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"Back Focus", "Working Distance", "Flatterner", "Reducer"... HELP!

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#1 Chrisofweden

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Posted 13 April 2021 - 06:07 AM

Hi!

 

As I described in a previous post, my Astro photo season is more or less over, due to the long long days up here at almost 60 degrees latitude.

From early may, there won't be any real dark skies again until late August!

 

So... the perfect opportunity to make plans for projects and what I'd love to achieve when autumn comes around - not least: new EQUIPMENT laugh.gif

 

Whilst I'm more than pleased my "rig" as it is right now (pls see my profile), I am seriously considering taking the (big) step into using a telescope and dedicated astro camera shocked.gif

 

To be very honest, I have found that more or less everything I've learned so far about my image train and what I can do with it has become irrelevant, including even the most basic terminology.

Nothing stays the same... and all of a sudden there are new terms to understand and they appear to not have a direct equivalent in my "old world"...  gone are ISO, variable aperture, focal length and zoom. In comes binning, correctors, reducers, back focus, working distance, collimation, well depth, filter wheels and drawers...

 

I decided to start at the beginning and I have tried to configure some hardware that I think will work together and thus allow me to learn how to use a set up that won't require an expert to put together and maintain. The configuration is also intended to give me a similar "performance" to my current set-up, including FoV etc.

Here it is:

  • Williams Optics Gran Turisimo 102mm APO Triplet
  • ASI 294MC Pro 
  • Williams P Flat6A 102 Corrector/0.8x reducer
  • ZWO EAF S Auto focuser

All run thru the ASI Air pro that I already have and mounted on the EQ6R Pro.

 

I guess many might suggest to go mono from the start, but I really don't see myself having the patience (nor the availability of clear sky imaging time) to do the whole filter rotation thing... just call me lazy.

On the topic of filters, I do realise that I will need to include a filter, not least to help tackle medium light pollution (Bortle 4). I simply can't work out how to get a filter into the above setup, given the back focus requirement of 55mm for the camera? There's no space left???

 

My "reach out" is for anyone that might have this (or a very similar) rig and could confirm that I'm not missing something critical (eg obscure thing like sensor size, pixel size and density etc etc) - will it not only work, but actually produce good results when I finally learn how to use it properly?

 

Thanks for reading and thanks for any help I can get!

 

Chris

 


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#2 PirateMike

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Posted 13 April 2021 - 06:57 AM

When you get your camera it will come with a 21mm extension tube. You just replace that with a 21mm filter draw.

https://astronomy-im...lter-drawer-m42

 

 

2021-04-13 07_56_48-ASI294MC Pro (color) _ ZWO ASI.png

 

I think that should do it.

 

 

 

Miguel   8-)

 

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Edited by PirateMike, 13 April 2021 - 07:01 AM.


#3 Chrisofweden

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Posted 13 April 2021 - 07:04 AM

Thanks Miguel!

 

I did check out the set-up examples on their site, but didn't find any that included the flatterner/reducer.

As I (think) I understand it, that thing alone takes up the entire 55mm? 

What am I missing?

 

/Chris



#4 Chrisofweden

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Posted 13 April 2021 - 07:18 AM

Wait a second....
Have I just totally misunderstood this and the 55mm is in fact from the back-end of the reducer? If so then my “challenge” with where to put the filter is resolved :)
Apologies for my (previously declared) newbiness!

Would still appreciate feedback on the set-up and it’s rating on a fit-for-purpose scale?

/Chris

Edited by Chrisofweden, 13 April 2021 - 07:19 AM.

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#5 jonnybravo0311

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Posted 13 April 2021 - 08:15 AM

I have the GT81 with the 6AIII reducer and the ASI294MM Pro with ZWO EFW. Here's how my imaging train looks:

 

GT81 -> 6AIII -> 16.5mm extender -> 11mm ring -> T2-T2 adapter -> EFW -> ASI294MM Pro

 

The GT102 with the Flat 6A is an older scope/flattener combo, so I'm assuming you're thinking about buying used. Unfortunately, I don't know the back focus distance for that combo. For my own setup, the back focus distance is 62.1mm. The 6AIII is adjustable, so to achieve the total, I get 55mm from the ZWO gear and twist the 6AIII to add 7.1mm.

 

As far as definitions, back focus is the distance where the image circle becomes flat. It's only relevant if you are using a reducer/flattener. Otherwise, it's just focus distance. You rack the scope's focuser in and out until the stars are in focus. Focus obviously still comes into play with a reducer/flattener. It's just that you need to ensure the camera's sensor is the proper distance behind the reducer/flattener so it gets a flat image.

