Main camera can't focus...help needed please

[dazzystar]

Hi All,

On my ASI183MC fitted to my ST102 on an EQ3 Pro, I'm trying to focus on a distant object this morning to test it out and all I'm getting is a kind of green / white image which is out of focus (see pic below). I've removed the camera and can see the object I want in the distance. Can anyone offer some advice please?

Cheers
Daz

[ONIKKINEN]

Looking at your settings its difficult to tell whether you would be in focus or not. You have a tiny little area on the sensor ROI active so the field of view here is much tighter than you think. You are also over-exposing quite a lot. Green peak is oversaturated.

Use the full sensor and drop your exposuretime until you get a proper exposure and it will be easier to tell whats going on.

And the green colour cast is to be expected, since you are using a camera that has twice the number of green pixels compared to red and blue so dont worry about it.

[dazzystar]

Thanks so much. I've adjusted those settings and here's what I'm getting now. I've tried rolling the focus tube in and out. The object (a small building) is visible when I remove the camera. I don't know what settings a proper exposure should be.

 

 

 

[ONIKKINEN]

21 minutes ago, dazzystar said:

Thanks so much. I've adjusted those settings and here's what I'm getting now. I've tried rolling the focus tube in and out. The object (a small building) is visible when I remove the camera. I don't know what settings a proper exposure should be.

 

 

 

Now that its not saturated it doesn't matter nearly as much anymore, if there was detail it would be visible here. You can see your exposure is usable now as none of the histogram peaks are to the right edge (saturated).

If you dont see any difference when rolling the focuser in and out you are probably far out of the focal plane and so this difference is difficult to see. How is the camera attached? If its attached at the same place as an eyepiece, it should reach focus at roughly the same place as that eyepiece. If you have taken the diagonal off, you will need to have some kind of extension between the focuser and your camera to replace the lost light path of the diagonal.

[dazzystar]

Thanks again for the reply. The camera is in place of an eyepiece and there's no diagonal. Should I put an eyepiece in (without a diagonal) and focus (or try!) and then replace it with the camera to see?

[ONIKKINEN]

2 minutes ago, dazzystar said:

Thanks again for the reply. The camera is in place of an eyepiece and there's no diagonal. Should I put an eyepiece in (without a diagonal) and focus (or try!) and then replace it with the camera to see?

The eyepiece wont reach focus either without the diagonal. Refractors have long backfocus distances behind the scope to give enough room for the diagonal so now without one there is no hope of reaching focus without extensions. You will need an extension that is the same length as your diagonals light path, now what that length is i dont know (since i dont have this scope) but you could try to measure it roughly and get one that is closest to that. I think it should be somewhere around 100mm? But do check this yourself before buying something.

[ONIKKINEN]

Or put the camera in the diagonal, actually?

Not sure if the focuser in the ST102 likes to have a diagonal and a camera attached to it though, but you could try to see if there is an image.

[dazzystar]

I put the diagonal in and an EP then focused. I then replaced the EP with the camera and at least can see something now (pic below). I've had to wind the focuser almost all the way in to get it into some kind of focus. Will I be okay to remove the diagonal and just have the camera without this or the nose when I come to use it on stars?

[dazzystar]

Here's the pic now!

[ONIKKINEN]

2 hours ago, dazzystar said:

I put the diagonal in and an EP then focused. I then replaced the EP with the camera and at least can see something now (pic below). I've had to wind the focuser almost all the way in to get it into some kind of focus. Will I be okay to remove the diagonal and just have the camera without this or the nose when I come to use it on stars?

You will not be able to get focus on anything without enough backspacing. So either the diagonal or an extension piece in place of the diagonal.

Would be better to ditch the diagonal and get the extender, as a poor quality diagonal is just an extra source of optical aberrations that is really not needed.

[Cosmic Geoff]

A picture of your intended setup would be useful.  I have a Startravel ST102 and find a planetary camera will come into focus with a diagonal or an extender.  To focus a DSLR, a T2 extender about 2" long  is needed.

[dazzystar]

Thanks guys. I really wish I knew about this months ago. I could have at least had the right equipment here for when we had clear skies. 

Here's an image of my setup below. If someone could tell me what extender I need or how to calculate which one I'd need that would be great!

 

[dazzystar]

and I'm not certain if this is relevant, but I had to wind the focuser almost all the way in earlier to get that in-focus shot. That was with the diagonal installed.

[fozzybear]

20 minutes ago, dazzystar said:

and I'm not certain if this is relevant, but I had to wind the focuser almost all the way in earlier to get that in-focus shot. That was with the diagonal installed.

Daz you are going to need at least 50mm extension attached to the focus tube up to you either threaded or push fit.

[dazzystar]

Thanks @fozzybear

Can you explain why I need it please and how the length is calculated please?

Cheers

Daz

[Cosmic Geoff]

40 minutes ago, dazzystar said:

Can you explain why I need it please and how the length is calculated please?

Short answer: because you do... your scope was not specificically designed for attaching a ASI183MC camera - it was designed for use with a diagonal and an eyepiece.

If your experiments so far don't indicate what size of extender you need to get within the focuser's range, you could buy a set of T2 estenders, eg this one: https://www.svbony.com/svbony-extension-tube-M42/

I bought a set of Svbony extenders which may not have been this exact one.  One of these extenders or a combination of them should get you sorted.  I assume your camera has a T2 thread if you unscrew the nosepiece (if it has a nosepiece).

[dazzystar]

Thanks. I wonder if I can 3D print something?

[michael8554]

Hi Daz

The ST102 has 500mm FL, so the ASI183MC sensor has to be 500mm from the objective lenses.

The ASI 183MC has 17.5mm Flange Distance.

500 - 17.5 = 482.5mm

Take off the diagonal and put the focuser in mid range.

Roughly measure 482mm from the scope objective.

That's where the end of the camera's M42 flange needs to be.

Measure from there back to the end of the focuser.

That's how much spacer you need.

Buy the nearest stock spacer, the focuser will take up the difference.

Michael

 

[dazzystar]

Thanks Michael for the reply. When you say 'Measure from there back to the end of the focuser.' What do you mean to the end of the focuser?

[michael8554]

You have placed the ST102 with the focuser at mid point.

And placed the front of the ASI 183MC about 482mm from the ST102 objective.

There's probably a large gap between the camera and the end of the focuser.

I don't have a ST102 or a ASI 183MC, so it's up to you to work out how to fill that gap.

Michael;