Information about Evostar 72ED and OVL Field Flattener for 1,25" Filter Wheel

[Kelian98]

Hello everyone !

I was looking for solutions to put a filter wheel with my current equipment and I am a bit confused.

I have a Nikon D3300 Astrodon Inside, a Astro Essentials T Ring for Nikon DSLR, the Skywatcher Evostar 72ED and the OVL Field Flattener (from the website First Light Optics).

Do you know if I could focus correctly with this 1,25" filter wheel https://www.firstlightoptics.com/zwo-accessories/zwo-5-position-1-25inch-manual-filter-wheel.html placed between the scope (Evostar 72ED) and the OVL field flattener (like he does) ? I asked the customer service and they said that I need these adaptors (below), but they are not sure if I can do the focus or not. Does somebody know ?

Their response:

“This to get the wheel to a male T thread on the flattener side https://www.firstlightoptics.com/adapters/baader-t2-t2-inverter-ring.html then this to get the M48 thread needed to connect the flattener https://www.firstlightoptics.com/adapters/flo-m48-to-t2-adapter.html

The flattener should separate into two parts if you need the shorten it ( the nosepiece will screw off)
On the telescope side of the wheel fit a 2" nose piece to the wheel https://www.firstlightoptics.com/adapters/flo-2-inch-t-mount-camera-adapter.html“

So I made some sketch to better understand what it is about.

Thanks !

[Louis D]

I'd be more concerned about having enough in-focus to be able to achieve focus with the 72ED.  I have one, and it won't reach focus with a binoviewer's added 100mm of optical path length.  You have 105mm without the flattener's path length.

[Kelian98]

Ok thanks for your answer ! So I can eraise the idea of the Filter Wheel with that configuration.

[Louis D]

4 minutes ago, Kelian98 said:

Ok thanks for your answer ! So I can eraise the idea of the Filter Wheel with that configuration.

On second thought, I forgot to discount the 1.25" diagonal's ~60mm path length.  It might work in straight through mode like your imaging setup.

My recommendation would be to connect the field flattener directly to the T-ring, attach it to the camera, and put the field flattener into the focuser and focus at infinity (the moon, let's say).  Measure how much in-focus you have left on the drawtube.  That's how much room you have to play with to add accessories.

[A320Flyer]

I have a ZWO 183MM Pro and the 8-position filter wheel and I can get focus with the OVL FF and room to spare. 

Bill

[Kelian98]

I will try that, thanks for your advices.

[Kelian98]

Bill, what are the adaptors that you use ? In order to know the optical path length. Thank you !

[RayD]

42 minutes ago, A320Flyer said:

I have a ZWO 183MM Pro and the 8-position filter wheel and I can get focus with the OVL FF and room to spare. 

Bill

Yes but the ZWO only needs 6.5mm back focus.  The DSLR needs 46.5, and the FW is shown as going in front of the FF.  

I don't think it will work with the DSLR and FW personally, but I would be very interested to see if you can manage to do it.

I would also say with the train as shown, you will have a large amount of vignetting, especially using 1.25" filters.

[Kelian98]

1 hour ago, RayD said:

Yes but the ZWO only needs 6.5mm back focus.  The DSLR needs 46.5, and the FW is shown as going in front of the FF.  

I don't think it will work with the DSLR and FW personally, but I would be very interested to see if you can manage to do it.

I would also say with the train as shown, you will have a large amount of vignetting, especially using 1.25" filters.

Yes I heard about the vignetting with 1,25" filters and the DSLR far away from the Filter Wheel. I will try to gather additionnal information and put a message if I manage to do it or if there is something wrong. Thanks the precision !

[alexbb]

You won't be able to reach focus in such a configuration. Neither backfocus is really optimal at 55mm, it is longer, not really sure how longer, my bet is for 60+mm. An then you're left with just a few mm of focus travel. No way you can insert a few adapters and a filterwheel there.

And second, the light cone leaving from the lens and reaching the sensor is always larger than the sensor size. The sensor is already larger than the filter glass so you will get severe, uncorrectable vignetting with 1.25" filters.

Moreover, the DSLR already has a filtering bayer matrix in front. Having additional filters would decrease the efficiency drastically. If you intent to put some narrowband filters in front, I can understand, but I would suggest screwing directly 2" filters directly in the light path or clip-in filters in front of the sensor. Don't want to put you off, but It's still quite inefficient even so.

[Kelian98]

10 hours ago, moise212 said:

You won't be able to reach focus in such a configuration. Neither backfocus is really optimal at 55mm, it is longer, not really sure how longer, my bet is for 60+mm. An then you're left with just a few mm of focus travel. No way you can insert a few adapters and a filterwheel there.

And second, the light cone leaving from the lens and reaching the sensor is always larger than the sensor size. The sensor is already larger than the filter glass so you will get severe, uncorrectable vignetting with 1.25" filters.

Moreover, the DSLR already has a filtering bayer matrix in front. Having additional filters would decrease the efficiency drastically. If you intent to put some narrowband filters in front, I can understand, but I would suggest screwing directly 2" filters directly in the light path or clip-in filters in front of the sensor. Don't want to put you off, but It's still quite inefficient even so.

Okay, thanks for this very clear explanation. I will continue to use my Explore Scientific 2" CLS Filter for now.