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Sedna

How to focus deep sky objects for the camera

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Hi everyone, I just got started with imaging using a DSLR and a Celestron Nexstar 8SE with a VX (equatorial) mount. One of the biggest challenges for me so far has been getting deep sky objects in focus for the camera when I cannot see anything through the camera. The obvious strategy, of course, would be to focus on a planet or something really bright in the sky, then turn back to a deep sky object to image. However, even with the go-to system, I am having trouble getting objects centered in the scope without looking through the eyepiece (and using the eyepiece of course requires getting the object in focus for my eye). This is partly because I am still bad at the alignment, and partly because I have a piggyback mount between the viewfinder and the SC tube, which means that objects are really off when I center them in the viewfinder.

Anyway, getting back to the question: how do you focus for the camera when you can’t see anything through it? I’ve tried taking a 20-30 second exposure, turning the knob, taking another exposure, turning the knob again, but this takes forever. There’s gotta be a better way :( Does anyone have suggestions? Thanks!

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Hi, 

You may try to focus it on a distant terrestrial object during the daytime and mark the draw tube with a pencil or a sharpie, you can also do that at night time with the moon or another visible object. Then if it isn't perfectly focused it will be almost there, so you can take a picture of the dso and  make the proper adjustments. 

Hth

Edited by Atreta

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50 minutes ago, Sedna said:

Anyway, getting back to the question: how do you focus for the camera when you can’t see anything through it? I’ve tried taking a 20-30 second exposure, turning the knob, taking another exposure, turning the knob again, but this takes forever.

This is the process I use, but it is greatly aided by a focussing mask. Start with a simple Y-mask (literally a "Y" held in place over the front of the scope - I've seen it done "in the wild" using twigs!). Your stars will now have an X through them and another line. This other line needs to precisely bisect the X. Any out of focus results in it moving one way or the other.

So, first shot: line probably not in right place.

Refocus and take a second shot: if it has moved closer to the centre of the X you are going the right way, do it a bit more (it is useful to have some sort of idea by how much you have turned the focusser). If it is on the other side of the centre of the X you have gone too far, refocus the other way but less than your first amount.

By this process you can very quickly "zero in" on accurate focus.

MOST IMPORTANTLY, before you start imaging properly, REMOVE THE MASK. - I'm really not being patronising saying that ... I've forgotten this step ... more than once!!

Longer-term, the focusser that comes with the 8SE is quite coarse. At some point in the future you may want to consider replacing it with a focusser with a 10:1 facility. Simple job (even for a DIY dummy like me) and has made a world of difference to my focussing.

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1 hour ago, Demonperformer said:

MOST IMPORTANTLY, before you start imaging properly, REMOVE THE MASK. - I'm really not being patronising saying that ... I've forgotten this step ... more than once!!

 

Ditto!

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Yes, using a mask is the simplest way to do it.  The Simple Y mask suggested above or a Bahtinov mask which many of us use.  You need to focus on a bright star and then lock the focus.  Move to your target and the focus will be the same. 

Carole 

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I use a Bahtinov mask as Carole has suggested. If you are using a Canon DSLR I would recommend Backyard EOS as it makes the whole process really easy.

Peter

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If you run the camera through a dedicated astronomy capture programme you will also have the option of focusing via a measurement of Full Width Half Maximum. Effectively this measusres a star's diameter. It can be more accurate than the Bahtinov Mask in my experience, but the B mask gets you very close indeed so I use that first. FWHM can be a pain in high resolution setups or in bad seeing.

Olly

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Worth trying APT, it's free and has focusing aids built in. I would still get a Bahtinov mask though! Have you not got a 'live' option on your camera, most Canon's have??

 

Cheers

Ron

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What I do is this :

Say you want to photograph the Fireworks Galaxy, Deneb or maybe Vega are quite close to that, so move scope to Deneb and you should see this in your camera live view screen. Focus this down to a small a dot as you can get. press the 5x magnification on your camera and do the same. If you have a bhatinov mask you can put that on and use it as well to get it even better focus.

This will also confirm your star alignment as well because Deneb should be pretty close to centre in your view screen.

Then slew over to the Fireworks Galaxy and start snapping :)

Its a case of learning which stars are bright enough to see in your view screen. Doesn't take long to figure this out.

Good luck !

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For when you get rough focus, for alignment help there might also be a grid you can enable on your DSLR display to show you where the centre is in Live View mode. Search your camera's manual for Grid Display. Some models have a dot or diagonal lines to show the exact centre.

Good luck :)

 

 

 

Edited by Swamp Gas

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13 hours ago, Demonperformer said:

This is the process I use, but it is greatly aided by a focussing mask. Start with a simple Y-mask (literally a "Y" held in place over the front of the scope - I've seen it done "in the wild" using twigs!). Your stars will now have an X through them and another line. This other line needs to precisely bisect the X. Any out of focus results in it moving one way or the other.

So, first shot: line probably not in right place.

Refocus and take a second shot: if it has moved closer to the centre of the X you are going the right way, do it a bit more (it is useful to have some sort of idea by how much you have turned the focusser). If it is on the other side of the centre of the X you have gone too far, refocus the other way but less than your first amount.

