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groberts

No image, what am I dong wrong?

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I started astronomy about a year ago with an EQ3-2 mount and Skywatcher 150PL and have had some success imaging using a ZWO webcam and more recently a Canon 700D DSLR with this scope.  I have recently upgraded my kit with an EQ6 mount and William Optics GT81 refractor, with the aim of moving onto DSO imaging.

 

The new mount is a dream to use and although my alignment is far from perfect, the mount has ‘found’ and tracked stars and clusters quite well. Likewise, the WO GT81 provides an outstanding visual image, with which I am very pleased. Unfortunately that is where the good news ends, imaging (or lack of it) is a disaster, as I have so far been completely unable to capture any sort of image – even a bad one!     

In order to use the aforementioned DSLR to the scope I also purchased a WO field flattener at the same time, which came in three parts: (i) male T camera adapter, (ii) Flat-6A Adapter (iii) Flattener / focal reducer x0.8 for F6A APO. Like all the WO items it came with no instructions so I have presumed each of these is screwed together and then fixed to the camera (picture attached): I slid this set-up straight into the focus tube.

Using this set-up, straight into my computer and alternatively using Live View on the camera, I have been completely unable to get any sort of image, what am I doing wrong?

As a good starter subject at this time of the year I have been trying to photograph M13, which was easily and clearly located by the EQ6 providing a good visual result. Even with the ISO turned up I have been unable to see any discernable image on the camera or on the computer, making any sort of focusing impossible and certainly no picture: if the object were focussed visually would that same setting also work for the camera?

I am convinced I must be doing something fundamentally wrong, perhaps the camera / flattener / focus set-up?      

post-34728-0-33527900-1406281517_thumb.j

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It's probably a focussing issue. The camera attached to the telescope will focus at a different point to an eyepiece attached to a diagonal.

  • Like 2

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could be with the flattener on camera it all doesn't come to focus,maybe use an extension tube to bring camera out a bit more

if you don't have an extension tube,try in daytime (pointed away from sun) to bring camera into focus,dont tighten camera to scope

and lift it slightly above focusser,if you start to get something,then an extension tube is needed

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Where focus is with the eyepiece is not going to be the same as with the camera unfortunately. I have also just started in using a flattener with a similar sized refractor and DSLR and the flattener demands pretty precise distance between flattener + DSLR sensor to get it right as well as overall focus using the focuser. What I would do (and it is a bit painstaking) is to not use live view, but to take 1 or 2 second exposures with the highest ISO (unless this saturates the image) and move in increments from focuser wound tight in, to pulled right out. If this still reveals nothing, then put in the middle and start adjusting the distance the camera sensor is away from the flattener - ie start by making the distance as small as possible. Most flattener -> sensor distances are around 55mm from what I understand, and I had to actually put some additional distance into my setup for proper alignment of this section.

Dark art indeed and good luck.

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The problem is that unless you are looking at a very bright object you will see nothing if the image is significantly out of focus.  If you can set up in the daytime with a view of the distant horizon you can get yourself fairly close and mark that point on the focuser.  When it comes to the night's session don't use the EP to align.  First of all find the brightest star you can e.g. vega.  Use the rough focus point you marked previously and you should hopefully see vega provided your finder scope is well aligned.  Nail the focus, spend a lot of time getting this right!.  Now run your alignment routine using the camera ideally with laptop.  You can also drift align using the camera and there is software to help with this.

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I would suspect that something has to be added to the optical path or removed from what is already there.

I "think" the normal is to remove the diagonal and add an extension, however I think the extension is shorter then the optical path of the diagonal.

You really do need something bright to get everything at a good starting position.

That big bright thing that every DSO imager complains about called the Moon is ideal, however when doing DSO imaging you will forget this and complain about it. :grin: :grin: :grin:

Is there a specific order that bits need to be attachecd?

You may get a quicker and specific reply by another post titled GT-81 Imaging Help Reqd, that way people that use the GT-81 who have experienced the same would likely give an answer immediatly.

Edited by ronin

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Use the bright moon one night to focus on. Best way to get an idea what the issue is. If the camera has live view use it.

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But it's forecast to be clear tonight and there isn't a moon!!

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Always found it best to learn to walk before I run so would start with camera, T ring and adaptor and leave out flattener / reducer until you get that working but you may well need an extension.

Also a good idea to muck about in the daytime to see what's needed to get eyepiece and camera parfocal.

Dave

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I should think that with the addition of the filter wheel you're running out of backfocus as the latest WO flat 6A11 with built in adjustment should normally cope with a DSLR.

Annoying that manufacturers still have an obsession with designing stuff around DSLRs.

I'll have a quick look at the ASI183 spec sheet to see what it says about back focus.

Dave

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I have the same FR WO x 0.8 and the back focus needs to be 55 - 57mm which is the distance from the sensor on the camera to the rear of the FR (not including the bit that screws in).  This is critical to getting a good flat field and not having strange shapes in the corners.  If the distance is too short you will need an additional spacer in the middle.  Looking back at some old info from when I used to use a Canon DSLR, the sensor to the front of the T ring is already 55mm, so looks like you are already OK with this.  

Having said that, the distance should not affect your ability to get an image even if you did have the wrong spacing.

I have found seeing anything on live view with a DSLR VERY difficult unless you are almost in focus and then you will only see the brightest stars.  So the knack is to focus on a bright star and use a Bahtinov mask in live view to check the focus.

Bahtinov%20mask.jpg?attachauth=ANoY7cpQ-

When in focus, the central spoke of this split star should be exactly in the middle.

Bahtinov%20split%20star.jpg?attachauth=A

My recommendation is to use the Moon initially as this is bright enough to see even when out of focus.  Focus on the Moon and then when you have that in focus, slew to a bright star which you should then be able to see and fine tune the focus.  Take note the position of the drawtube and perhaps even mark a line on it, so when you go back to it subsequent times you have a pretty good idea where focus is.

I hope this helps.  If you can't get the spacing spot on initially, then you'll have to crop the edges off the image to get rid of the mishapen stars in the corners.

HTH

Carole 

Edited by carastro

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