Field Flattener and back focus distance question

[Neko]

Hi folks, 

I wonder what happens if one can not reach the ideal back focus distance with a field flattener? 

For instance, my QHY8L has 20.1mm back focus, but the Evoguide 50ED FF requires 17.5mm  

then, how the images would look like? those 3mm of the sensor being too close to the FF would it 

make it not possible to focus? or will focus but wouldn't correct the whole field?  

Withouth the FF  I can reach focus but maybe about 50% of the frame (or more? ) is distorted, obviously

So, what's the theory behind it? Not reaching the ideal backfocus makes the whole field distorted or

just can not correct a certain % ? 

Sorry if that is a too dumb question ^^;  

 

[Tomatobro]

Have a look at https://optcorp.com/blogs/astronomy/how-to-set-the-correct-back-focus

[Neko]

32 minutes ago, Tomatobro said:

Have a look at https://optcorp.com/blogs/astronomy/how-to-set-the-correct-back-focus

I'm aware of this. I followed the instructions for the Hotech ff on my Borg. Just wondered the effect. In the site they show those graphs, and according to it would distort all the field ?  

Guess a good way to test it would be modifying the distance in my Hotech+borg combination to see the effect.   But weather is bad enough for long to waste next clear night on this 🤣🤣 

Thanks for your answer anyway

[EyeGuy]

3 hours ago, Tomatobro said:

Have a look at https://optcorp.com/blogs/astronomy/how-to-set-the-correct-back-focus

Man - that is seriously useful. Thanks for posting this. Suddenly makes sense.

Barry

[michael8554]

I suspect you already know the answer - unfortunately the QHY8L isn't going to work with that OTA and FF combination.

Michael

[happy-kat]

There's more back focus available with the starizona evoguide field flattener I think.

[Neko]

On 02/05/2021 at 23:11, happy-kat said:

There's more back focus available with the starizona evoguide field flattener I think.

Yes, but the cost is about twice as the sw one, and also there's no availability so far for starizona one..