Is this normal?

[Grifflin]

Hi Everyone

Recently took delivery of my first refractor...SW 80 ED...and was putting it on the mount during the day to work out balance, focus etc.  When looking for the focus point I aimed it at a distant tree line (miles away) but, even with the focus tube all the way out I could not reach focus.  I added in some spacers (about an extra 50mm worth) and I could see that it was starting to get there but it looked like it still need more 'distance'.  Maybe another 10 or 20mm.  Now I know that I can get another spacer to add that distance in and I feel it will probably work but somehow it doesn't feel right that it should be so far out.  However, that could just be my lack of experience and this is something which is perfectly normal.  Grateful for anyones thoughts before I go and buy some spacers.

For info, there is no focal reducer being used here and I am using a ZWO ASI294MC camera connected to a laptop using SharpCap.

Many thanks

Neil

[Laurieast]

I have the SW Evostar 80 ED, I use a Canon EOS 500D, the focus tube is out 46mm. Then a T adapter, T Ring, Camera. 
I have to go out to 72mm with a 2x barlow in that trail. 

Back focus in this camera is about 46mm

Different camera, but it does all add up.

~ 180mm  altogether.

This was last used on NGC2903 at 33 million light years away, so pretty much set at infinity 😉

 

Edited May 23, 2021 by Laurieast

[Neil H]

Hi I have a 127/1200 refractor if I use my asl224mc I have the 2 extensions that came with it and a 35mm and the 2inch to 1.25 adapter if pop an low power eyepiece in to get focus then remove the eyepiece when you fit the camera you will need to rack in focus point is closed to scope than with eyepiece 

watch this video on sharpcap the guy shows you he focus point of camera to eyepiece so this may help

I found an image of mine when in focus

Edited May 23, 2021 by Neil H
More info

[Grifflin]

1 hour ago, Laurieast said:

I have the SW Evostar 80 ED, I use a Canon EOS 500D, the focus tube is out 46mm. Then a T adapter, T Ring, Camera. 
I have to go out to 72mm with a 2x barlow in that trail. 

Back focus in this camera is about 46mm

Different camera, but it does all add up.

~ 180mm  altogether.

This was last used on NGC2903 at 33 million light years away, so pretty much set at infinity 😉

 

Thanks Laurence.....Ok...yep ... that makes sense....looks like I probably need about another 30mm.  Many thanks for taking the trouble to take the picture. :0)

[Grifflin]

1 hour ago, Neil H said:

Hi I have a 127/1200 refractor if I use my asl224mc I have the 2 extensions that came with it and a 35mm and the 2inch to 1.25 adapter if pop an low power eyepiece in to get focus then remove the eyepiece when you fit the camera you will need to rack in focus point is closed to scope than with eyepiece 

watch this video on sharpcap the guy shows you he focus point of camera to eyepiece so this may help

I found an image of mine when in focus

Thanks Neil....Yeah...looks like I need a bit more space...thank you for the pictures as well ...it does help to visualise.  I haven't watched the video yet but will give it a go later.  Many thanks.

[Grifflin]

@Laurieast @Neil H Perfect....all focused up now....thank you for your help 🙂

[Nik271]

You could use a star diagonal to add distance to back focus if all else fails.

In fact the reason you need so much extra extension is that the focusers of these telescopes are designed with the visual user in mind who are expected to use a diagonal.

[Grifflin]

On 25/05/2021 at 20:03, Nik271 said:

You could use a star diagonal to add distance to back focus if all else fails.

In fact the reason you need so much extra extension is that the focusers of these telescopes are designed with the visual user in mind who are expected to use a diagonal.

Thanks Nik...makes sense.  Also good to know as I do tend to switch around between imaging and visual.