single frame. However it comes out just fine once I stack everything together. Because of this I do need to platesolve and deal with the framing I have, however you can definitely look for star patterns inside the DSO and if you're using a goto mount a 3-star alignment will give you plenty of chances to both focus and get an accurate goto



#16 kisstek

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Posted 03 June 2020 - 11:46 PM

Interesting; I'd not noticed that.

 

One of the reasons I suggested the OP start with the Moon, then a bright star with live view, then with the Bahtinov mask, was as a way to move through progressively more difficult, as well as more accurate, methods.  We sometimes focus {ahem} too soon on the "best" way to do something, without providing guidance on how to gain the experience to get there.

When I first started out with a DSLR on my C6, I ran into a chicken and egg problem: you need to point at a star in focus to align your spotting scope. But you need the spotting scope aligned to point at a star! And you need to be pointing at something so you can focus so you can see anything!

 

So I'd start out pointing at the Moon. Whether I was in focus or misaligned, it was bright enough to center anyway. Then focus. Align the spotting scope. Slew to a planet and hope it's in the FoV. Refocus, center, fine tune alignment. Then slew to a bright star, focus, center, fine tune the alignment again. Then slew to whatever I was originally aiming for. Or more often that not, take everything down since I spent the whole evening working on the focus and alignment and it was now time to go to bed! :-(

 

So I was doing the same as you suggest. Start big and progressively refine.



#17 jstrandberg

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Posted 04 June 2020 - 04:53 PM

I had this problem all the time and here is how I solved it.  With my EXOS2GT after I polar align I have to do a one or two star alignment to get the GOTO function synched. These are usually very bright stars. So I put the Bahtinov mask on my lens once I have the first bright star centered in the viewfinder and the mount tracking.

 

Now, maximize the magnification of your live view screen. You will still see only a very small Bahtinov image of the star (although with your scope it might be big enough to do an accurate focus with out the need to do the following steps that I need to do).

 

Even a small image is enough to get a pretty good focus with the Bahtinov mask. Take a 5 or 6 second shot of the star with the Bahtinov mask on.

 

Now, pull up the picture on your camera image screen and start cranking up you magnification until you can clearly see the Bahtinov pattern on the star. Sometimes you will find this focus is good already.  If not, carefully note the position of the central spikes. Now, ever so slightly, adjust the focus and note the direction you turned the focus (clockwise or CCW). Take another 5 to 6 second exposure and examine this one. Note the position of the center spikes. Are they closer to centered (better focus) or farther away (worse focus). If the focus is better, repeat the step turning the focus the same direction as original, if it is worse, turn the focus the opposite direction of your original attempt.

 

Usually I can get the focus nailed with two or three attempts.




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