Baader Hyperion Zoom IV back focus issue

[Spile]

If anyone is using the BHZ IV and Barlow with a Skywatcher 200P or similar Chinese brand, how are you mounting it or do you use a smaller 2" extender as well as the one that comes with the telescope?

Because I have to retract the EP about 5mm when I use it with the Barlow, I thought it would be a good idea to add a parfocal ring to the 2" adapter.

The problem now is that I don't have enough inward travel  to use the EP without the Barlow.

 

 

[Don Pensack]

So use the eyepiece as a 1.25".  It's really a 1.25" eyepiece anyway.

[Spile]

On 14/08/2021 at 17:23, Don Pensack said:

So use the eyepiece as a 1.25".  It's really a 1.25" eyepiece anyway.

I had hoped the same but the issue remains regardless which adapter is used. In fact the problem is exacerbated IIRC with the 1.25".

[Don Pensack]

3 hours ago, Spile said:

I had hoped the same but the issue remains regardless which adapter is used. In fact the problem is exacerbated IIRC with the 1.25".

Not necessarily.  2" to 1.25" adapters come in many different heights, from -12.7mm to +16.5mm

So you could change to a lower height adapter and get the 1.25" eyepiece just as close as the 2" eyepiece.

Ultimately, though, there is nothing prohibiting you from using the eyepiece as a 2" in the scope without the adapter at all, and using the 2" adapter when using the Barlow.

The focuser works without the 2" tall adapter.  You could even use the telescope as a 2" without any adapter and drop a normal 1.25" adapter into the focuser if you needed a lot of in-travel.

Edited August 15, 2021 by Don Pensack

[Spile]

11 hours ago, Don Pensack said:

Not necessarily.  2" to 1.25" adapters come in many different heights, from -12.7mm to +16.5mm

So you could change to a lower height adapter and get the 1.25" eyepiece just as close as the 2" eyepiece.

Ultimately, though, there is nothing prohibiting you from using the eyepiece as a 2" in the scope without the adapter at all, and using the 2" adapter when using the Barlow.

The focuser works without the 2" tall adapter.  You could even use the telescope as a 2" without any adapter and drop a normal 1.25" adapter into the focuser if you needed a lot of in-travel.

I've tried all the combinations (1.25" / 2" / with/ without current adapter. That is why I wanted to find out what other users of the same combination were doing or had got to work before I got another adapter.

Confirmed by Baader...

"Skywatcher has a shorter 2" eyepiece holder available for this problem. But for now you might be able to remove the parfocal ring and live with the inconvenience until you can obtain the shorter adapter. "

 

Edited August 16, 2021 by Spile

[Don Pensack]

As I said--a lot of adapters out there.  You can pretty much choose where you want the scope to focus.

[Spile]

A shorter extender https://www.firstlightoptics.com/adapters/astro-essentials-2-35mm-focus-extension-tube.html will mean that the focus tube is further out.

When using the Barlow, I would still need to use the longer adapter AND the short adapter above.

It might be easier (and cheaper) to carry on doing what I already do - Use the long adapter and draw out the EP 5mm when I use the Barlow.

 

Edited August 17, 2021 by Spile

[Don Pensack]

One should remember that no matter how short the upper section of an extension tube is, it will still raise the eyepiece out of the focuser by at least the full length of the lower barrel of the eyepiece.

That will require a LOT of in travel at the focuser to get back to focus.  So if you need 1/2" of out travel to focus, it may not be the best solution.

It might be better, where focus travel is more limited, to utilize a barrel extender that threads onto the eyepiece but which still inserts fully into the focuser, and then

pull the eyepiece out to the focus point, knowing there is still enough in the focuser for safety.

 

I know this does not solve the problem of moving a filter from one eyepiece to another, but does get around the huge refocusing problem when an extension tube is used.

 

Creative solutions to the hassle of transferring filters from eyepiece to eyepiece (as a side note for the above):

--filter on the front of a coma corrector or Barlow.  This applies to newtonians or any scope with a Barlow.

--filter on the bottom of a 2" to 1.25" adapter.  This is a preferred method for planetary filters, where multiple high power eyepieces might be used.

--filter on the front of the star diagonal (refractor, SCT, MCT)

 

 

[Spile]

20 minutes ago, Don Pensack said:

It might be better, where focus travel is more limited, to utilize a barrel extender that threads onto the eyepiece but which still inserts fully into the focuser, and then

pull the eyepiece out to the focus point, knowing there is still enough in the focuser for safety.

Is that not what I am already doing  - see above post?

[Don Pensack]

2 hours ago, Spile said:

Is that not what I am already doing  - see above post?

No.  Your extension tube pulls the eyepiece out of the focuser a minimum of the length of the bottom barrel of the eyepiece.

If that works, then fine.  it might not work for other eyepieces, though.

Most of the time, you won't need that much pull-back distance, but only, say, 1/2", which is much shorter than the length of the lower barrel on the eyepiece.

In that case, adding a barrel extender (it merely extends the barrel of the eyepiece, but it is the same diameter as the barrel) works better, since you can simply pull the eyepiece out 1/2" and tighten it down

and know there is enough barrel down inside the focuser for safety.

[Spile]

I decided that the best option for me is also the simplest and cheapest - the 35mm focus extension tube. That works with the BHZ and my 42mm 65° EP. I will use it along with the the longer extension tube that came with the Skywatcher when I use the matching BHZ Barlow   

Other options were too expensive and offered no benefit and a narrower tube which could cause additional light fall off problems with my low power EP...

Standard M54 to M48 adaptor 5mm optical path + 15mm extension tube + 36.6mm ClickLock optical path = 56.6mm extension/optical path length.

10mm backfocus version of M54 to M48 adaptor + 15mm extension tube + 36.6mm ClickLock = 61.6mm optical path length/extension total