Newbie puzzlement

[JOC]

I've just seen the lovely picture in the current thread of M42 - I haven't got the framing of the nebula like that yet myself - I've been closer and seen the trapezium close up.  It got me to thinking maybe I need to use a different, not so close up lens in the EP.  However, then I thought to myself.  If the T ring goes onto the camera body, and then straight onto the telescope, i.e. no lens - what provides the magnification for the celestial object?  Sorry if this is silly question.

[Davey-T]

It's a common misconception that telescopes magnify things, all they do is capture some light and bring it to a focal point, if you then insert an eyepiece it magnifies the image by spreading the light out.

A camera straight into the telescope captures the light caught by the telescope, how big and bright this appears to be depends on the aperture and focal length of the scope.

Dave

[Cornelius Varley]

Think of the telescope as a telephoto lens. The term "magnification" doesn't apply to imaging. Imagers use the terms image scale or arcsecond per pixel resolution.

[SilverAstro]

19 minutes ago, JOC said:

and then straight onto the telescope, i.e. no lens - what provides the magnification for the celestial object?  Sorry if this is silly question.

The focal length of the telescope - the longer the focal length the larger the image scale so the more of the frame will be filled with M42

You mean no eyepiece  lens ? Then see above, however : there is also a method of projecting the image from an eyepiece onto the camera sensor ( no camera lens ) to give high magnification,  is this what you have been doing so far ? )

[JOC]

Hi SilverAstro, so far I've done nothing except to start thinking about it.  More so since my T-ring arrived in the post today :-)

Davey-T, the OP arises because that fact had suddenly occurred to me, i.e. that I know the telescope doesn't essentially do the magnification - I am under the impression (crude explanation wise) that it is a light gathering tube and that it is the eyepiece lens that provides the magnification.  I must admit that I had forgotten that the telescope tube provides its own focal length - I believe on my new telescope this is 1200mm (the main mirror is 200mm) giving I assume a F6.0 focal ratio.

SilverAstro So at the moment it appears I am limited to attaching the camera body (Canon 1100D) to the T ring and then the T ring to the 2" holding accessory (it looks like a giant eyepiece holder).  No 'magnification' will be possible beyond that provided by the main telescope tube itself, in other words any image taken would be as though I was viewing the image without an eyepiece being inserted - yes?  Will this provide sufficient increase in size of the/any celestial object to make trying to take an image of anything except the moon (which is obviously nicely large) possible?

[spaceboy]

My guess is you have a 200P dobsonian telescope?? If your thinking of imaging deep sky objects with that scope your going to find it incredibly difficult unless it is the goto version and even then it's not going to be easy.

[JOC]

spaceboy, yup exactly right and you are not the first to suggest that I'm on a hiding to nothing.  However, the kit came with the camera adapter (not that I bought it for that reason) and I reasoned that since I already had the DSLR and the T ring was a relatively inexpensive accessory that I might experiment and maybe learn a little in the process of finding out what is and isn't possible - maybe I might get some shots of the moon at the very least?  In terms of DSO's from what I read it might be so successful, but yes it is the Goto (flextube) version and therefore it should find and track providing I get the stars correct when I set it up.

[Pompey Monkey]

Hi JOC, Don't be put off - you can do a lot with a tracking Dob. Take a look at his thread: 

 

[cjdawson]

Certainly, keep going, you are not on a hiding to nothing.  Sure, long exposure 15 mins autoguided shots are not going to happen. But there is a huge learning curve to astrophotography, and plenty that you can do with a decent dob before getting to a place where you might want to change kit.  The basics are the same, so you'll learn about targetting, focussing, tracking and post processing.  You should be able to do lunar and planetary without too much trouble.  DSO's, well, the thread for that is linked be @Pompey Monkey

 

Here's my laymans way to thinking about connecting a camera to the telescope.  Currently you have a DSLR, with your DSLR, you most likely have the 17-55 EFS lens that came with the camera.  The numbers 17-55 means 17mm, to 55mm focal length.  As it's a "zoom" lens, you can adjust the lens between those numbers.  As you do this the width of the field of view changes.  With the 17mm you get a wide angle image, with the 55mm you get a narrow image in comparison.  There's a chance that you'll have the next lens in the series as well, this is a 55-250mm zoom lens.  So with this one, you can get to a 250mm focal length.  Things appear to have been magnified, but this isn't the case.  All that you have done is told the lens to fill the field of view with a much smaller part of the lens glass.  This is where the F ratio comes in.  The chances are that your 17mm will be at F4.5, but when you zoom to 55mm, the F ratio will go down.   This means that the lens mechanics is blocking some of the light. With the 250mm, you'll most likely find something similar.  

