Filter and spacers sanity check please

[michaelmorris]

I've just taken delivery of a lovely new ZWO ASI 24MC Pro camera. ?.  It will be used with my 80ED with focal reducer with an M48 rear thread.  The spacing required between the camera sensor and the back of the focal reducer is 55mm. 

Unfortunately my skies at home a pretty light polluted so I'll need a light pollution filter when using this camera at home.  Our local council have nearly finished swapping out all of the old sodium streetlights for LEDs.  The best light pollution filter for me is going to be the new IDAS D2 light pollution filter because this is good at reducing the effects of LED light bulbs. 

Now, the ZWO camera comes with a nifty little adaptor for using 1.25" filters.  However, the smallest IDAS D2 light pollution filter size is 2" (M48).  ?

So here is my plan.  I will use a 6mm M42 to M42 extension tube I already have; FLO's own M42 to M48 adaptor (10mm) and ZWO M48 extension tube (16.5mm)

https://www.firstlightoptics.com/adapters/flo-m48-to-t2-adapter.html

https://www.firstlightoptics.com/zwo-accessories/zwo-m48-to-m48-165mm-extender.html

Camera >M42-M42 >M42-M48 > Filter >M48-M48 > Focal reducer

17.5mm      6mm          10mm       5mm      16.5mm

Does this sound sensible or have i got something wrong (or is there a better way of doing this)?

(PS - I could screw the filter onto the back of focal reducer but the front of the filter doesn't have more that 2 - 3 mm of thread on it and I don't want to hang the whole camera + extensions on so little thread.)

[michaelmorris]

Ignore the above posting as I've just realised there is a simpler (and cheaper option) as the camera already comes with a 16.5mm long M42 to M48 adaptor (Doh!)

Camera >M42-M48 > Filter >M48-M48 > Focal reducer

17.5mm      16.5mm    5mm      16.5mm

This puts me 0.5 mm over the ideal back focus distance but my guess is this should be okay?

[RayD]

I reckon you're not going to be far away as you need to add a little (physical length) for the filter anyway, so if the filter is 2mm, you're adding 0.6mm, which puts you pretty close.

[bdlbug]

I had exactly the same issue a few weeks ago puuting the D2 into my optical path and as you mention in your original post the threads on the filter are extremely shallow and would potentially fail under load or flex - So I resorted to creating a DIY 'built in' filter in the spacers/adapters and deconstructed the filter using a special tool from Amazon. From what you have discussed above it seems you are still putting the M48 filter body into a mechanically loaded train of spacers - " Camera >M42-M48 > Filter >M48-M48 > Focal reducer "

Have a read of my discussion on this topic - it may be useful - although it was very scary dismantling such an expensive filter !!

thanks

[david_taurus83]

Am I right in thinking if adding a filter in front of the flattener that it is has no effect on the required spacing? I have ordered a D2 as my WO Flat6A II has an internal M48 thread in front of the lense.

 

@BDL_Bug Have you managed to try the new filter out yet? Reports/reviews on this filter are very scarce! I have sodium lamps above my garden but LED appearing on the main roads locally.

[RichLD]

7 hours ago, david_taurus83 said:

Am I right in thinking if adding a filter in front of the flattener that it is has no effect on the required spacing?

Yes, that's correct.

[bdlbug]

@david_taurus83,

8 hours ago, david_taurus83 said:

Have you managed to try the new filter out yet? Reports/reviews on this filter are very scarce! I have sodium lamps above my garden but LED appearing on the main roads locally. 

Yes I have imaged with this filter, only one session so far - but the sky transparency was not very good, it got worse as I moved through filters to red, I also think I need to review the back focus as the addition of this filter, substrate thickness 2.5mm - from Teleskop-Express website technical data - as my focus didn't seem as sharp - as usual add something new and it takes a bit of time fine tuning to get back to a performance point you're happy with - anyway  
My first light experience is in this topic below that I posted a couple of weeks ago

Bryan

[michaelmorris]

In the end I realised that I already had a a 2" filter holder in the form of a FLO Adapter for Sky-Watcher Focal Reducers

I unscrewed the Focal reducer from the focuser draw tube, fitted by Baader ClickLock adaptor to the draw tube, then screwed the filter into the adaptor.  IT works a treat and there is no load bearing on the filter.  The Baader Click Lock hold the whole focal reducer,camera train very solidly.  The only real disadvantage is that the Baader Click lock adds so much length to the imaging train that I'm now only left with about 1.5 mm of inward travel on the draw tube with the camera at focus. ?