By this process you can very quickly "zero in" on accurate focus.

MOST IMPORTANTLY, before you start imaging properly, REMOVE THE MASK. - I'm really not being patronising saying that ... I've forgotten this step ... more than once!!

Longer-term, the focusser that comes with the 8SE is quite coarse. At some point in the future you may want to consider replacing it with a focusser with a 10:1 facility. Simple job (even for a DIY dummy like me) and has made a world of difference to my focussing.

Hi Demonperformer, thanks for the feedback. I tried making a really simple focusing mask today. Cut out a cardboard circle with the same circumference as my SC tube and then cut out three triangles from this cardboard. I wrapped it in saran wrap so that I don't get my SC tube dirty (e.g., if little bits of frayed cardboard fall onto the glass). Still need to try using it.

I guess I'm still wondering how I use this focusing mask if I cannot see the object through my camera without taking a long exposure. It seems I would still need to do a long exposure, adjust focus, long exposure, adjust focus, etc. which can be quite time consuming .... hmmmm. And yes, the problem with the focuser on the 8SE seems to be that I have to turn it past 360 degrees at least several times to switch from eye focus to camera focus. I guess I could mark the correct place on the knob and just remember how many 360 degree turns need to be made before rotating the focuser to that point? Still seems very cumbersome. 

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10 hours ago, Ronclarke said:

Worth trying APT, it's free and has focusing aids built in. I would still get a Bahtinov mask though! Have you not got a 'live' option on your camera, most Canon's have??

 

Cheers

Ron

Hi Ron, my Canon has a liveview mode, but if I'm looking at a deepsky object in liveview mode, I see absolutely nothing on the LCD screen, even with the ISO cranked all the way up.

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7 minutes ago, Sedna said:

Hi Ron, my Canon has a liveview mode, but if I'm looking at a deepsky object in liveview mode, I see absolutely nothing on the LCD screen, even with the ISO cranked all the way up.

Do you have the live view exposure simulator set correctly and the exposure time set to 20 seconds or so but never bulb when focusing.

Alan

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A Bahtinov mask is one of the cheapest things you can buy for astro imaging, but can make such a big difference to your results.  I usually use the bright alignment start for the focusing before slewing to the imaging target.

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For focusing you really need to invest in a Bahtinov mask. Even with a light pollution filter I can get a bright star like Vega in the camera Live View. I'd do this:
 
Target the moon. Get it into your field of view with an eyepiece and then put on your camera. Switch to live view and get the moon in focus as best you can. Make a note of the drawtube position. Either mark it somehow or measure from back of scope to end of drawtube. You now know the near focus position for your camera. If you start off in this position in future you should always be able to see a bright star in live view when you slew to it. Though live view can be used with a Bahtinov mask to fine focus, it's better to take a short exposure of a bright star as you will get better spikes and you will not be focusing on the 'seeing'. Below is a 10 second shot of Vega when I was getting in focus last week. I use a laptop for imaging but the same results can be seen in just the camera LCD.

focus1.png

focus2.png

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Let's break this problem down.

Are you saying your finder is not centred on the main ota view? Why is that? You need to fix that so that a star is visible in the centre of the finder and at least somewhere in your dslr's optical viewfinder.

The multi-turn SCT focusers make it very hard to swap between eyepiece and camera with any accuracy, unless you have an expensive digital readout attached like Robofocus.

So GoTo and centre a bright star near your target (using your aligned finder to help), and focus using LiveView at max ISO and with the magnifier on.

Even if your Star Alignment is not good, having GoTo'd your nearby star and centred it, your VX mount hopefully has a command that allows you to Synch on it.

The mount's starmap is now correct for the general  area you are pointed at.

GoTo your nearby target and it should be visible on a 1 minute exposure at highest ISO.

Michael 

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9 hours ago, Sedna said:

I guess I'm still wondering how I use this focusing mask if I cannot see the object through my camera without taking a long exposure. It seems I would still need to do a long exposure

Yes, this is true. The advantage of the mask is that it enables you to zero in on the sweet point a lot faster that trying to compare the sharpness of the stars this time with how sharp you think you remember them being last time. It also gives an indication of how much more adjustment is needed. The line has travelled 1/3 of the way from where it was to the centre during shots 1 & 2? Try doubling what you did. It's now almost in focus ... do you need to go a little bit further or a little bit back? If the line is still on the same side, it is further, if it has crossed the centre of the X it is back. If you pick a bright star for your focussing, it may only require a 5s exposure to get the diffraction pattern and a mask cuts down the number of attempts you need and you know when you have got it right.

7 hours ago, david_taurus83 said:

For focusing you really need to invest in a Bahtinov mask.

I don't want to get drawn into the benefits of a bahtinov pattern over a Y ... if there are any. I will just say that I have always found the Y to do the job and it is cheap (a bit of scrap cardboard) and simple to make (cutting out all the slits of a B is a pain. If the Y works for you, you don't need a B, if for some reason you can't achieve the result you want with one, then I totally agree that a B is a cheapish way of achieving the result. Although be aware that the B mask blocks more light than a Y and so the image will be dimmer.

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