Now, let's make the camera lens glass bigger, say 200mm across.  That's a large amount of glass.  Now, lets put the focal length to 1200mm with that you are still talking about an F/6 lens.   That's what will happen when you put your camera on your telescope.

 

As for how do you play with the magnification, well again you dont.  You can get a couple of devices however.  One is called a Focal Reducer, the other is a focal extender.  What they do is bend the light to simulate a shorter or longer focal length.  By playing with those you can adjust the 1200mm reading to be something else.  For example on my telescope I have an F/6.3 focal reduder, that changes my F/10 scope to F6.3 or to put that into mm, my 2000mm lens becomes a 1260mm lens.  There is a cost for these devices, as they do loose a little light as a result of the extra glass in the system.  Reflections can also happen too.  In addition to that, the field of view will be changed as well.

 

To sum it up, the longer the focal length of your "lens" (camera lens, telescope, microscope!) the more "magnification" that you experience.

 

Btw, a 2000mm lens attached directly to a telescope with a 200mm apeture will produce an image that is about the size of a full moon.  This is about the right size for DSO's that are about the size of the great orion nebula.  And that's alot of them!

[JOC]

cjdawson - I am inspired - from your description it seems I might experiment with my goto tracking dob and see what I can get.  You've put two and two together and exactly worked out which camera kit I have - yup, both of those camera lenses here.  I must admit, and despite years of photography behind me {embarrassed smiley needed} ,  I hadn't stopped to think that the apparent 'magnification' you get from a telephoto lens is just the lens bending the light to fill the FOV with less of the image - it's certainly a different and more descriptive way of looking at it. 

I should love to get a picture of something like the M42 Nebula like those in the thread that Pompey Monkey linked to.  That thread makes huge reference to stacking images of fair exposure duration.  I presume that's done using a piece of software with overlays X amount of images as layers one on top of another.  I comprehend the theory of layers, but have never experimented with them in ordinary photography (for example to make HD 3D like images from RAW files).  So does the forum have a basic - this is how you start thread?  Sort of: you need this (free?) software to do the stacking, do I need to take RAW images, they really need to be of X exposure duration, X ISO setting, you will need X number of images to be in with a chance, how do you align the images for stacking - does the software spot matching steady stars and do the overlaying itself?  Do all the exposures need taking on the same night?

It was interesting in that other thread to see folks with whole telephoto kits effectively piggy-backed onto their telescope set-ups, rather than the camera 'looking' through the eyepiece holder.  My forum picture of the moon (which I was quite pleased with) was taken using the full 250mm zoom, my sturdy tripod and a cable release, it was the only way I could get the camera steady enough.  The moon was staying put, but getting the camera to do the same was far from easy.  However, I want to try attaching my camera body to the telescope with the T ring and trying that method and I guess with the DSO's emitting such low light levels that total stillness isn't quite as critical as it was with the moon?

[SilverAstro]

3 hours ago, JOC said:

Hi SilverAstro, so far I've done nothing except to start thinking about it.  More so since my T-ring arrived in the post today :-)

Ah sorry, my misunerthingy , I didnt read the ref to the trapezium correctly, thought you were already framing it in your camera perhaps by eyepiece projection or by the afocal method. Three methods - (A) Prime Focus, where you just use the telescope main objective and the camera body - (B) Eyepiece Projection,  telescope with its eyepiece, and the camera body only  - (C) Afocal, telescope with eyepiece, and camera with its camera lens.

Lots of excellent advice and pointers by the others here, especially to the No-Eq topic where they are making many GoTo guided exposures of DSOs typically of between about 15sec and 60sec each exp. and yes you are right, software (Deep Sky Stacker a good free example) is used to stack ( add ) them together to reveal the DSO.

 

 

 

[ronin]

As the scope sounds to be the goto+tracking variant then you can get some images but you are a bit time limited. Although you are tracking there is still field rotation to take into account, and that really means that when stacking there comes a point when the software cannot perform sufficent alignment of the stars.

So pick reasonably bright objects, maybe use a bit higher ISO, simply to reduce the first to last expoisure times. Sort of 800 not 400, unsure about 1600.

If you are thinking that he moon is sort of a limit owing to size then actually M42, Orion Nebula, is bigger then the moon, about twice the size. Strange action of eye+brain is that it interprets brighter as bigger. Being honest I know about this but the moon still seems bigger, a lot bigger.