[Hughsie]

Hi Michael et al,

Apologies for raising this subject again but I am getting very confused. 

I am thinking about purchasing the ASI294MC pro cooled. I would use this with my William Optics Z103 and Flat 6Aii reducer. According to the ZWO manual the back focus is 6.5mm but here everyone is talking about 55mm. I checked with Rother Valley Optics and they say 6.5mm from from of the camera to back of the reducer and suggested a T2 6mm extension tube would be ok. So where does 55mm come from?

Next I watched the review of the camera on Astrobackyard and Trevor was using a 2 inch light pollution filter in his imaging train. I would want to do the same but again the advice I have is it’s not possible to use the reducer and filter in the same image train.

I have read through the earlier comments and ‘get it’ but before I revert back to being a Luddite and breaking out the box brownie camera I thought I would ask the questions. Please help it’s driving me nuts!

 

Cheers, John

 

[newbie alert]

2 hours ago, Hughsie said:

Hi Michael et al,

Apologies for raising this subject again but I am getting very confused. 

I am thinking about purchasing the ASI294MC pro cooled. I would use this with my William Optics Z103 and Flat 6Aii reducer. According to the ZWO manual the back focus is 6.5mm but here everyone is talking about 55mm. I checked with Rother Valley Optics and they say 6.5mm from from of the camera to back of the reducer and suggested a T2 6mm extension tube would be ok. So where does 55mm come from?

Next I watched the review of the camera on Astrobackyard and Trevor was using a 2 inch light pollution filter in his imaging train. I would want to do the same but again the advice I have is it’s not possible to use the reducer and filter in the same image train.

I have read through the earlier comments and ‘get it’ but before I revert back to being a Luddite and breaking out the box brownie camera I thought I would ask the questions. Please help it’s driving me nuts!

 

Cheers, John

 

Hi John, the 55mm distance is usually the distance from the reducer to the camera sensor on most camera's.. 

Zwo say your camera has a 6.5mm back focus which I think is the distance from the camera body to the sensor..so 55mm minus 6.5 is 48.5 of free space to make up with your filter wheel and adapters..filter wheels are usually 20mm ish .. but if in doubt contact flo, Martin was really helpful with me setting up my Atik.. think the thickness of the glass in the filters need to be added on in the equation too

[david_taurus83]

The Flat6A II has a female M48 thread in front of the optics on the same side as the M63 thread that you screw into the drawtube. You would mount your light pollution filter there. As it is in front of the optics it has no effect on the back focus distance.

 

My setup as follows:

ASI1600 6.5mm from camera body to sensor

20mm filter wheel (you are looking at colour camera so replace FW with 20mm spacer)

Glass adds back focus so add 1mm for the filters and protective screen over the sensor.

10mm M42 spacer

16.5mm M42 to M48 adapter that came with the camera

0.8mm delrin spacers that came with the camera.

 

That's 54.8mm so pretty close. You would then set the adjusting ring on the flattener to the recommended spacing for your scope. Here's where you need to experiment a bit. I set the spacing using a digital caliper. A word of caution though, finding the exact spacing for perfect round stars has been known to send individuals insane...

 

[Hughsie]

Thanks David. Very clear reply. Quick question. For my scope I need to adjust the flattener to 4.6mm. Is that distance included in the 55mm?

[david_taurus83]

59 minutes ago, Hughsie said:

Thanks David. Very clear reply. Quick question. For my scope I need to adjust the flattener to 4.6mm. Is that distance included in the 55mm?

No, the 55mm is from the back of the flattener. Get it as close as you can and then you set the distance in the middle of the flattener to 4.6mm. If you could only get your spacing to 54mm then you could make up the 1mm in the middle by going 5.6mm. Make sense?

Don't get too hung up on getting perfect stars in the corners. I still cant but I refuse to waste any more clear nights fiddling with spacing. I'm shooting a project and if the image looks like I need to adjust after stacking then I do so and try again for the next project. It's a hobby that needs to be enjoyed, not lose sleep over! Even though the hobby requires losing sleep lol

[Hughsie]

Understood mate, thank you again.