Do you want to replace your telescope stock focuser with ESATTO? Our robotic focuser is available in different models offering the free aperture, weight load capacity and draw tube travel you need for your telescope and camera. After you choose what version of ESATTO you want to use, the first important thing you have to check is if you can reach focus by replacing your stock focuser with ESATTO. If your telescope has a long focus extraction (usually refractors, Schmidt-Cassegrain, Ritchey-Chretien, etc.) this will also help you to understand if you will need to add extensions. If your telescope has a short focus extraction (usually Newtonians), this will allow you to understand what version of ESATTO will allow you to reach focus.
In this video Filippo shows you how to check if you can reach focus with ESATTO when you want to replace your stock focuser:
Since focus extraction from the tube depends on your telescope optical scheme and stock focuser optical thickness, first of all you have to check the optical thickness of your stock focuser when your camera is focused to the stars. In order to do this, point your telescope to the stars and focus the camera. Then measure the distance from the focuser visual back to the focuser-tube interface. For this example, we took measurements by using a refractor and a DSLR camera with 55mm backfocus (this is the distance from sensor to the female T2 thread in front of the camera): the focuser’s optical thickness (with the camera focused to the stars) was 200mm.
For this example we chose to use the 3″ version of ESATTO that has 58mm optical thickness. Since the ESATTO 3″ has a dawtube travel of 25mm that is shorter than the stock focuser, we’ll have to add the proper extensions in order to reach focus, as you can see in the picture below. And we’ll have to consider their optical thickness too. From right to left, these are the optical thickness of the elements:
- Adapter to connect ESATTO 3″ to telescope: 20mm
- PL3600284 extension for ESATTO 3″ on telescope side: 65mm
- ESATTO 3″: 58mm
- ESATTO 3″ draw tube travel: up to 25mm
- PL3600285 extension for ESATTO 3″ on camera side: 20mm
- PL8410002 2″ visual back for ESATTO 3″: 12.5mm
By adding optical thickness of ESATTO 3″ to the ones of the adapters (by considering the ESATTO draw tube in full-in position), we have:
20mm + 65mm + 58mm + 25mm + 12.5mm = 180.5mm
And this means that, in order to reach the 200mm optical thickness of the stock focuser, we’ll have to extract the ESATTO 3″ drawtube of:
200mm – 180.5mm = 19.5mm
that is possible since the ESATTO 3″ offers 25mm draw tube travel. This example helps you to understand how to check if you can reach focus when you replace your stock focuser with ESATTO, by using a refractor telescope that usually comes with a longer drawtube focuser than the ESATTO. If your telescope has a short focus extraction from the tube (like a Newtonian) most probably you won’t need extensions. Instead, you will have to compare the ESATTO optical thickness to the one of your stock focuser and check if this allow you to reach focus. If ESATTO (with tube adapter) is thicker than your stock focuser, this may prevent you to reach focus properly. In this case, we suggest you to focus your camera to the stars and measure the optical thickness of your focuser as we did in the example above to check if you will be able to reach focus with ESATTO.
