Getting a better pic of Orion etc.

[happy-kat]

Then just take mount out side with just camera need to fashion a bracket. Mine use vixen dove mount not sure what your mount uses. 

[SilverAstro]

The rotors of the alt. bearing are fixed to the tube, I think, with the stators, usually a drive rod and an idler bearing (??) (two idlers on 'tother side)  fixed to the azimuth box. So half the alt. bearing is removed with the tube when split.

So JOC would have to fashion a dummy middle bit of tube equivalent. EDIT  Unless,  as I said, ! attach to the tube not to its optics

 

[happy-kat]

Or just piggy back on the tube. I hadn't known the motors were arranged like that.

[SilverAstro]

  @JOC you might like this outstanding stack (2nd pic down) of 70 x 10sec taken, if I am reading right, by fixed camera on tripod (at least that is the op's op) by @StargeezerTim dunno the camera or lens etc. looking at the colours I guess astro-modded ?

[JOC]

OK, what focal length and exposure did I use?  

I don't know - I had the zoom on and had just tried to frame Orion alone in the sky with it - as it turned out because of the brightness of the screen I couldn't really tell in the view finder if I had it all in view.  I will therefore have moved the focal length, but would not have been aware that I could have made a commensurate adjustment to the f-stop as a result of changing the focal length.  I left the resulting exposure to the camera to decide - I don't know what it picked, but it felt around the 10 second mark.

Can I use my tracking Goto mount - yes, I'm sure I could, I was just experimenting to see if I could take some adequate pictures with just the DSLR on its own tripod. i.e.  A quick way of doing some interesting without the hassle of setting up the telescope.  I literally only played for 10 minutes.  I'd spotted the stars were out, but was off out myself and didn't have time to go telescoping, but didn't want to miss out on the stars entirely so I whizzed out with the camera and thought I'd see how difficult it was to take a wide-field view of the stars - the answer is clearly that it's not easy.

I have all the T-rings for the telescope attachment to the Goto Dob and have already taken single pictures of the moon using the camera directly attached to the focus unit - that's what my Avatar is.  I've tried it attached to the Baader Morpheous, but didn't get such a satisfactory result.  So far (in my very, very limited experimentation) I've had less success at taking the stars through the telescope/camera combo.

 

[happy-kat]

On your pc right click on file explore on one of the image files and look at properties this will reveal the focal length you used with the kit lens.

[SilverAstro]

2 minutes ago, JOC said:

I don't know what it picked, but it felt around the 10 second mark.

Can I use my tracking Goto mount - yes, I'm sure I could, I was just experimenting to see if I could take some adequate pictures with just the DSLR on its own tripod.

Yes indeed !

We were just thinking about the "Getting a better pic of , , " and running a bit wide in circles round it

[SilverAstro]

or look in the EXIF ( which I tried to do but the uploaded jpg has been abridged )

[happy-kat]

We're not suggesting at all to use camera with telescope. Just think you could hyjack your go to mount to keep Orion in the field of view for you using camera and camera lens.

[SilverAstro]

16 minutes ago, JOC said:

OK, what focal length and exposure did I use?  

but it felt around the 10 second mark.

Ok I found it again, you said by the 500 rule = 10sec

so that means you intended to be in the region of 55mm , tick

Two benefits then of shortening the focal length - you can increase the exposure towards 20sec -ish  =2x more light

and increase the aperture towards F3.5  = even more light

Just how much more 3.5 is over 5.6 I leave as the homework question

all we need now is less cloud and more starlight !! Probably explored this as far as we can till then ?   Good luck.

 

[JOC]

I've found a Canon utility that I'd installed on the portable it can read the EXIF on the pictures.  It says that I was using the 18-55mm lens, that I was using it at 55mm that the aperture was set for f5.6 and the automatic shutter speed was at 13 seconds (I wasn't far off - it felt around 10 seconds - for a cold night when I wasn't really paying attention that wasn't a bad guess.  I had set the camera to Aperture Priority mode and an ISO speed of 800.  

The problem I see in this statement:

16 minutes ago, SilverAstro said:

Two benefits then of shortening the focal length - you can increase the exposure towards 20sec -ish  =2x more light

is that I envisage with a fixed mount (like my tripod) that increasing the exposure time to say 20 seconds would allow too much movement in the star field and would give the stars little tails (I don't think a shorter focal length would counteract that if I understand things correctly) - I was trying for a sort of trade-off between getting enough light and not running the exposure for too long.

