Using a ASI120mm-s with an OAG

[Ivor]

Having restarted my interest in astrophotography I'm trying to reuse as much of my old kit as possible. I've bought a ASI2600MC and I'm trying to follow the setup in the image below except my OAG is 13.5 so fortunately the AFR IV flattener/reducer let's me move the focus plane to make up the distance 55mm. Focus appears good throught the FLT110 however I need to properly test this with a decent stack of subs. My original guding camera was the starlight express v1 and I was hoping to leverage my ASI120MM-S as a replacement but I just can't seem to get it in focus. I've tried swapping the extender with the OAG but still can't get it to focus when I move the 120mm up and down in the OAG. I feel I'm missing something obvious so if someone could steer me in the right direction I'd be grateful.

 

[newbie alert]

First get your spacing right to the imaging cam... Measure the distance between the imaging sensor to prism, this distance should be the same from prism to guide cam sensor...although the 120 may not be sensitive enough, but it's worth a try

[Starflyer]

By "starlight express v1" do you mean a Lodestar?  If so stick with that for your OAG if you can, it's way more sensitive than the 120.

[Ivor]

If I understand what you're implying correctly the guide camera needs a total back focus of 55mm as well. The back focus of the 120mm-s is 12.5mm and the prism stem is ~42mm if I then add the distance from the middle of the OAG to the back of the AFR IV it exceed 55mm.  I think I must be misunderstanding something as the ASI174MM has a backfocus of 8.5mm which doesn't add up to 55mm either.

Lodestar that's the one. I'll try the Lodestar again however the micro usb connector is temperamental hence I was considering the 120mm-s

[michael8554]

You mention a AFR IV flattener/reducer, which has this backfocus quoted in the spec:

Lens-to-chip distance : 66-86mm

So set your ASI2600MC within that range, which presumably alters the reduction      Power : 0.75x ~ 0.8x

Now measure from the centre of the prism to the chip.

Set the Lodestar or ASI120MM-S  sensor the same distance from the prism.

Michael

 

 

[tooth_dr]

14 hours ago, Ivor said:

I feel I'm missing something obvious so if someone could steer me in the right direction I'd be grateful.

I never bothered measuring mine, I just stuck it on and moved the OAG camera in and out until it focused.  The question is - does it get closer to focus when fully in or fully out on the stalk?  That will tell you if you need to add a spacer or remove one.  Assuming the main imaging train is in focus before you do this. 

Edited November 6, 2022 by tooth_dr

[Ivor]

1 hour ago, michael8554 said:

You mention a AFR IV flattener/reducer, which has this backfocus quoted in the spec:

Lens-to-chip distance : 66-86mm

So set your ASI2600MC within that range, which presumably alters the reduction      Power : 0.75x ~ 0.8x

Now measure from the centre of the prism to the chip.

Set the Lodestar or ASI120MM-S  sensor the same distance from the prism.

Michael

 

 

The AFR IV is technically great but numbering system is over complicated (I'm convinced the scaling was converted from inches at the last minute), simpliest way I had it explained was to target a total of 128.5, below is how I used to have it setup with my ST8300M. I effectively did the approach you mentioned and moved the 120mm-s up and down in the OAG but I down get any form of image just some out of focus dust bunnies, which is why I thought the 120mm-s was just too far way to acheive focus. By setting the 120mm-s at the same distance as the main camera surely that assumes the cameras have the same back focus?

 

 

 

1 hour ago, tooth_dr said:

I never bothered measuring mine, I just stuck it on and moved the OAG camera in and out until it focused.  The question is - does it get closer to focus when fully in or fully out on the stalk?  That will tell you if you need to add a spacer or remove one.  Assuming the main imaging train is in focus before you do this. 

I have tried this with the 2600mm in focus at night as well as during the day and I just get a grey image with large dust bunnies on the 120mm-s. I've tested the 120mm-s with the fisheye it comes with and I get a clear image so I know the camera isn't faulty. I thought you weren't meant to positioned the prism beyond the edge on the main imaging chip, is this not the case?

[tooth_dr]

5 hours ago, Ivor said:

I have tried this with the 2600mm in focus at night as well as during the day and I just get a grey image with large dust bunnies on the 120mm-s. I've tested the 120mm-s with the fisheye it comes with and I get a clear image so I know the camera isn't faulty. I thought you weren't meant to positioned the prism beyond the edge on the main imaging chip, is this not the case?

You shouldnt move the prism stalk once aligned with the edge of the chip, you are moving the camera on the actual stalk.  It sounds like you need more extension, can you even make up some sort of temp solution to determine approx how much you need?  IMHO the time spent on analysing figures, you could have practically solved the problem or at least come to a solution (viable one or not)

[newbie alert]

Are you using a zwo oag? as I think the original versions had a small prism size 

Also worth checking you have the prism facing the right way

[tooth_dr]

3 minutes ago, newbie alert said:

Are you using a zwo oag? as I think the original versions had a small prism size 

Also worth checking you have the prism facing the right way

That’s a very good shout, prism might be the wrong way. 

[newbie alert]

7 hours ago, tooth_dr said:

That’s a very good shout, prism might be the wrong way. 

Don't ask me how I know Adam😲

[tooth_dr]

18 minutes ago, newbie alert said:

Don't ask me how I know Adam😲

I've done it too, it looks like it should go the opposite way!

[newbie alert]

1 hour ago, tooth_dr said:

I've done it too, it looks like it should go the opposite way!

If there's a 50/ 50 guess I'm positive I'd put it the wrong way around 

[tomato]

19 hours ago, tooth_dr said:

You shouldnt move the prism stalk once aligned with the edge of the chip, you are moving the camera on the actual stalk.  It sounds like you need more extension, can you even make up some sort of temp solution to determine approx how much you need?  IMHO the time spent on analysing figures, you could have practically solved the problem or at least come to a solution (viable one or not)

I agree with the idea of creating a temporary support so you can take the camera further out to see if that's what you need to achieve focus. With the laptop next to the OAG I've even hand held the guide camera to see where the focus point is, at least then you know which way to go. As it turned out, I needed to go in more than the camera and focuser would allow, not ideal but fixed by unscrewing the protective window on the camera, although I should see if I can take the prism stalk in a little.