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NV: Tips for Viewing Globular Clusters

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41 replies to this topic

#26 careysub

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Posted 31 October 2021 - 03:28 PM

This summer I discovered and purchased a NOS Meade #918A Prism Diagonal that really is a prism.  If you look these up online the same Meade # now contains a mirror instead of prism.

This model seems entirely discontinued now. But I would assume that any 90 degree prism diagonal, like this one:

 

https://agenaastro.c...r-diagonal.html

 

would be similar. RIght?



#27 chemisted

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Posted 31 October 2021 - 03:49 PM

This model seems entirely discontinued now. But I would assume that any 90 degree prism diagonal, like this one:

https://agenaastro.c...r-diagonal.html

would be similar. RIght?

Yes, that Celestron is an excellent diagonal. Not too long ago, on another forum, TOMDEY posted an extensive analysis of diagonals and that one got incredibly high grades.


Edited by chemisted, 31 October 2021 - 04:06 PM.

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#28 Sarkikos

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Posted 25 November 2021 - 10:06 PM

Will a prism diagonal pass the longer wavelengths that the NV amplifiers need?

That question popped into my head after reading the first post.  :waytogo:

 

Mike



#29 Sarkikos

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Posted 25 November 2021 - 10:12 PM

Yes, that Celestron is an excellent diagonal. Not too long ago, on another forum, TOMDEY posted an extensive analysis of diagonals and that one got incredibly high grades.

I need to check my diagonal bin.  I might have received one of those with a telescope I bought.  thinking1.gif

 

[Opening plastic bin and looking through diagonals..]  Yep, I've got one!

 

Something else to store away for my NV future.

 

:grin:

Mike


Edited by Sarkikos, 25 November 2021 - 10:17 PM.

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#30 Sarkikos

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Posted 26 November 2021 - 01:54 PM

So, to optimize throughput for NV, the choice is between an inexpensive Celestron prism diagonal and very expensive silver diagonals?

 

Choose wisely.  lol.gif

 

Mike


Edited by Sarkikos, 26 November 2021 - 01:54 PM.


#31 Deadlake

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Posted 26 November 2021 - 02:16 PM

So, to optimize throughput for NV, the choice is between an inexpensive Celestron prism diagonal and very expensive silver diagonals?

Choose wisely. lol.gif

Mike

It’s more then that I’ve been trying to but a Baader 2” BBHS mirror for last year with no luck…..

Is there a 2” version that works?

Edited by Deadlake, 26 November 2021 - 02:17 PM.


#32 chemisted

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Posted 26 November 2021 - 03:31 PM

I'd be curious to hear why you want a dedicated 2" for globulars. They require an image scale that would typically use the 1.25" format.
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#33 Deadlake

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Posted 26 November 2021 - 03:43 PM

I'd be curious to hear why you want a dedicated 2" for globulars. They require an image scale that would typically use the 1.25" format.

Televue 55 mm EP.



#34 chemisted

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Posted 26 November 2021 - 03:56 PM

The 55mm is great for other targets but I have never used mine for globulars.
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#35 Deadlake

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Posted 27 November 2021 - 03:02 AM

Is there a thread for which diagonals work well with NV.
The default answer seems the 2” BBHS diagonal however it’s impossible to buy, what other mirror diagonals work well with good reflectance in the red/infra-red part of the spectrum.

#36 Highburymark

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Posted 27 November 2021 - 12:26 PM

I just use Baader T2 diagonals with refractors for NV - easy to swap between 1.25” and 2” clicklock eyepiece holders when using 55mm plossl for nebulae and Delites for globulars.

#37 Sarkikos

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Posted 27 November 2021 - 12:43 PM

Is there a thread for which diagonals work well with NV.
The default answer seems the 2” BBHS diagonal however it’s impossible to buy, what other mirror diagonals work well with good reflectance in the red/infra-red part of the spectrum.

Yes.  This thread.  An alternative to mirror diagonals is prism diagonals.  Prisms should pass the entire range of light available for NV.

 

Mike


Edited by Sarkikos, 27 November 2021 - 12:45 PM.


#38 Deadlake

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Posted 27 November 2021 - 01:46 PM

Yes.  This thread.  An alternative to mirror diagonals is prism diagonals.  Prisms should pass the entire range of light available for NV.

 

Mike

Prism do indeed behave as you describe, however if you have an APO which is well corrected the prism will cause the red spectrum to be shifted as per this thread:

https://www.cloudyni...e/#entry7334377

So a mirror is the best approach foe me.


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#39 Jeff Morgan

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Posted 27 November 2021 - 10:08 PM

The 55mm is great for other targets but I have never used mine for globulars.

A quick mind spots the Red Herring.

 

Even for Omega Centauri one would be in the 1.25” format. 



#40 hoof

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Posted Yesterday, 12:09 AM

Prism do indeed behave as you describe, however if you have an APO which is well corrected the prism will cause the red spectrum to be shifted as per this thread:

https://www.cloudyni...e/#entry7334377

So a mirror is the best approach foe me.


It’s unlikely even the best APOs correct properly the near infrared spectrum that NV sees. They typically correct for the visible part of the spectrum, either to match the eye’s or camera’s frequency range. Typical NV devices see into the infrared, which is likely to be poorly corrected even in the best APOs.

So i wouldn’t worry too much about the effects of the prism, unless you plan on using an IR cut filter to keep the NV from seeing the less-corrected near-IR part of the spectrum.

OTOH, using a dielectric diagonal will in effect act as an IR cut filter, so might work better with APOs simply by not reflecting parts of the spectrum the NV sees but the APO doesn’t correct properly :)

#41 Deadlake

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Posted Yesterday, 03:08 AM

It’s unlikely even the best APOs correct properly the near infrared spectrum that NV sees. They typically correct for the visible part of the spectrum, either to match the eye’s or camera’s frequency range. Typical NV devices see into the infrared, which is likely to be poorly corrected even in the best APOs.

So i wouldn’t worry too much about the effects of the prism, unless you plan on using an IR cut filter to keep the NV from seeing the less-corrected near-IR part of the spectrum.

OTOH, using a dielectric diagonal will in effect act as an IR cut filter, so might work better with APOs simply by not reflecting parts of the spectrum the NV sees but the APO doesn’t correct properly :)


Would a mirrored scope (Newtonian or SCT) have the same uncorrected near-infrared spectrum as well? Presume this is the case.

#42 hoof

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Posted Yesterday, 02:50 PM

Would a mirrored scope (Newtonian or SCT) have the same uncorrected near-infrared spectrum as well? Presume this is the case.

The light bending properties of a Newt are mirror-like, which reflects at the same angle (vs incoming angle) regardless of frequency.  That's why looking at a bathroom mirror doesn't show a prism type effect.  A Newt with regular aluminum/silver coatings for both the primary and secondary should have the same correction at 1000nm (near infrared) as at 300nm (UV) due to that.

 

Glass surfaces refract differently based on light's frequency, that's where the prism effect occurs and thus why things like "APO" are necessary.  Reflected surfaces do not suffer from this effect.

 

An SCT has both (the corrector is glass), so it's possible the correction is slightly different than with a regular newt.  But I doubt it would be anywhere as much as with a refractor.




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