The point about using the mount for the camera alone - got it!  :-D

[happy-kat]

I personally use the 400 rule

So 400/55 = 8 (give or take)

I do this because some directions in the sky rotate more noticeably faster, the West and East are more forgiving in showing noticeable field rotation and keeping below 70 degrees helps too.

Star trails are a personal thing I don't tolerate them but if you do not zoom in it is amazing what you wont see.

[SilverAstro]

1 hour ago, JOC said:

 The problem I see in this statement:

"Two benefits then of shortening the focal length - you can increase the exposure towards 20sec -ish  =2x more light "

is that I envisage with a fixed mount (like my tripod) that increasing the exposure time to say 20 seconds would allow too much movement in the star field

Ah but that is exactly what the rule is for, what it is illustrating is that the shorter the fl the slower the movement across the sensor/frame/field/pic is, and thus the longer can be the time allowed before noticeable trailing begins to be apparent.

How about this version of it :

T=K/fl

where K is a Konstant , fl is the variable and T is the dependent variable = allowable exposure Time.

K is 500 or 400 or whatever your personal tolerance is upon pixel peeking your test exposure.  ( <edit :  of Orion in this case to take account of h-k's good point about the celestial sphere !)

Your 500 rule that you adopted is good because your stars are well not trailed ( but the focus is a bit odd !)

Is my case rested ?

 

[JOC]

9 hours ago, SilverAstro said:

but the focus is a bit odd

The focus is what I didn't have the time outside to work on - I had to go out and was rapidly running out of time outside - I did dash back inside 3 times and check the outputs on the PC before I got those and in the finish did the quick Google before I got what I did - it was more tricky as an exercise that I first thought.

9 hours ago, SilverAstro said:

the shorter the fl the slower the movement across the sensor/frame/field/pic is, and thus the longer can be the time allowed before noticeable trailing begins to be apparent.

My gut still doesn't like this ^^^^^  It is screaming 'ye canna change the laws of physics' - thought I suspect if you understand things this it is probably the laws of physics that make things work. LOL

Is it because the angle the light enters the lens and where it strikes the sensor is affected by the focal length? 

I've even got a piece of paper and drawn lines on it still and have problems with it.  If you draw a line across the sky from a point where an object starts it relative movement to where it finishes it.  That line will be twice as long in 20 seconds as it is in 10 seconds.  What I can't see is why changing the focal length of the camera lens affects the 'line' for want of better term that is drawn across the sensor of the camera.  I can possibly see why changing the aperture might - if I think about lines entering a hole at two opposing angles then they cross and draw a picture.  If the hole is smaller the lines have to enter at less steep angles and so the line drawn on the sensor might be shorter, but my mind is having problems with focal length being the determining factor - though possibly a longer focal length also means that the light entering has to be at a less steep angle, but I can't see the reverse being true - which is what is suggested.  Can it be explained with a drawing?  I function on visuals far easier :-D

[SilverAstro]

1 hour ago, JOC said:

 I function on visuals far easier :-D

I dine on equations So without a blackboard and swinging my arms about that you cant see :-

Do you trust the rule of 500 ? Yes because you used it !
Did it give you the right result ? Yes because you had no star trailing.
So (ignoring crop factor which is constant for you, and various things, and if or not it should actually be 500 or some other number) let us keep it simple for now

ignoring insignificant decimals!
With a (zoom)lens of 50mm fl : 500/50 = 10 sec exp
with a (zoom)lens of 40mm fl : 500/40 = 12 sec exp
with a (zoom)lens of 30mm fl : 500/30 = 17 sec exp
with a (zoom)lens of 18mm fl : 500/18 = 28 sec exp

What does that tell us ? That you can up your exposure the shorter your fl.  A similar progression would be obtained for a Rule of 400 or of 600.

Re focus, when I said "odd" it wasnt a crit. I know it was your first quick go and under the circumstances you got it pretty close !
I meant that I would have expected a slight mis-focus to result in symmetrical 'polo mints' but you seem to have curious 'C' shaped ones. Odd, perhaps an experienced APer can comment ?

