Image train and backfocus

[LordLoki]

Hi all, 

I just snagged myself a Flattener and reducer. Now I am wondering  where is the right place to put it. Also what should be where distance wise for correct backfocus

I tried to find out what the backfocus should be but did not really find a good source. 

Should the reducer be attached directly to the focuser then extensions filterwheel camera and from where to where do I Measure backfocus? Is there a good way or tool to calculate it? 

My gear 

Flattener https://www.teleskop-express.de/shop/product_info.php/info/p3628_TeleVue-0-8x-Flattener-Reducer-TRF-2008-bis-600-mm-Brennweite.html

ZWO asi 1600mm

Atik efw 3 filterwheel  with baader 2" filters 

And scope is explore scientific Ed 80

 

[KP82]

The FR/FF goes right between the focuser and your camera. It's a standard 2" push fit design, so just slot it into your 2" eyepiece clamp on your focuser and then attach the camera along with the filter wheel to the T2 thread on the FR/FF.

The backfocus on your 1600mm is 17.5mm and the atik efw3 adds another 41.5mm. So you're already at a total backfocus of 59mm which is more than 55mm required by your FR/FF as shown on the specs page. However since this is a universal FR/FF, the optimal backfocus isn't always 55mm. The closer the focal length of your scope is to the lower end of the working range (400 - 600mm for this particular model), the longer the backfocus is going to be. You will have to work out the exact number yourself by trial and error. According to atik website you can remove the front plate of the efw3 to reduce its backfocus to 34.5mm, then insert additional T2 extension tubes to reach the desired backfocus.

[LordLoki]

Thanks a lot for all the info's already. Only thing I am not sure about is where to put the 55mm extensions do they go between FF/FR and filterwheel?

[Adreneline]

6 hours ago, KP82 said:

The backfocus on your 1600mm is 17.5mm

Sorry to jump in here but the backfocus on my ASI1600MM is 6.5mm with the extender ring removed. It shouldn't be too much of a challenge getting to the 55mm nominal but some experimentation might be needed to find the optimum spacing taking into account the effect of filter thickness.

HTH

 

Edited January 4, 2021 by Adreneline
Clarification

[KP82]

1 hour ago, LordLoki said:

Thanks a lot for all the info's already. Only thing I am not sure about is where to put the 55mm extensions do they go between FF/FR and filterwheel?

The 55mm backfocus is the distance from the base of the T2 thread on the FR/FF to the surface of the camera sensor. In your case if you remove the front plate of the efw3, the backfocus is going to be 34.5 + 17.5 = 52mm. To reach 55mm,  a 3mm T2 extension is required between the FR/FF and efw3. However because you need the supplied the M42 adapter to connect the the efw3 to the imaging train, that adapter may already measure 3mm and you don't actually need any extra extension tubes (if not, fill the remaining gap with appropriate extensions).

[LordLoki]

Hi all,

Thanks again for all the Help there is a lot to figure out with this hobby

So i made you a quick snap of the Adapter stuff i have and what i think the threads are based on infos i found on the internets

I also made a list next to it with what i think i have (green) and what i need (blue)

Based on this link: https://optcorp.com/products/atik-efw3-filter-wheel-7-position-for-2-inch-filters i believe the backfocus of the EFW3 alone is 21.8 which also roughly fits the thickness of the wheel. I think the 41.5 is including some sort of adapter?

So on the Camera Side of the Filterwheel where i have a male M54 i would install this one: https://www.teleskop-express.de/shop/product_info.php/info/p8959_TS-Optics-T-Adapter-from-M54-to-T2-thread--male----photo-and-CCD-adapter.html then attach the camera with the T2/T2 Ring to it 

Then on the other side of the Filterwheel with the Female M54 thread comes this one (not sure if its right does not say much about which end is female and which male) https://www.teleskop-express.de/shop/product_info.php/info/p5895_ATIK-Adapter---Reducer-from-M54x0-75-to-T2--for-EFW-filter-wheels.html

And then between the previous adapter and the FF/FR another T2/T2 Extension https://www.teleskop-express.de/shop/product_info.php/info/p11904_ZWO-T2-Female-Thread-Extension-11-mm.html

 

Does this make sense? 

And that then goes directly into the focuser (only need to get that stupid T2 to M48 ring off the FF/FR)

[KP82]

All looks good to me. 57.6mm is actually a good starting point for the subsequent trial & error to find the exact backfocus because the focal length of your scope is 480mm which is closer to the lower end of the 400 - 600mm working range and will require slightly more backfocus than the default 55mm theoretically.