If you have a 200P at f/5 then that is a 1000mm focal length, taking a simple value of 1 degree for M42 that makes an image of:

S = 1000* tan(1) = 17.5mm, so actually quite big on a DSLR sensor, moon would be around half that, say 8mm diameter. Pleiades is another that is around 1 degree across so that come up well.

[JOC]

Hi Ronin, that's trigonometry!!  I've never, ever seen a practical use for trigonometry since first learning it in school many, many moons ago.  My kids often want to know why they have to learn certain bits of maths and trigonometry has been queried - I now have a practical example - so many thanks for that as well as the other info.  My flextube 200P is a 1200mm focal length F6 (200mm mirror) so my sum is S=1200*tan(1) = 20.9mm so a wee bit bigger on the sensor than even your example. 

I must admit I tend to think of the moon as being miles larger in the sky than the orion nebula, surely it must be......it is so much closer!  It is really hard to believe M42 is the same size or even larger.

In fact your two targets M42 and Pleiades sound two good starting points when I finally get some decent weather - they are the only two objects that I have found so far!!

[cjdawson]

3 hours ago, JOC said:

cjdawson - I am inspired - from your description it seems I might experiment with my goto tracking dob and see what I can get.  You've put two and two together and exactly worked out which camera kit I have - yup, both of those camera lenses here.  I must admit, and despite years of photography behind me {embarrassed smiley needed} ,  I hadn't stopped to think that the apparent 'magnification' you get from a telephoto lens is just the lens bending the light to fill the FOV with less of the image - it's certainly a different and more descriptive way of looking at it. 

I should love to get a picture of something like the M42 Nebula like those in the thread that Pompey Monkey linked to.  That thread makes huge reference to stacking images of fair exposure duration.  I presume that's done using a piece of software with overlays X amount of images as layers one on top of another.  I comprehend the theory of layers, but have never experimented with them in ordinary photography (for example to make HD 3D like images from RAW files).  So does the forum have a basic - this is how you start thread?  Sort of: you need this (free?) software to do the stacking, do I need to take RAW images, they really need to be of X exposure duration, X ISO setting, you will need X number of images to be in with a chance, how do you align the images for stacking - does the software spot matching steady stars and do the overlaying itself?  Do all the exposures need taking on the same night?

It was interesting in that other thread to see folks with whole telephoto kits effectively piggy-backed onto their telescope set-ups, rather than the camera 'looking' through the eyepiece holder.  My forum picture of the moon (which I was quite pleased with) was taken using the full 250mm zoom, my sturdy tripod and a cable release, it was the only way I could get the camera steady enough.  The moon was staying put, but getting the camera to do the same was far from easy.  However, I want to try attaching my camera body to the telescope with the T ring and trying that method and I guess with the DSO's emitting such low light levels that total stillness isn't quite as critical as it was with the moon?

@JOC Glad you are inspired.  shoot in RAW.  Turn the setting on, and forget that jpg exists.  Now that's out of the way. There are several programs that will do stacking.  A free one to start with is Deep Sky Stacker.  What it will do is take the images (we call them subs, short for sub frame), align them, and stack them.  The images do not all need to be taken on the same night.  For best results you need to use a technique call plate solving to get the scope to line up exactly on a different night.  That said, you will also have some "field rotation" in your images, DSS I think can cope with that too.

Once you have stacked the image in DSS, save the output tiff, and load it into photoshop to do the levels and curves... not to mention the other processing that you will end up doing.

As others have said, you'll be limited on your exposure time, so grab more sub frames to make up the difference.  More the merrier, but there are diminishing returns that you need to factor in.

[SilverAstro]

Yes ! RAW and remember to keep all your RAWs, for in the future you will be able to go back and re-process them all with all newly found skills, having not lost anything meanwhile in the lossy jpg format !

Field rotation - interesting point, I dont mean to be picky, I know that DSS can handle field rotation between subs, (and all sorts of other misalignments), but can it handle (some?) field rotation within a sub as well ? just curious, I have read about processing algorithms that can minimize focus and tracking errors in frames so I suppose there may be some de-rotation possibilities within subs to allow for longer alt-az exposures ???

photoshop - or practice in GIMP (free)  ! sorry, fan of freeware here, does it show ?!

[JOC]

Given the comments above I've already noticed that when I view through the telescope whilst it is 'tracking', that in the finish I need to move telescope a smidgen myself (my system apparently copes with this) to keep things in the centre of the eyepiece, I assume this is due to this 'field rotation' issue referred to above and in several other threads that I've read.  When I take these images for stacking should I make this manual adjustment between sub frames (to bring the focal object back to what I think visually looks like dead centre) or does the stacking software pick out the patterns of the surrounding stars and do the alignment itself? 

