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lune lupine

just can't seem to get sharp focus

27 posts in this topic

0hmmm. can't seem to get tight focus on stars; finally got hold of a bhatinov mask and took shot (see below). unless i'm missing something, this looks ok yet when i take mask off stars look like small indescript blobs. 

star used was rigel - panned over to orion nebula and well kind of meh sadly.  i tethered camera to laptop, zoomed right in 10x and still no better. used canon eos utility. should i upgrade to eos backyard or what else can i try? checked my collimation too and looks grand under high magnification. help!!

mask image.JPG

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Have you got an image you can upload so we can see?

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heres one. moon was very bright and close by tonight if that makes any difference. also scope was cooled very at least an hour

orion.JPG

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The moon is the bane of RGB imaging, it destroys contrast.

There is some very slight blue fringing on the stars but nothing much, you may just be at the limit of seeing.

 

A while ago I posted a couple of shots with and without moon; https://stargazerslounge.com/topic/231826-what-a-difference-a-week-makes-ced-214/

And the target was nowhere near the moon either!

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hmmm. so how do i get those nice pin sharp sharp stars though? really confused as i thought even at seeing limits wouldnt i just see dots of light but not blobs if you see what i mean? is my mask image out of whack or are there better stars than rigel to attempt to focus on when trying to get pictures of orion's neb?

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Maybe a stupid question, but what is your focuser like? And what scope is it - is the camera hanging off the focuser? Just wondering whether the focuser needs tightening to prevent it slipping when the scope is moved - a DSLR is a fair weight and I have read of slippage a few times over the years. Might just be worth a check - could be slipping over time and as you move the OTA?

 

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It's not just overexposure is it whereby your getting star bloat? If you reduce the exposure time do the stars then become sharp? Off course you will not then see the nebula but this is where you would need longer exposures for that and layer them in post processing. Your bhatinov mask image looks bang on focus BTW.

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Hi lune lupine,

Did the temperature drop a lot during the night? If so then this shrinks the ota slightly and alters the focus. I have had issues with this in the past. After a couple of hours of imaging I normally put the bhatinov mask back on and redo the focus, this is annoying because I have to find and frame my imaging target again but at least I know t is in focus.

Also, regarding your bhatinov mask image, I use a lower ISO/sorter exposure - this will help you see the core of the diffraction pattern and make very precise focus adjustments.

Dan :happy7:

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Are using a dew shield? Your front plate could be dew which would give blobby stars.

Also did you sort out visual use and are now getting good star focus?

Edited by happy-kat

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Hi. I think at that scale -assuming that's a full frame- you're doing fine. It seems that the longer the focal length, that fatter the stars. If you look at long focus snaps with e.g. celestron mirror telescopes (cassegrain?) , they always show stars as discs. HTH.

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have a canon 760d dslr and telescope is celestron evolution 8. I might be able to get access to a webcam and see if that makes any difference to the images getting. Is Canon EOS backyard worth getting as well?


@ alacant; would a focal reducer help me at all - seen them on the market. my telescope is f10 if i remember correctly

@happy-kat; no dew - was using dew shield but checked and was looking good

 

thanks for all the advice folks, really appreciated.

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For inspiration on what you might image with your mount you might like to look at the No EQ DSO challenge thread.

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45 minutes ago, lune lupine said:

would a focal reducer help me at all

I always advocate the use-what-you-have approach. I'm not sure if a focal reducer would get you smaller stars but it would give you a wider FOV. One thing your telescope would be great for ATM is Jupiter. It's not necessary to have accurate guiding and you have the focal length to make it a decent size. Take videos instead of single snaps.

 

27 minutes ago, happy-kat said:

image with your mount you might like to look at the No EQ DSO challenge thread

To be realistic, I think it's gonna be tricky getting a dso at over 2000mm focal length, eq or not. But of course that's what challenges are for. 

HTH and clear skies to try...

Edited by alacant
of corse of course

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The Bahtinov mask image looks fine.  I think something else is going on here.  

You have obviously moved the scope a bit between Rigel and M42 (and yes Rigel is a good star to focus on).  I am wondering whether either the focusser is slipping or sloping.  I had a slipping focusser (with a refractor) when I first started and despite locking the focusser (are you doing that?) images still were not in focus, and it took me a while to fathom out that the focusser was slipping (the focusser had to be tightened up). 

