4 replies to this topic

[astroflak88]

I just received my SHG 700 and want to image using my Esprit 80ED but I cannot reach focus. I don't think i have enough inward focus. Any recommendation what I should do? I tried a 2X barlow to image at F/10 but same issue, not enough inward focus.

[R Botero]

It’s not inward focus you need. You need extenders to reach focus just behind the slit. Have you pointed at the sun without the SHG to find where the sun comes to focus? Use a piece of paper and you’ll see the image of the sun form on it. It’ll be outside of your telescope, well past the back of your focuser. You’ll need extenders all the way there.

 

Roberto. 

[NuovaApe]

Top tip Roberto. I've done this with all my scopes.

 

1. Point scope at sun (safely...). The Moon should do.

2. Piece of paper at rear of OTA to measure where the focus is

3. Measure from some datum (back of OTA, back of focuser)

4. Write the distance on a small sticky label and stick it on scope/focuser

My datum is the last surface of my fully racked in focuser.

When I pick my scope of the day the label tells me where my camera or telecentric needs to be.

I add enough extensions to be 2cm short of focus.

With the focuser racked all the way in, I attach this length of gear.

Rack focuser out by 3cm, then wind it in.

Getting focus can be extremely annoying with trial and error extension tubes.

I needed to write it down in a place I couldn't lose it.

 

M42/T2 tubes (screw fixings) are much lighter/sturdier than 2" focus extensions with thumb screws.

I had 10" of these stacked 2" focus extensions. Apart from weighing a ton, they flexed/drooped.

 

I bought a sheet of 20 sticky labels. I've used 4. Enough for 16 more scopes.

 

Ed.

[astroflak88]

Thank you, your recommendation helped alot. 

[Scotty]

I use an 80mm 2" external T2 extension on my ED80/ SHG and find it can be slipped into the focuser and gives good focus with around 60mm inside the focuser. This minimises sag and can also provide a safe location to add 2" filters like the Astronomik L1 UV-IR, or the Antlia duo band CaK/ Ha.

Edited by Scotty, 23 June 2025 - 06:06 PM.