 

Hope this helps.



#6 Chrisofweden

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Posted 13 April 2021 - 08:56 AM

Thank you for confirming my learning point of the day :)

Given this new understanding, I have now also looked at the adjustable reducer, but came to the conclusion that the real advantage of it comes when you have several cameras with different focal length requirements.
In my case, I might well plug my Sony mirrorless to the scope, but the requirement there is the same 55mm, so if I get it right for the dedicated camera, it will also be right for the Sony.

/Chris

#7 dx_ron

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Posted 13 April 2021 - 09:14 AM

I don't know anything about your particular camera, but the reason many flatteners have 55mm backfocus is because that's exactly what you get with a typical T-ring adapter + the camera body back to the camera sensor. So a typical dslr image train is just flattener/reducer - T-ring - camera. If you need a filter, many flatteners include threads where a 2" filter fits inside the flattener, or many filter makers have a few lines of "clip-in" filters that sit inside the camera body.

 

For filters, you should check if the sensor window in the 294MC has any filtering or is just "AR" (anti-reflective). If AR, you will need an IR/UV cut filter, because the sensor will respond to those wavelengths but they won't be in focus, so you need to filter them out. I would skip "light pollution" filters. As you gain familiarity with your system and the results you get on different kinds of targets, you might want to think about a narrow duo-band filter like the L-Extreme. There are several threads here that can give you a good idea what to expect from that and what types of targets to use it on (emission nebula).


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#8 Chrisofweden

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Posted 13 April 2021 - 09:50 AM

Thanks for the further input and explanation!

 

 

Regarding filters and what might be "best" for my ambitions, you hit the nail on the head and this was what I meant when I rather inacurately wrote about combatting Light Polution, which I think the filter will also do, besides enhancing the image overall?

My dream is to capture some of the bigger/brighter nebulae in even more detail than I have been able to so far with my Sony and my (limited) understanding is that such a filter would greatly enhance my chances of success smile.gif

 

If/when I do use the Sony together with the scope, I won't be using any band filters, since the exposure time on the unmodded camera would become way too long.... this is one of the main reasons for moving to a scope and dedicated camera in the first place. That and the fact that the ASI Air doesn't support Sony cameras so I lose some of the features of it - for example live view and even stacking. 

 

Thanks to all who have contributed here, it's really appreciated and has helped me to formulate my plan. I have 4 or 5 months now to actually put it all into action so I'm ready when darkness comes back to the region!

 

/Chris

 

   



#9 PirateMike

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Posted 13 April 2021 - 09:58 AM

Wait a second....
Have I just totally misunderstood this and the 55mm is in fact from the back-end of the reducer? If so then my “challenge” with where to put the filter is resolved smile.gif
Apologies for my (previously declared) newbiness!

Would still appreciate feedback on the set-up and it’s rating on a fit-for-purpose scale?

/Chris

No problem, we were all "newbies" once.

 

 

Miguel   8-)

 

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Edited by PirateMike, 13 April 2021 - 10:00 AM.


#10 jonnybravo0311

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Posted 13 April 2021 - 10:42 AM

Thank you for confirming my learning point of the day smile.gif

Given this new understanding, I have now also looked at the adjustable reducer, but came to the conclusion that the real advantage of it comes when you have several cameras with different focal length requirements.
In my case, I might well plug my Sony mirrorless to the scope, but the requirement there is the same 55mm, so if I get it right for the dedicated camera, it will also be right for the Sony.

/Chris

Take a look at the back focus requirements for the GT102 with the Flat 6A to ensure it really is 55mm. For the GT81 with the Flat 6AIII, it's 62.1mm. For example, the Flat 7A denotes a back focus of 70.6mm when used with the GT102. The only chart I've found for the combo of GT102 and Flat 6A shows a 60.95mm back focus. Therefore if you were to use this with your Sony, you'd need a T-ring to get you the 55mm... but you'd also need a 6mm spacer to get you to 61mm (yes, I rounded up because that 0.05mm is not going to make a difference).



#11 Chrisofweden

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Posted 13 April 2021 - 12:14 PM

Thanks again...
The rabbit hole just keeps getting deeper...
/Chris

#12 Redhook

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Posted 13 April 2021 - 01:53 PM

You can use filters and be lazy, just get an automated filter wheel.

Sent from my Pixel 4 XL using Tapatalk


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