Or the locking of the focusser is actually shifting the focus out of whack, are you re-checking focus after locking it? 

Something to consider.  

Canon Utility is basic but fine, should not affect focus, though there are focus aids with APT etc though I have never used them.  What you have already is quite sufficient to get you started.  

Carole

Edited by carastro

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20 minutes ago, alacant said:

To be realistic, I think it's gonna be tricky getting a dso at over 2000mm focal length, eq or not. But of course that's what challenges are for.

Absolutely, my comment was use the mount thinking with the dslr even if the kit lens to get going with the whole imaging process seeing as the equipment was already owned. There are at least two members using a similar mount on the thread just not with an sct or mak.

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54 minutes ago, carastro said:

Or the locking of the focusser is actually shifting the focus out of whack, are you re-checking focus after locking it? 

+1 for this, I have to leave the star just out of focus then tighten the lock screw that brings the focus ok as I tighten. 

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Nothing wrong with the mask picture..and indeed of m42.. id say your camera isnt modded so not picking up on the h alpha wavelength..

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Thanks for all the advice. Really appreciated. One thing that puzzles me tho is advice is to lock the focus/tighten the focus. How do I do that with telescope? Also is there a better named star than rigel to focus on before panning over?

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Any of the bright stars are suitable for focus.

I don't know what telescope you are using, but there should be a nut at the back of the focusser that you need to tighten.  

Refractor or Newtonian's normally have something like this, the focus lock is where the red arrow is.  No idea if you can lock Cassegrain/SCT  scopes. 

DSCF0564 (Small).JPG

Edited by carastro
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15 hours ago, newbie alert said:

Nothing wrong with the mask picture  ..and indeed of m42..

+1. I think you're in focus; the telescope you are using produces stars like that.

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Yup, I think your B mask pattern is bang on and you have resolved the Trapezium stars successfully. Nothing wrong with either of those. In deep sky imaging SCTs do sometimes tend to give large soft stars though they can resolve galaxy detail well. I don't know why this is.

Does your Celestron have a mirror lock? If so I'd use it.

There is another thing sometimes said about focusing an SCT: find out which way your mirror moves when you turn the focus knob. Which way pushes it up the tube and which way pulls it down? The old trick is to make the final focus tweak in the 'push the mirror up the tube' direction. If you focus by letting it come down the tube it may not come back quite all the way and then continue to settle downwards after you've focused.

Olly

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Thanks everyone. Lots to chew over. I took a few more blob star shots last night as it happens. On some of them was a slight blue tinge. Presumably if this was removed would sharpen them a little. Is this masking or similar technique.

All this is a bit ironic really as when I got telescope never really considered photos of nebula and now I'm obsessing about them!!

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If you don't want to spend much cash StarTools has great tools for star bloat and blue bits.

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was able to get rigel focussed nicely last night I think  finally thanks to all the great advice above and panned over to orion again for another one shot wonder at 30s iso800...

think I'm ready to start thinking about this stacking business; if i'm sticking to single images like those below - would I best aim for similar time exposures when stacking or go for a mixture; say some at 30 - 20 and 10 for arguments sake.

 

orion8 (1 of 1).jpg

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For now, go for the same exposure times.

As for your focus issues, sometimes the skies are just too poor to get decent results. A Bahtinov image will probably look allright, but when you exposure for a few seconds or more the distortions caused by the atmosphere will blob your stars all over the place, making them bigger in the process. An indication that this might be happening is the slight red/blue colour separation, an effect usually seen at high magnification on planetary views, even with top grade optics.

A good indication of the sky state is to take a look at bright stars, starting at the horizon, or as low as you can see. At the horizon, due to being viewed through a lot more atmosphere, the stars will be jumpy, flashing, colourful and twinkling. As you look higher they start to be less twinkly on good nights. Sometimes though, even at the zenith high overhead they are still blinking away, and those nights rarely produce anything worthwhile apart from practice.

Some capture software will give you a figure for star size/focus called FWHM  (Full Width Half Maximum).  If you have this, take note of the way it varies from night to night even on the same stars.  When you start stacking pictures, select the best ones, and use the very best as the reference image, this one will likely have the lowest FWHM figure.

At any rate, you are about to embark on a exciting journey :)  Steep learning curve and lots of frustration, but in the end the results are amazing and well worth the effort.

Hope you enjoy it!

Tim

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