[D4N]

44 minutes ago, JOC said:

Given the comments above I've already noticed that when I view through the telescope whilst it is 'tracking', that in the finish I need to move telescope a smidgen myself (my system apparently copes with this) to keep things in the centre of the eyepiece, I assume this is due to this 'field rotation' issue referred to above and in several other threads that I've read.  When I take these images for stacking should I make this manual adjustment between sub frames (to bring the focal object back to what I think visually looks like dead centre) or does the stacking software pick out the patterns of the surrounding stars and do the alignment itself? 

What you are seeing is pointing inaccuracy in the goto system rather than field rotation.

It is actually quite good if the target moves a little bit between subs as this provides dither, dithering causes the hot pixels to appear to move compared to the stars so they can be removed with kappa sigma clipping.

The stacking software will deal with it just make sure your target is somewhere near the middle where image correction is best.

6 hours ago, SilverAstro said:

Yes ! RAW and remember to keep all your RAWs, for in the future you will be able to go back and re-process them all with all newly found skills, having not lost anything meanwhile in the lossy jpg format !

Field rotation - interesting point, I dont mean to be picky, I know that DSS can handle field rotation between subs, (and all sorts of other misalignments), but can it handle (some?) field rotation within a sub as well ? just curious, I have read about processing algorithms that can minimize focus and tracking errors in frames so I suppose there may be some de-rotation possibilities within subs to allow for longer alt-az exposures ???

photoshop - or practice in GIMP (free)  ! sorry, fan of freeware here, does it show ?!

DSS is quite bad at dealing with star trails, it will often fail to stack images if the stars aren't round.

Startools has the ability to fix non-round stars.

 

I like Gimp, at least the 32bit version is pretty good.  Well I like a lot of image processing software so it might be fairer to say I just don't like Adobe's pricing structure

[happy-kat]

A altaz mount which you are using moves in tiny left right up down movements this means the object stays in the field of view but is rotating because the earth is rotating. This rotation is what limits the imaging exposure length. 

[cjdawson]

Things can move in your eyepiece for several reasons.

 

1. Field rotation, To see this. Look a star pattern in the east at the beginning of a session.  Make a note of how it appears in the eyepiece, relative to the bottom of your telescope.    Now, wait a few hours and observe the same object again, you will notice that after 6 hours it will have rotated 90°. This rotation is caused by the motion of the stars relative to your scope.  To stop it happening there are two options.... a. get a field derotator (it's a device that moves your camera so that it keeps the image on the same orientation compared with the image sensor) b. switch to an Equatorial mount, these mounts adjust the orientation of the mount so that they can take the object without any rotation in the field of view.     An EQ mount is only really required for imaging.

 

2. tracking issues.

These can be broken down into several types.

1. Incorrect tracking speed

2. Periodic error

3. Poor alignment

 

The Poor alignment, and incorrect tracking speed can be corrected easily.  Poor alignment can be fixed by double checking your setup and adjusting as needed, rerunning alignment routines until everything is good.  The incorrect tracking is more a calibration issue than anything else.  A good power supply is normally a big help there.

Periodic error.  This is a mechanical issue, and there is no simple solution.  For those with an EQ mount, the solution is to employ autoguiding techniques, this won't work for and AltAz setup (well, not the same anyway)  Autoguiding will take into account things slight corrections in alignment, wobble from Periodic error and also movement due to atmospheric refraction.    There is another way around this issue too, short exposures.

[JOC]

Many thanks cjdawson, happy-kat and D4N.  It is fair to say that I am still getting to grips with all my new kit, we've only had a couple of short sessions so far (Christmas busy-ness, clouds, and getting somewhat cold (temps have been pitching well below zero here and my fire warm inviting decorated home has seemed quite attractive), so I am still working on sorting out the goto/tracking system (this means a certain lack of ability in finding these named stars to calibrate on - I now have some drawings to work from and I'm a quick study) and finding good areas in the garden to view from (need to try out my new water butt stand to see if it fits the base as intended!) - I am hopeful that, with a little more practice, I can get I set up quicker and get some better tracking out of it.  Once I've mastered it in the garden and if I can get hold of a portable power source, I'll see if I can find a point in my field - a shift of about 130m will make a world of difference to a darker viewing location. 

