Barlow for CPC925

[1CM69]

Hi all,

I'm considering a CPC925 but the specs state that:

"The CPC 925 GPS was specifically designed for visual infinity focus when using 2" eyepieces..."

 

I am a little confused as I was hoping to use a x2 Celestron 1.25" Barlow 

Does this mean the 1.25" barlow is no good and that I actually need a 2" Barlow?

 

Regards..,

[Louis D]

14 hours ago, 1CM69 said:

Hi all,

I'm considering a CPC925 but the specs state that:

"The CPC 925 GPS was specifically designed for visual infinity focus when using 2" eyepieces..."

 

I am a little confused as I was hoping to use a x2 Celestron 1.25" Barlow 

Does this mean the 1.25" barlow is no good and that I actually need a 2" Barlow?

 

Regards..,

I think it means that when used with a 2 inch star diagonal, the OTA will be operating at it's spec'ed focal length where it will have the least aberrations.  Many SCTs' specified focal length assumes the backfocus needed for a 1.25 inch star diagonal, which has a shorter path length than a 2 inch diagonal.  For those OTAs, it means you have to move the primary forward to get enough backfocus for a 2 inch diagonal which increases the focal length and starts to increase spherical aberration because the spacing between the primary and the corrector/secondary is no longer optimal.  Spherical aberration gets even worse when using a binoviewer without an OCA because of the enormous amount of required backfocus.

So no, you don't need a 2" barlow, though you might want to add a short extension tube somewhere when using a 1.25 inch diagonal to get to the optimal amount of backfocus.

[1CM69]

2 hours ago, Louis D said:

I think it means that when used with a 2 inch star diagonal, the OTA will be operating at it's spec'ed focal length where it will have the least aberrations.  Many SCTs' specified focal length assumes the backfocus needed for a 1.25 inch star diagonal, which has a shorter path length than a 2 inch diagonal.  For those OTAs, it means you have to move the primary forward to get enough backfocus for a 2 inch diagonal which increases the focal length and starts to increase spherical aberration because the spacing between the primary and the corrector/secondary is no longer optimal.  Spherical aberration gets even worse when using a binoviewer without an OCA because of the enormous amount of required backfocus.

So no, you don't need a 2" barlow, though you might want to add a short extension tube somewhere when using a 1.25 inch diagonal to get to the optimal amount of backfocus.

Perfect, thanks so much for the indepth answer even if at the moment I only understand about half of what you said. ?

[Stu]

16 hours ago, 1CM69 said:

Hi all,

I'm considering a CPC925 but the specs state that:

"The CPC 925 GPS was specifically designed for visual infinity focus when using 2" eyepieces..."

 

I am a little confused as I was hoping to use a x2 Celestron 1.25" Barlow 

Does this mean the 1.25" barlow is no good and that I actually need a 2" Barlow?

 

Regards..,

Is this for visual use? I ask because the 925 has a pretty long focal length (2350mm) so a 10mm eyepiece will give x235 magnification. Using a barlow is likely to give you far higher magnifications than the atmospheric conditions will support.

For imaging it would make sense, but visual not really necessary.

[1CM69]

3 minutes ago, Stu said:

Is this for visual use? I ask because the 925 has a pretty long focal length (2350mm) so a 10mm eyepiece will give x235 magnification. Using a barlow is likely to give you far higher magnifications than the atmospheric conditions will support.

For imaging it would make sense, but visual not really necessary.

Purely for imaging, thanks for replying

[Stu]

15 minutes ago, 1CM69 said:

Purely for imaging, thanks for replying

No problem. Would you like me to move this to imaging discussion? In 'Eyepieces' it is more likely to get answers from visual guys.

[1CM69]

31 minutes ago, Stu said:

No problem. Would you like me to move this to imaging discussion? In 'Eyepieces' it is more likely to get answers from visual guys.

I guess that would be a much better location, thanks. 

[Stu]

8 minutes ago, 1CM69 said:

I guess that would be a much better location, thanks. 

Done, I've left a link where you posted originally too.

[ronin]

The barlow usage should be for imaging planets = Jupiter, Saturn and Mars. A barlow is not applicable os a 925 for imaging DSO, if imaging DSO's etc then you will need a reducer not a barlow.

Amongst the best results (Jupiter) I have seen with a 925 was when the person used a 2.5x TV powermate.

[1CM69]

3 minutes ago, ronin said:

The barlow usage should be for imaging planets = Jupiter, Saturn and Mars. A barlow is not applicable os a 925 for imaging DSO, if imaging DSO's etc then you will need a reducer not a barlow.

Amongst the best results (Jupiter) I have seen with a 925 was when the person used a 2.5x TV powermate.

Thanks, I have ordered a reducer as well. 

I'll have a look at a powermate

[kirkster501]

I used a Televue 2" Barlow with my CPC1100 with no issues with backfocus with visual and a diagonal. 

For planetary AP with a webcam have to remove the diagonal to reach focus bit that makes sense anyway, why do AP with a diagonal?!

[Louis D]

4 hours ago, kirkster501 said:

why do AP with a diagonal

They can only degrade the image, so best to leave them out of the imaging train.

[kirkster501]

On 19/08/2016 at 15:01, Louis D said:

They can only degrade the image, so best to leave them out of the imaging train.

... yes that's what I was saying, why bother using them.