Focus Issue with ASI 174 mini and OAG-L: Page 2

Focus Issue with ASI 174 mini and OAG-L

Dinol

Today I had time to build an artificial star and start to postiion the OAG-L prism such that the imaging cam and the main cam are focused at the same time. This time I put the prism as far out as possible. In that position the guide cam comes in focus at the 4.5 position of the OAG focuser. That should give me enough leeway to always allow focusing the guide cam.

BTW, all 'extensions' had already been removed from the 174 mini.

Obviously it will be cloudy the next nights, so I have no idea when I will be able to test this...

pmodi

I just got an OAG-L and a 5 position ZWO filter wheel. I'm using ASI2600MC Pro and ASI 220 mini. After Installing everything, I have achieved focus on my main camera, but ASI 220 is pushed all the way in, and I can just barely get it in focus (daytime pointing at a hill ₃ miles away) with helical focuser down to zero. I tried adjusting the prism stalk, but it is as far down it can go. I haven't had a chance to try it at night due to cloudy weather.

My concern is I do not have a lot of wiggle room in current setup. Please advise.

w7ay

pmodi I tried adjusting the prism stalk, but it is as far down it can go.

You want to go the other way -- up. I.e., hypotenuse of a right angle triangle is smaller than the sum of the two sides. So, as the stalk is pulled all the way out (see @Dinol's post from Sept 2023), you have the shortest optical path from your OTA to the guide camera sensor.

If that still does not get you enough, add a 1mm or so shim to the main camera (i.e., between OAG and the main camera flanges). It will make your guide camera sensor appear (relatively) closer. Be sure to remove the same amount from any backfocus spacing if your OTA requires a strict backfocus distance.

Chen

pmodi

w7ay Thank you. That worked.

Lymelight

w7ay it seems Dinol and pmodi had the same issue, ie guide camera fully seated and barely achieving focus, but for Dinol you said extend the prism lower and for pmodi you said "You want to go the other way-- up". That confused me.

I have the same issue using a 120MM Mini. I am fully seated and just barely near focus with teh helical on "0". My intention is to lower the prism. Of course, it's easy enough to also go the other way and iterate, but as you know that entails dismounting everything and remounting it for every trial. Dust, cross-thread risk, and time wasting.

Could you confirm please: looking in through the front to the 2600 sensor, do I need the prism down further or up higher? Would ZWO ever consider a stalk that was adjustable from the outside with a locking set screw?

Thank you,
Alan

PS I've also read that a 174MM has an additional 6mm of play for achieving focus, 12.5mm vs 6.5mm, so if correct that is another solution for me and others...

Tech@ZWO

Lymelight The point to make the guide camera and main camera both get focus is to ensure the distance from the prism to the guide sensor is the same as the distance from the prism to main sensor.
In this case, consider of the the length of itself, need to add enough adapter between the OAG-L and the main camera，refer to our 55mm back focus solution, the distanc from the OAG-L to the main sensor should be around 32.5mm, if distance on your device is less than this, it may cause the guide camera won't be focus.
If it reaches this or longer than, mostly due to the prism being too low, and needs to move the prism up higher.
Before adjusting the prism, you can also directly pull the mini camera to verify.
Here is a guide on how to adjust the prism.

Lymelight

Your own sites show how to preserve 55mm using the OAG-L and the Adapter into the 2600 with its 17.5mm setback. I have followed this EXACTLY. Prior to inserting the OAG-L in teh system, I had the 16.5mm and the 21mm extenders. When I added an M42 filter drawer, I swapped it 1:1 with the 21mm extender. With the OAG-L, I had to dump the filter drawer and use the 15mm Adapter. In every case, that added to 55mm. Where does 32.5MM now come from?

While helping me understand that, I had already gone back out in daytime and
(1) loosened the screw, and found my prism was indeed fully down, it cannot extend down anymore and
(2) verified I couldn't focus the 120MM; lifting it in the barrel only made it worse and
(3) with the prism down this far I shot flats and have definite clipping and
(4) I lifted the prism up to eliminate the clipping and that made the 120MM focus even worse, ie, now, fully seated the objects weren't even close to in focus as they were when the prism was fully down.

My full image train is:
Meade 14" ACF 2855mm
Optec 0.62x reducer
Optec No 19384 M48-Adapter for 55mm Back-Focus
ZWO OAG-L with 120MM Mini inserted fully seated, helical at "0"
ZWO M54-OAG-L Adapter
ZWO ASI2600MC Pro

Where have I gone wrong?

Lymelight

Lymelight Answered my own question re: the 32.5mm: that is the M54 Adapter (15mm) + the 17.5mm in the ASI2600 itself. Yes, that is my setup.

Lymelight

Prism down (clipped) and prism lifted up. Prism up made 120MM Mini focus worse, unachievable.

Lymelight

(And to be clear, for steps 1-4 the main OTA was in focus first through the 2600 before I made the 120MM the main camera and attempted my focus there.)

Tech@ZWO

Lymelight
Do not need to move too much, align the prism edge to the main sensor's long side, and move up a little bit.
Then, insert the guide camera to the end and adjust the OAG-L helical focuser to get focus. you

surfinash

Hello - I have a slightly different challenge. I got my 174MM mini yesterday and was testing it. Focus was excellent, however, I noticed the dark vignetting / clipping on the vertical edges on the side (please see the image). Any ideas on what may be causing this? I am using OAG-L.

Here is the gear:

Williams Optics FLT-132 with Flat-8 reducer / flattener (camera side M48 opening)
OAG-L + EFW (7x2) + ASI 2600MM

Tech@ZWO

surfinash rotate the guide camera to make sure the sensor's long side align to the prism's long side. you can check it before insert the guide camera.

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