Too much back focus

astroct

w7ay
Hi Chen

Yes I am getting the same pattern as the second image in the link that you provided ie sensor is too far away from the reducer.

It looks like with my current EFW and OAG I will not be able to reduce the back focus. I’ve had a look at the tilt plate and removing it will mean I’ll have to get the new EFW where the cam is bolted to it. Can you suggest anything else ?

Cheers
Carlos

w7ay

astroct Can you suggest anything else ?

If you are handy with a drill press, you can drill four holes in your EFW to mount the EFW directly to the camera (just like how you mount the newer EFW). You then save the thickness of both the 2mm adapter and the 5mm thickness of the tilt plate and recover 7mm worth of back focus. (But you of course lose the utility of the tilt plate to correct for any sensor tilt.)

You can use the hole locations from the documentation in the newer ZWO filter wheels as a drilling template (first check which set of 4 holes are used in the ASI2600 -- the newer filter wheels have two different sets of 4 holes).

(By the way, like others who use the direct attachment method, you will also end up with some light leak, so you will need to seal the EFW/camera interface with black electrical tape if you use the four hole attachment method.)

Chen

astroct

w7ay
Hi Chen

That is an option. Which I rather not do lolz.

What I don't understand is I am at the correct back focus requirement as stated on this page (using the old 36mm EFW and OAG) but I am still getting elongated stars that rotate around the optical axis showing my spacing is too long?

[https://astronomy-imaging-camera.com/tutorials/asi2600-guide-4-connection-methods-to-get-55mm-back-focus-length.html]

Cheers
Carlos

w7ay

astroct What I don't understand is I am at the correct back focus requirement as stated on this page (using the old 36mm EFW and OAG) but I am still getting elongated stars that rotate around the optical axis showing my spacing is too long?

Did you measure the distance from the metal back of the reducer to the flange of the ASI2600, and then adding 17.5mm, to see if it is really about 55mm (and if your reducer's documentation says it needs 55mm)?

If it is, then I would check the instructions of your reducer again. Quite often, when reducers are designed to work with multiple OTA, they have some extra spacer, or some way to change an internal drawtube so that the same reducer can be matched the native focal length of different OTAs.

Chen

w7ay

astroct

Carlos,

As a sanity check, remove the OAG and connect the EFW to the coma corrector. If you are using the original 36mm EFW, you will also need M48 to M42 adapter too.

With the extra length of the OAG out of the way, does the stars at the edge of the frame show that the backfocus is now too short?

Equivalently, add a spacer to your current situation. Does it make the problem worse?

Chen

w7ay

astroct

Carlos,

I did a google search and a number of threads on the subject says the optimal backfocus of your coma corrector depends on the focal length (just as I mentioned earlier).

With a 600mm focal length, the backfocus is 51.66mm, with 800mm focal length, the backfocus is 53.66mm, and it is only the 55mm value for 1000mm focal length. For focal lengths that are longer than 1000mm, you again need a backfocus of less than 55mm (54.66mm for focal length of 1200mm). 55mm is the longest and only works at 1000mm focal length.

Here is one of the threads:

https://stargazerslounge.com/topic/352068-f4-800mm-with-skywatcher-aplanatic-spacingback-focus/

and it references a YouTube video that gives the backfocus (the numbers that I just mentioned) for a couple of different focal lengths

https://youtu.be/6YLcXEUgjbM?t=1033

So, mystery solved. But it means that if you have a focal length that is different from 1000mm, then drilling those 4 holes might be your only option. Even using a new ZWO filter wheel won't help.

Or, use a guide scope instead of an OAG. If your focal length is not too much longer than 1000mm, a guide scope should work just fine as long as there is no differential flexure.

Chen

astroct

w7ay
Hi Chen

Previous to the 2600MM I was using the 183MM which required a 55mm back focus. It was working fine then.

astroct

w7ay
Hi Chen

My focal length is 800mm so I need even less back focus of 53.66 mm. So the only option I see so far is guide scope instead of OAG. Thanks for all your help anyway. Much appreciated !

Cheers
Carlos

wvreeven

astroct The back focus given by ZWO is a recommendation. Cameras do not require backfocus but flatteners and reducers do. A Redcat, for example, is designed such that it doesn’t require a back focus so when the camera is focused, it has pinpoint stars over the entire FOV.

So you need to look up the back focus requirements for your flattener or reducer and use that.

astroct

wvreeven

Thanks. I understand that. The problem is how to achieve the required back focus of 53.66. The options now are to remove the tilt plate on the camera and drill holes in my EFW or to remove the OAG and replace with appropriate spacers.

wvreeven

astroct As a sanity check I would, as Chen also suggested, try without the OAG and at least verify that you get a proper flat field. If yes, you can consider the various options. One step at a time and the first step IMHO is to verify the required back focus distance.

astroct

wvreeven
Cheers yes I will do that, remove the OAG, I just don't have the necessary adapters/spacers to go from an M42 (EFW) to the flattener (M48) to replace the distance occupied by the OAG. Have ordered.

astroct

Hey guys,

I have decided to get a TOAG that is 6mm thinner than the ZWO one so hopefully will give me more room to play with. Thanks for all your help and suggestions. This has been a good learning experience !

Cheers !