ADC Effective Optical Length?

james1ca

Does anyone know the actual optical length of the ZWO ADC? Since the ADC has optical elements (prisms) that means the metal backfocus or physical length is not the same as the effective optical length and you need to know the optical length to help with backfocus calculations. It would probably be okay to just know the thickness of the prisms (at their center or neutral position) and I would then assume that their index of refraction is about 1.5. Please remember to make it clear whether the thickness is for just one prism or for both.

Support@ZWO

Physical lenght +2.57mm optical pass length.

james1ca

Thanks. But, that still seems to be an uncertain value. Is that 2.57mm the thickness of the prisms or the thickness divided by the index of refraction? I assume the latter (divided by the index of refraction) but in that case the actual effective length of the ADC would be:

Physical length - 2.57mm (subtract that value, don't add)

Thus, the optical length of the ADC is shorter than the physical length.

So, if the ADC takes up 29mm of the metal backfocus (physical length) then it really only accounts for 29mm - 2.57mm ≈ 26.4mm of the target backfocus and if you were wanting 55mm then you'd need 55mm - 26.4mm = 28.6mm of additional spacing. Giving a total metal distance of 29mm + 28.6mm = 57.6mm (without any other filters or optical elements in that path).

Support@ZWO

No, not substract, add.

james1ca

Well, it depends upon what you are adding to or subtracting from. If you are trying to figure out the metal backfocus that is required for the system then you need to ADD that offset to the total. Adding a filter or other optical element moves the focus point further away from the objective.

However, if you just want to calculate the effect length of the ADC it will be reduced by that same amount (so you'd subtract that from the metal length of the ADC).

In either case, the total metal backfocus of the system will be increased by that offset. So, you can calculate the correct backfocus either by adding that offset to the total (giving a new target increased by the offset) OR you can subtract that offset from the metal length of the ADC and then add enough additional spacing to reach the original target spacing.

Kind of hard to describe, that's why it's probably better to review what I wrote in my second reply, where I give an example of calculating the backfocus.

I've been down this road many times and several years ago there was a long discussion on Cloudy Nights on how to calculate the changes needed for backfocus when you add a filter. A LOT of people had this backwards and it was only after many posts and a clarification from Dr. Don Goldman at Astrodon (filters) that this debate was concluded.

Here is a link to part of that discussion:

https://www.cloudynights.com/topic/482487-elongated-star-at-corner/page-2#entry6313991

And then almost two years later when the question came up again it was finalized to everyone's agreement:

https://www.cloudynights.com/topic/541574-imaging-train-spacing-subtract-filters/page-2#entry7295477

By the way, I'm the james7ca in those discussions.

Support@ZWO

Yes, we add two prism, need to add about 2.57mm.