Jump to content

Narrowband

Backfocus calculation for FSQ85 + reducer


Recommended Posts

Hi

I'm hoping that someone who has already done the maths can check my understanding of the backfocus calculation for an FSQ85 + reducer. Specifically, I want to know the width of the Wide T Mount needed to achieve optimum backfocus when used with a QSI-690 and Baader LRGB filters or Astrodon NB filters.

It's not easy finding this information but this is my understanding so far:

- Tak x0.73 reducer requires an optimum backfocus of 72.2mm (reducer to CCD imaging plane).

- Tak supply the CA-35 (SKY90) adapter that provides 16.0mm backfocus.

- The QSI-690 provides 50.2mm of backfocus when fitted with the standard T mount adapter plate, not including LRGB or NB filters.

- When fitted with a Baader 1.25" LRGB filter the backfocus of the QSI-690 is reduced by a third of the width of the filter. Baader 1.25" filters are 2.1mm thick so that's 0.7mm. The optical backfocus of the QSI-690 is therefore reduced to 50.2-0.7=49.5mm.

- Similarly, when fitted with Astrodon 1.25" NB filters the backfocus of the QSI-690 is reduced by a third of the width of the filter. Astrodon 1.25" filters are 3.0mm thick so that's 1.0mm. The optical backfocus of the QSI-690 is therefore reduced to 50.2-1.0=49.2mm.

So, the Wide T Mount needs to provide 6.7mm (72.2-16.0-49.5) of backfocus when using a Baader filter and 7.0mm of backfocus (72.2-16.0-49.2) when using an Astrodon filter.

Have I understood this correctly?

I would ask Ian King for an explanation but he's away this week :smiley: . He has already supplied me with a custom Wide T Mount for use with the reducer but I'd like to understand how the width of the Wide T Mount was derived and what difference (if any) would result when used with both Baader and Astrodon filters. Maybe the Wide T Mount is optimised for the Baader case (6.7mm) and for Astrodon case I'd have to use delrin spacers to make up the width to 7.0mm.

Thanks for any advice!

Regards

John

Link to comment
Share on other sites

WIth that chip you won't need to worry about the difference in optical path between the Astrodon and Baader filters. On a chip reaching right out into the edge of the image circle it might just make a perceptible difference but you have some latitude with the small chip.

You seem to have identified your CA35 but have you actually measured it? I'm guessing you have but there are, I'm told, assorted different CA35s - as you'd expect from Takahashi!  :confused:

Olly

Link to comment
Share on other sites

Hi Olly

Regarding the CA35. I have both incarnations for the FSQ85 - the CA35 (TSA-102) when used native and the CA35 (SKY90) when used with the reducer. There are a few Tak PDFs available that show the different adapters that are needed for the various setups. That helped a lot.

I now have 2 more issues to deal with:

- The custom Wide T Mount supplied by IKI doesn't seem to screw all the way in to the QSI T mount adapter plate and I don't want to force it. All the other Tak adapters screw together very easily so I think there's a defect with the Wide T Mount screw thread.

- The rings used to secure the FSQ to the Losmandy plate are so low that the focuser doesn't clear the Losmandy plate so I can't move the scope forwards. Not a problem until you hang a huge QSI off the back and you try to balance in DEC!

Oh, the joys of AP  :smiley:

Regards

John

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.