 

[SilverAstro]

Lets try this,
if during your exposure time the star travels 1deg ( I know silly amount) and your lens has a field of view of 1deg it will cause a trail across the whole of your frame.
now change your field of view (by shortening its fl) to 10deg, the star will traverse just 1/10th the frame
for an even shorter lens say 50deg field, then a trail of only 1/50th of your frame results. The shorter the focal length the shorter the trail for a given exposure interval.

now do a think in pixels :

with your 1deg field long lens the star may traverse 10pixels during an exposure, but change your field of view to 10deg, the star will trail by just 1/10 of what it did before = 1pixel Yea!result!!

Howzat ?

 

[JOC]

22 minutes ago, SilverAstro said:

Lets try this,
if during your exposure time the star travels 1deg ( I know silly amount) and your lens has a field of view of 1deg it will cause a trail across the whole of your frame.
now change your field of view (by shortening its fl) to 10deg, the star will traverse just 1/10th the frame
for an even shorter lens say 50deg field, then a trail of only 1/50th of your frame results. The shorter the focal length the shorter the trail for a given exposure interval.

now do a think in pixels :

with your 1deg field long lens the star may traverse 10pixels during an exposure, but change your field of view to 10deg, the star will trail by just 1/10 of what it did before = 1pixel Yea!result!!

Howzat ?

I think I've got it.  It's the same reason I can keep the moon in the frame with the 32mm EP on the telescope, but it soon vanishes when I am up at 8mm - the closer I am (the smaller the field of view of the object) the quicker it disappears?  At higher focal lengths I am 'in closer' and so things appear to move further relative to the view I am/the camera is seeing!!  At shorter focal lengths the view is wider and the percentage taken up by the movement is lower w.r.t. the overall field of view - Yay!!

The clever bit is then the ability I guess to zoom in on that one pixel and still only see one pixel.

'C' shaped dots - could that be due to this rotational thing that goes on up there?  Stars don't just move left to right do they?  It all also makes an arc which is why my telescope whilst helpful at short exposures isn't ideal for full blown imaging

[SilverAstro]

1 hour ago, JOC said:

I think I've got it. 

whilst helpful at short exposures isn't ideal for full blown imaging

I knew weed get thur in the end !

full blown imaging ? what you mean DSOs and the like  ,,, who has been telling you porkies ! Have you been listening to the Eq mounted mob ? Tsk tsk. Lots of good images on the interweb by peeps with Dobs

It is the same principle that we have just been discussing. Sus the exposure duration for minimum field rotation (in the same way we did for fixed camera trails) typically between 20sec and 60sec, though the latter is pushing the hard end a bit,   then stack many such frames.

check out this topic

a long one but whizz through it and pick out some pics to see what can be done with Alt-Az mounts. Ok so most are using tripod mounted altaz in that topic, the same principle applies to your ground level mounted Alt-Az and there are some fab. examples with Dobs  in the Aus. forum "IceInSpace" and elsewhere.

 

[happy-kat]

C shaped dots could be evident more towards the edges of the frame and due to the quality of the optics. You can minimise it by either cropping the edges from the finished image or using a higher aperture. Wide open aperture will show all the blemishes of a lens.

[happy-kat]

Rotation causes the edge of the frame to get less and less nice stars and any nebula in the centre of the image will eventually get blurred. C shaped stars on single frame of short exposure is lens quality.

[JOC]

@SilverAstro, @happy-kat OK, I've been outside and my memory card now contains around 19 frames in both RAW and JPEG formats that look a bit like this one.  All done with that zoom lens set at 18mm, F3.5, 1600 ISO and 10 Second exposures.  I has a bash at focusing as tightly as I could on the moon as I couldn't see any stars in the viewfinder.  I believe the lens/sensor is probably not the cleanest on the planet and the moon has cast some light into the melting pot, but as an exercise in doing the next bit will 19 frames like this do?  How have I done with the learning?

[happy-kat]

Much better focus. 

At18mm you could have used 20-25 seconds exposure, for next time.

I'll need to download it to check the histogram.

[JOC]

BTW this is my sighting shot whilst I worked out where to point the camera, overall isn't it a pretty picture?  Do you like my vantage from my back garden?

[happy-kat]

Very much clear view clear sky what is there not to love

[happy-kat]

Actually take loads of those, sequator will create an image with that foreground.