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Old 30-03-2021, 03:57 PM
evltoy (Wayne)
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SW ED80, Barlow & back focus

Hey All,

I have a skywatcher ED80 that I mainly use for Astrophotography. As my kit only came with a 28mm 2" eyepiece I decided to buy a Saxon 2" 2x ED Barlow Lens with 1.25" Adapter.

https://www.ozscopes.com.au/saxon-2-...5-adapter.html


My thought process was that I could use this barlow with the 28mm eyepiece (which I can) and also connect my DSLR to it via a 2" to M42 adapter.

The problem I have found out (after purchase) is that I cant get my DSLR to focus with it. I have tried different image-train connections:

ED80 -> Barlow -> DSLR
ED80 -> Barlow -> Diagonal -> DSLR
ED80 -> Diagonal -> Barlow -> DSLR

I have also played around with the above config with 5, 10 & 20mm M42 adapter rings before the DSLR with no luck

Have I bought a barlow that can only be used for visual only?

Cheers
Wayne
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  #2  
Old 30-03-2021, 04:25 PM
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multiweb (Marc)
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If you still can focus I'd put the barlow close to the DSLR. Can't you drop the diagonal for imaging? Easier to balance and less weight on the focuser.
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Old 30-03-2021, 05:06 PM
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Nikolas (Nik)
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Don't use the diagonal for imaging
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Old 30-03-2021, 05:40 PM
astro744
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You are caught in no man’s land between not enough outward travel without diagonal and too much with.

I just tried my ED80 with Andrews 2x 2” diagonal (similar to GSO but Saxon looks to have a longer barrel) and without diagonal I needed to add a 3” extension tube. The diagonal is adding about 4” but in any case you should omit it for photography.

Try racking out fully and then also pulling the Barlow out (with camera attached) until you get focus. (Try on a distant tree). You will then see how much extension you need.

I also tried Barlow + diagonal + camera and couldn’t rack inward far enough. I then tried diagonal + Barlow + camera and was able to get focus with 10mm to spare (inward limit).

Your Barlow seems to have a longer barrel and this may be contributing to you not having enough travel. A 3” extension should be all you need without diagonal in place. It is handy to have both 2” and 3” extensions in one’s kit for things like this especially if you have multiple telescopes or you add a focal reducer/flattened which then changes things again.
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Old 30-03-2021, 06:19 PM
evltoy (Wayne)
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Quote:
Originally Posted by astro744 View Post
You are caught in no man’s land between not enough outward travel without diagonal and too much with.

I just tried my ED80 with Andrews 2x 2” diagonal (similar to GSO but Saxon looks to have a longer barrel) and without diagonal I needed to add a 3” extension tube. The diagonal is adding about 4” but in any case you should omit it for photography.

Try racking out fully and then also pulling the Barlow out (with camera attached) until you get focus. (Try on a distant tree). You will then see how much extension you need.

I also tried Barlow + diagonal + camera and couldn’t rack inward far enough. I then tried diagonal + Barlow + camera and was able to get focus with 10mm to spare (inward limit).

Your Barlow seems to have a longer barrel and this may be contributing to you not having enough travel. A 3” extension should be all you need without diagonal in place. It is handy to have both 2” and 3” extensions in one’s kit for things like this especially if you have multiple telescopes or you add a focal reducer/flattened which then changes things again.
Oh wow! Thank you very much for taking the time to trial those different combinations with your equipment. I was just telling the wife that I was gonna spend some hours tonight and try and work out what combination was going to work. You have saved me a ton of time and effort.

I do have about 2-4” in total of m42 rings in various sizes that I can set between the Barlow and Camera. I’ll play with this tonight.

On another note, I don’t use the diagonal in my imaging train, but in this instance I was just working out where my issue was

Thanks again for helping me out
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Old 30-03-2021, 07:24 PM
astro744
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You’re welcome. I don’t image myself but occasionally use the my ‘scopes as long telephoto lenses for aircraft or the moon during an eclipse. Although I invested in both 2” and 3.5” extension tubes (sorry called it 3” before) from Tele Vue, my most used part when photographing is the ACM-2000 for 2” and ACM-1250 for 1.25” only telescopes as these have an inbuilt extension.

Prime focus:

https://www.televue.com/engine/TV3b_...d=42&Tab=_spec

Extension tubes:

https://www.televue.com/engine/TV3b_page.asp?id=93
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Old 31-03-2021, 11:34 AM
evltoy (Wayne)
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G'day,

With my excitement I went out last night to test my back focus issue.

Testing on the moon, I had no luck with 4" (and working down by 5,10,20mm) of extension to nothing while fully racked in and out on the ED80.

For some reason I think I may have too much back focus to start with and need to have the DSLR closer to the barlow lens.

An idea I have is to unscrew the barlow lens from its housing and get a 2" screw type -M42 adapter and start from as close up to the lens and work the DSLR back.

What are your thoughts on this idea? or do I just accept that this barlow is only going to be good for visual and I need to look at a short 2" 2x barlow?

Cheers
Wayne
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Old 01-04-2021, 04:25 PM
astro744
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There may be just enough difference in the barrel length and also focal length of the GSO Barlow compared to the Saxon Barlow to make one work and the other not. Can you measure the distance from bottom of Barlow to top (without 2”-1.25” adaptor in place) and also from bottom of Barlow to top of 2” barrel.

You could try your suggestion but I’m hoping some imagers will chime in and say yes or no. If you’ve got the bits then you’ve got nothing to lose in trying.
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Old 31-08-2021, 05:58 PM
evltoy (Wayne)
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Hey All,

I'll like to bring up this ol' chestnut as I believe I found the issue on why I cant get focus.

I now have 2 2x Barlows; the one I said in this thread above and now an ES 2x focal extender (2 inch) thinking there is any issue with my first one.

With excitement I took out the ED80 and put in the new ES focal extender straight in and inserted both an eye piece or my DSLR. To my dismay I have the same problem... cant focus on either one, racked in or fully out.

This is now a real joke! and I can say this hobby is costing me more than owning a boat!

Anyways I managed to find I believe is a fix! I took the barlow out of the ED80. I then screwed a 30mm 2" extender to a 2" T2 nose piece (30mm). I then screwed this extension to the front of the barlow and inserted it back into the ED80. I have now effectively pushed the front barlow lens 60mm back from the ED80 focuser.

and guess what! I now get focus on both eye pieces and DSLR. But it gets better. I get focus on BOTH barlows with this setup!

So from this, it looks to me that any barlow cant just be inserted straight into an ED80!
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Old 31-08-2021, 07:35 PM
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Merlin66 (Ken)
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Wayne,


The ED80's I have need a 80mm spacer to achieve focus if you don't use the diagonal.
You can see the spacer in the attached image of my MiniSHG set-up.
Attached Thumbnails
Click for full-size image (Mini_SHG_ED80_01.jpg)
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Old 31-08-2021, 09:04 PM
evltoy (Wayne)
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Thanks Ken for the pic.

Is that 80mm from outside edge of the draw tube / focuser or 80mm with about 30mm of inserted into the focuser?

Cheers
Wayne
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Old 01-09-2021, 07:31 AM
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Merlin66 (Ken)
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Wayne,
The spacer is 80mm and has a nosepiece about 30mm that sits inside the focuser.
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Old 01-09-2021, 07:58 AM
evltoy (Wayne)
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Thanks ken

I give this a crack tonight
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