ASI290MC cannot achieve focus with Barlow lens

[MylesGibson]

Hopefully someone can help me out. I recently got a ZWO ASI290MC camera and decided to give it a go tonight, between cloud breaks as I was eager to give it a go, and just get it set up and working!

I use an Altair Starwave 102ED-R as my scope and I need a tube extension on the ZWO camera to achieve focus. Not a problem, go that working fine. I aimed at the centre of the moon and put my barlow lens in. Could not achieve focus at all.

I then tried using the tube extension. Still no focus. I then unscrewed the lens from the barlow and attached it directly onto the nosepiece of the camera. Still no focus. Each time, I changed the gain and exposure to make sure the target would be bright enough and adjusted the focuser all the way in and out and just could not achieve focus with the barlow lens.

Anyone got any ideas how I can do this?

[vlaiv]

Barlow lens usually moves focus plane further out so you would need additional extension to reach focus.

You can check this by aiming scope at the bright star and using longer exposure. Star will be a circle rather than point. Take a shot and note how big that circle is. Move focus a bit and observe how star diameter changes. If it is getting bigger you are "going the wrong way" with focuser, but if it's getting smaller - you are moving focuser in proper direction. Continue doing so until you turn circle into a point.

If you run out of outward focus travel - you'll need additional extender to make it work.

I highly doubt that you have issue with inward focus travel - but that can happen when using focal reducer rather than barlow lens, as focal reducers move focal plane inward rather than outward.

If you don't have suitable extender, you could perhaps try with diagonal mirror? I don't recommend imaging with diagonal mirror all the time, but for testing purposes it will add quite a bit of optical path - about 60-70mm for 1.25" and about 100-110mm for 2" version. That should give you plenty of distance and you should be able to reach focus with barlow.

[MylesGibson]

@vlaiv

I've already used a focal extender. I've also put the barlow lens on the end of the extender to push it even further out, still unable to get focus

[vlaiv]

9 hours ago, MylesGibson said:

@vlaiv

I've already used a focal extender. I've also put the barlow lens on the end of the extender to push it even further out, still unable to get focus

Have you tried to figure out if you are missing out focus travel or in focus travel - trick with the bright star?

What telescope is this?

[Pete Presland]

Have you tried it with your diagonal in position?

[Neil H]

Hi use the ASL224MC to get focus with a Barlow I need an extra extension on top of the 50mm I have without the Barlow I use the 50mm but have to remove the 4mm eyepiece holder as I don't have enough back focus for camera 

Your camera is 12.5 mm focal length so try pulling the camera out of the tube and see if it gets focus with Barlow if it does you need an extension tube 

Edited June 29, 2020 by Neil H

[MylesGibson]

21 hours ago, vlaiv said:

Have you tried to figure out if you are missing out focus travel or in focus travel - trick with the bright star?

What telescope is this?

Unfortunately since testing this out, all I have had is cloud so haven't been able to try this

20 hours ago, Pete Presland said:

Have you tried it with your diagonal in position?

Not yet. I can give that a go with the barlow and extension too!

18 hours ago, Neil H said:

Hi use the ASL224MC to get focus with a Barlow I need an extra extension on top of the 50mm I have without the Barlow I use the 50mm but have to remove the 4mm eyepiece holder as I don't have enough back focus for camera 

Your camera is 12.5 mm focal length so try pulling the camera out of the tube and see if it gets focus with Barlow if it does you need an extension tube 

So you need 2 extension tubes? And what 4mm eyepiece holder? Thus far, any pulling out does not achieve focus and I am already using an extension tube

[Pete Presland]

When i was doing some "field of view" tests the other week for the Venus/Moon conjunction (which was completely cloud out). I could not manage to get focus with my Helios 120mm and ASI290MM/DMK21au618 i could not get focus with either without using a diagonal or extension tube.

[michael8554]

Hi Myles

The 102ED-R has a 714mm focal length.

The sensor in the ASI290MC is 12.5mm in from the end of the body.

So with a ruler measure 714 + 12.5 = 726.5mm from the front element of the OTA.

That's roughly where the sensor needs to be located.

Measure the distance to that point twice from the end of the focuser  - once fully in, once fully out.

Pick a figure about halfway that can be filled by commonly available sizes of spacers/extensions.

That's what will work without a Barlow of course.

Michael

[Neil H]

Hi my visual set up is focuser ( low profile) 50mm extension and a self centering eyepiece holder which is about 4/5 mm on top of the extension , when I use the camera I use the 50mm extension with eyepiece holder removed so I have a 35 mm extension coming so I can use this instead of the 50mm so I have more back focus with the self centering eyepiece holder, but when I tested with the Barlow I could not focus till I pulled camera out so may land up using both extension but clouds have stopped play for now so testing is on hold 

I think I may get an old loo roll tube to block stray light and slide this in and out of 50mm extension so when I get focus I can mark the tube to work out length of extension is best 

 

[MylesGibson]

So basically, it is sounding like I will need an extension tube on top of the extension tube!