To anyone reading you might think 'well if she hasn't mastered basic viewing why does she want to try something even more difficult'?  Well, I guess if I'm being honest, there is a certain amount of 'because I can' in any answer.  However, I am also aware that it doesn't help to 'run before you can walk'.  So at the moment I'm in the learning curve phase.  Ultimately I would love to bring some images back to the house to show the folks inside what I've been looking at esp. if it was possible to get some images like those from similar set-ups in the other thread.  I am also finding precious little time at the moment to go outside and use the kit (as already noted) so I am spending my time inside reading and finding out.  Now there is new software to experiment with (I happen to rather like 'neat' clever software) if I can get a passable set-up running outside and attach my camera I might be able to take some trial exposures and once I've got appropriate files to play with to try the action of stacking these 'subs' just to find out how it all might work/how difficult its all likely to be - it's another thing I can then experiment with when its cloudy.  At the moment I'm not aiming for a finished picture, but its all part of a general learning curve and I like the notion that I might ultimately be able to 'do' something with a practical end-point with the telescope; more than just enjoying the 'here and now' moment of watching the stars. After all I think I've picked a nice bit of kit as a 'first ever' starting telescope and it just appeals to me to find out what might be possible with it.

So the background to this is that it's the astronomy that is new to me.  SLR and DSLR photography I've done for over 30 years, I love IT and 'clever' software (I'm a 'tinkerer' and generally learn to use software by 'tinkering' with it, hitting problems then using the internet and help screeens to find and implement solutions).  I too am a fan of freeware (there are some lovely folks out there), I have GIMP downloaded, but must confess that I get on incredibly well with the online Pixlr Editor for adjusting levels - I'm quite new to electronic photo manipulation generally though - layers I comprehend through exposure doing some GIS work, but I've never played with HD photography and RAW files with it.  Oh, and I've got a science background, but more in Chemistry than physics!!!  I thought with a little background you might be able to continue to provide more focussed advice!   :-D  I will of course be back after a suitable time interval and will be pleased to show you what I've done with all your contributions!

[cjdawson]

Glad to be of help.  A wise man said on this forum recently that there's nothing difficult about any one part of Astrophotography.  The difficulty comes from the fact that you have to learn to do all of the parts at the same time.  The complexity of getting everything right makes it a challenge.    Sounds to me that you have your head screwed on, and are learning to walk before you can run.  It's great to have the ambition for what you intend to do, it will help shape the decisions on the next upgrades, and the parts that you might want to get hold of in order to achieve that goal.

[JOC]

OK, well you gave me the confidence to attach the camera and just see if I could just make the photography side of things work, i.e. using the telescope as a telephoto lens.  Well we had clear skis late this afternoon and today's half moon was sunlit, so folks as my very first sojourn into photography + telescope and to show that I've been paying attention I give you the result of today's beginners lesson - 'The moon in the late afternoon' (about 16:18), with the telescope in the signature and my Canon T3 Rebel (1100D) and a wireless cable release.  I focussed through open aperture mode on the camera with it zoomed in electronically twice (but the focusser on the telescope with its twistable 'wheels' seems a little 'coarse' for truly accurate focussing).

I'm OK with the result - I don't know if it is as good as the one I use as my forum picture which I took as noted above with the camera, 250mm zoom, steady tripod and wireless shutter release, but at least I've proved all the kit joins up OK and that the theory of taking an image through the telescope works just as you promised it would  Hopefully this is first of something a little bit more ambitious next time around.

[JOC]

I also ran the autocorrect for white balance from microsoft's standard picture editor just for a bit of a speedy solution - it adds a bit more contrast which may improve things

 

[cjdawson]

YAYYYYYYYY well done for having the courage to give it a try.

Your first lunar image is fantastic.   Personally, I prefer the one without the contrast enhancement.  Feels like a more natural shot.  But hey, everyone is a critic ;-)

As we're in winter, give The Great Orion Nebula a go.  you'll need to do a long exposure, I'd recommend trying a 15 second exposure.  at ISO 800, then try at ISO 1600.   If they come out good.  Try taking several photos all of the same exposure length.  After that we can talk you thought stacking the images, and you'll be amazed at the result

[JOC]

Hi cjdawson, the great Orion nebula is a subject I am getting quite good at finding, but all I see with my eyes is a fuzzy pale grey mass with the trapezium in it.  I would love to have a go and see how this 'stacking' works and if the camera ultimately sees more than I do.  So the next decently clear evening I'll do as you suggest and report back when I have the files ready.  FWIW I found the double star Castor tonight and was really surprised at how long the tracking held the image in the dead centre of the frame for, it was certainly in the order of tens of minutes, so exposures totted up in tens of seconds certainly appear to be possible.  I shall be quite interested to see the final results.  Many thanks for all the support everyone.