How to Acheive Focus with ASI533MCPRO

[themcfallguy]

Hi,

As a newbie I'm trying to achieve focus and thought perhaps best to do this during the day against some rooftops.
With or without filters it's completely white image. 
Focal Length 420 on 72ed.
Gain M = 100

setup includes ASI AIR running live video, focus or exposures.

Skywatcher 72ED
Reducer for 72ED
MFW
ZWO533MCPRO

sitting on star adventurer2

Also a Guide Scope 50ED
Reducer for 50ED
ZWO 120 MM Mini

I understand I need to first achieve back focus which is either 55mm or 56mm. I've tried a number of adapters but the image is not focused and becoming very time consuming any help is much appreciated.

Edited January 13, 2022 by themcfallguy

[Pitch Black Skies]

Have you tried bringing down the gain?

Also, make sure it is in fact directed at the rooftop and not slightly above it looking at the sky.

Edited January 14, 2022 by Pitch Black Skies

[Budgie1]

So, for the back focus (going on your photo) you have 6.5mm from the front of the sensor to front face on the camera, the EFW is 20mm and you've added the 16.5mm spacer = 43mm back spacing. If you add the 11mm spacer in front of the EFW and use the thin round spacers to get your extra 1mm then you'll be on the required 55mm.

With the EFW, if you're using 1.25" filters then this does need to be as close to the sensor as possible, otherwise you start getting vignetting.

For daytime exposures, reduce the gain down, as you only want to get a rough focus then use gain zero, and reduce the exposure times down to the 1 or 2 seconds, maybe even shorter.

When you start imaging at night, you'll need to check the focus again because it can change with the ambient temperature. So, if you're focusing manually, I would recommend a Bahtinov Focus Mask to make life much easier.

[themcfallguy]

@Budgie1 Thanks for the help.

I made the changes mentioned see images below and reduced the gain to low then 0 but still a completely white image.

 

 

[themcfallguy]

I also moved the focuser on the telescope all the way in. I have a Bahtinov Focus Mask  in place with and without the white image.

[PeterCPC]

10s exposure is way too much for daylight conditions. Try 1/2s or probably less.

[gilesco]

1 minute ago, PeterCPC said:

10s exposure is way too much for daylight conditions. Try 1/2s or probably less.

For this kind of thing during daylight hours I often end up using AsiStudio on a PC or ASICAP on a phone or tablet with OTG USB adapter, and then just enable "Auto GAIN" + "Auto Exposure" once it has settled to a reasonable setting I then disable the Automatic side of things and make a note of what they're actually set to.

[Pitch Black Skies]

That isn't in Live Video..

If it's still the same after switching to Live Video, put the dust protector on and see if the image turns black.

[themcfallguy]

Gave it a go this evening on the moon couldn't achieve focus probably too bright. Will go again tomorrow

[geordie85]

Try taking out the additional spacer. Personally I don't believe that it's as little as 6.5mm from the sensor to the front of the camera. My reason for this is because I had everything exact and couldn't reach focus. I used a shorter spacer then I got perfect focus without any problems.

[themcfallguy]

remove the 11mm spacer?

[Pitch Black Skies]

28 minutes ago, themcfallguy said:

Gave it a go this evening on the moon couldn't achieve focus probably too bright. Will go again tomorrow

Is this in video mode?

What is the gain?

What is the exposure?

It looks in focus, just over exposed.

Edited January 13, 2022 by Pitch Black Skies

[themcfallguy]

@Pitch Black Skies the video is video mode.

the gain is 10, anything higher goes bright outline.
exposure attempted was 1 sec and 5 sec.

[Pitch Black Skies]

Just now, themcfallguy said:

@Pitch Black Skies the video is video mode.

the gain is 10, anything higher goes bright outline.
exposure attempted was 1 sec and 5 sec.

Exposure is too long.

[Pitch Black Skies]

Here's a screenshot of mine last night at unity gain. You need to be going into the milliseconds for the moon.

[geordie85]

10 minutes ago, themcfallguy said:

remove the 11mm spacer?

Yeah that's what I did. Although I am using the 183mm

[Elp]

Finding out the specs of your optical train helps out which may require trying different sources like different forums, the manufacturer etc. I found my field flattener actually has a 54.8mm backfocus distance from a technical drawing from William Optics, the sensor to front face distance i found from a technical drawing from zwo, adding a filter into the train I found adds around 1mm to the train from reading forums etc. Obviously you're in the right place to ask for help.

If you're struggling to achieve the correct distance something like this will help a lot as it provides options, obviously check what thread size you need first:

https://www.firstlightoptics.com/adapters/astro-essentials-t2-m42-extension-tube-set-4mm-5mm-6mm-7mm-8mm-9mm.html

A set of good Vernier calipers will help you measure more accurately, I actually measured each ring individually and marked them on the side with tippex the mm distance each provides so I can change them in the field without having to measure them, I also have a few m42 to m48 adaptor rings in the system which make up the decimal point distance i need to get the full back focus.

Regarding focus a bahtinov works (unfortunately for lunar you have to use your own judgement, luckily with an Asiair you can take a preview image and zoom right into the image on your mobile/tablet and judge pretty well), the focus utility within the asiair also works well but I believe that's more for focusing on stars, usually all I use when setting up focus when changing filters.

[Budgie1]

1 hour ago, geordie85 said:

Try taking out the additional spacer. Personally I don't believe that it's as little as 6.5mm from the sensor to the front of the camera. My reason for this is because I had everything exact and couldn't reach focus. I used a shorter spacer then I got perfect focus without any problems.

The ZWO diagram for the ASI533MC & the ASI183MC Pro says it's 6.5mm from the sensor to the front face of the camera. You may be thinking of the 17.5mm distance normally quoted when the 11mm spacer is fitted but on the OP's setup he has the EFW fitted directly to the camera.

So I was working on 6.5mm + 20mm for EFW + 16.5mm spacer + 11mm spacer = 54mm from the rear of the reducer/flattener lens to the sensor. Then make up the remaining 1mm with the spacer washers which normally come with the camera or EFW.

Maybe the back focus on the Evostar 72ED is different to the 80ED & 100ED I have?

Edited January 13, 2022 by Budgie1
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