Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Zenithstar 73 Canon EOSm100 backfocus questions

Astrophotography DSLR
  • Please log in to reply
3 replies to this topic

#1 Steeve B

Steeve B

    Lift Off

  • -----
  • topic starter
  • Posts: 3
  • Joined: 28 Aug 2020

Posted 09 May 2021 - 10:27 PM

Hi There

 

I just got myself a ZS73 with a Flat 73 adjustable. At the moment I'm waiting for my camera to come back from being astro modified but I have a question regarding the back focus to the flat 73.

When reading the WO Flat61/73 back focus diagram I see that I need 67.7mm between the edge of the flattener to the image plane.

 

Now this is the adapter I have to reach this back focus:

 

Canon EOSM100 flange distance= 18 mm, EOSM to T2 adapter (27.5mm) + Zwo M42 to M48 spacer (16.5mm)

 

That gets me to 62mm.

 

Now I also have an Optolong 2" L Pro filter in the image train connected to the flat 73. With the filter installed the flattener adjuster sits at about 5mm out due to the thickness of the filter by default.

 

So I end up at 67mm as a starting point...

 

My question are

1) Am I calculating this correctly?
2) Will I be able to get the correct back focus for a flatfield image?



#2 oatmeal

oatmeal

    Vostok 1

  • -----
  • Posts: 107
  • Joined: 12 Apr 2020
  • Loc: Sonoma County, CA

Posted 09 May 2021 - 11:23 PM

For your scope, you should have the smooth black back piece of the flattener dialed out to 11.4mm, which you then lock into place with the toothed rotating ring. Once you have done this, you will have a 55mm working distance from the back mating surface of the flattener to your sensor. 

 

I don't know about your T2 EF-M adapter. If it doesn't land you at 55mm of backfocus spacing on its own, it likely wasn't designed specifically for AP. For EF mount (44mm flange distance), most M48 adapters are 11mm thick, giving you exactly 55mm backfocus spacing. Another option - if you already have an EF-M to EF adapter, would be to use that with an EF-M48 adapter, to give you exactly 55mm backfocus. These EF M48 adapters can be had for $10-$25. This one worked well for me.

 

And if you plan to use a filter, realize that you filter will effectively shorten the optical distance by about 1/3 the thickness of the filter, meaning you need more backspacing distance (e.g. a 3mm thick filter will require an extra 1mm of backspacing, a 1mm thick filter 0.33mm, etc.), which you might need to accomplish with thin fine adjustment spacing rings.


Edited by oatmeal, 09 May 2021 - 11:26 PM.


#3 Steeve B

Steeve B

    Lift Off

  • -----
  • topic starter
  • Posts: 3
  • Joined: 28 Aug 2020

Posted 10 May 2021 - 01:00 AM

For your scope, you should have the smooth black back piece of the flattener dialed out to 11.4mm, which you then lock into place with the toothed rotating ring. Once you have done this, you will have a 55mm working distance from the back mating surface of the flattener to your sensor. 

 

I don't know about your T2 EF-M adapter. If it doesn't land you at 55mm of backfocus spacing on its own, it likely wasn't designed specifically for AP. For EF mount (44mm flange distance), most M48 adapters are 11mm thick, giving you exactly 55mm backfocus spacing. Another option - if you already have an EF-M to EF adapter, would be to use that with an EF-M48 adapter, to give you exactly 55mm backfocus. These EF M48 adapters can be had for $10-$25. This one worked well for me.

 

And if you plan to use a filter, realize that you filter will effectively shorten the optical distance by about 1/3 the thickness of the filter, meaning you need more backspacing distance (e.g. a 3mm thick filter will require an extra 1mm of backspacing, a 1mm thick filter 0.33mm, etc.), which you might need to accomplish with thin fine adjustment spacing rings.

Thanks for that. So what I have wont work then... The cheapest route for me would be  get an EF to EOSM adapter and then the EF - M48 adapter to get me to 55mm. My filter is a Optolong L-Pro 2inch which is 2mm thick... so I would need an extra 0.66mm of backspacing. The best one I can find is a 0.6mm fine adjustment ring... so that should get me close enough right?



#4 oatmeal

oatmeal

    Vostok 1

  • -----
  • Posts: 107
  • Joined: 12 Apr 2020
  • Loc: Sonoma County, CA

Posted 10 May 2021 - 01:58 AM

That should get you in the ballpark, yes, though each optical system is subject to some variation, and it may not be that exact. Before you buy a spacer ring, try a fraction of a counter-clockwise revolution on your adjustable flattener. Take some test shots, and see if the results are acceptable to you.

 

As it seems you don't have the EF-M to EF adapter in hand, there may also be a way to finagle some other, cheaper solution using a mishmash of rings, spacers and M42-M48 adapters to get you in the 9.5-10mm range, but it'd be piecemeal.


  • Steeve B likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: Astrophotography, DSLR



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics