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Large achromat for doing NV

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#1 GOLGO13

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Posted 12 February 2020 - 03:44 PM

I decided to go for trying out a big achromat with NV (and for general low power wide field with the eyepiece). While I'm OK with the 6 inch F4 newt for NV, I'm more of a refractor guy and I think this could be a nice capability. 

 

Getting a 6 inch F5...something I've wanted to try for a long time anyways. 

 

I know in the past folks brought up the need to use filters in these situations. Which I do anyways so I figure it should work fine. Any other thoughts on the use of this with NV?

 

One downside is I sold my FTX recently to fund other items. And that would have been perfect for alt/az mounting. I'm guessing my Vixen porta mount is not going to be able to handle such a scope. But my Vixen GPD2 will handle it no problem. I'll have to worry about a alt/az mount for it some other day.

 

I think this scope with my 34mm 68 degree should be pretty magical. But I also think the NV should be quite good with it.



#2 Eddgie

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Posted 12 February 2020 - 04:04 PM

The main thing you will want to ensure you have is a 695nm or similar IR pass filter,   At low power, the CA on these scopes does not look all that bad, but when you stick an image intensifer into the focuser, watch out!  Stars will bloat out hugely, and there will be no best focus.  If you start on one side and rack through focus, you will these varius size circles collapse and reform at different times on either side of focus, and this is your red, blue, and green color crossings all crossing over at different times.

While the eye does not see the red blur in an Achromat, the image intensifier will see it as the brightest of the circles.  

 

So, filter it heavily to sharpen the stars.  

Other than that, a lot of people use a big redocuer and love these scopes.  Hope you enjoy it!



#3 GOLGO13

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Posted 12 February 2020 - 04:08 PM

What about on the HA side of things? Do those filters work fine? Or do you need to use both types of filters together?



#4 GOLGO13

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Posted 12 February 2020 - 04:09 PM

The main thing you will want to ensure you have is a 695nm or similar IR pass filter,   At low power, the CA on these scopes does not look all that bad, but when you stick an image intensifer into the focuser, watch out!  Stars will bloat out hugely, and there will be no best focus.  If you start on one side and rack through focus, you will these varius size circles collapse and reform at different times on either side of focus, and this is your red, blue, and green color crossings all crossing over at different times.

While the eye does not see the red blur in an Achromat, the image intensifier will see it as the brightest of the circles.  

 

So, filter it heavily to sharpen the stars.  

Other than that, a lot of people use a big redocuer and love these scopes.  Hope you enjoy it!

 

I have a Baader 685 IR pass filter.



#5 Jeff Morgan

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Posted 12 February 2020 - 04:32 PM

I have a Baader 685 IR pass filter.

 

Sounds like a job for h-alpha.



#6 bobhen

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Posted 13 February 2020 - 07:20 AM

I use a 102mm F5 achromat and love it.

 

You will need a 685 Pass filter. I use 6 and 7 nm Ha filters and they work just fine as well.

 

If you get the Celestron/Synta 150 F5, (or the 120 F5 or 102mm F5 versions) you should consider getting a GSO 2” focuser to replace the original focuser. The shorter light path of the GSO will allow you to use a 2” .7 Antares reducer. So used at prime focus you can get these F5 refractors down to around F3.5 or so with good results.

 

I have often thought about getting one of these 150mm F5 refractors myself. So if you get one, let us know how you like it.

 

Below is my 102 F5 with the GSO focuser and NVD Micro. Very easy set up and a killer, wide filed, deep sky machine.  

 

Bob

Attached Thumbnails

  • IMG_31012.jpg

Edited by bobhen, 13 February 2020 - 10:45 AM.


#7 GOLGO13

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Posted 13 February 2020 - 09:52 AM

 

I have often thought about getting one of these 150mm F5 refractors myself. So if you get one, let us know how you like it.

 

 

Sounds like I should be fine. This one has a WO focuser on it...and it looks fairly nice. So hopefully the back focus isn't an issue. I don't want to replace it if possible. I can always do afocal if need be.

 

I'll post some thoughts on it. We've had non-stop clouds lately.



#8 GOLGO13

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Posted 17 February 2020 - 09:03 PM

Was able to try out the 6 inch F5 refractor today. The conditions were not great, but I was able to try it out.

 

Surprisingly the scope worked ok on my Vixen Porta mount with wooden legs. Adding Celestron Vibration pads fixes any jiggle. My personal opinion is everyone should have these vibration pads.

 

Anyways... In prime focus with the .7 reducer it didn't quite come to focus. However, this is where Afocal comes in. Afocal with my 40mm plossl was good on the Orion Nebula. Connecting the .7 reducer to the 2 inch to 1.25 inch adapter, and using the 40mm plossl afocal allows further reduction and focused fine.

 

I won't know how it compares to the 6 inch F4 newt until I can test it out further.

 

Binoviewer had no trouble with the new scope. Not that I'd be using it like that a ton, but it's good to know it works.

 

So far so good. Best news to me is the fact that it works on the Porta mount. However, I'd like to pickup a UA unistar or something at some point. I sold my FTX to fund all the sky and telescope magazines I acquired and the binoviewers. Kind of funny I have a scope that could have used it now..haha. But I'm comfortable with the sale. While the FTX was a great mount, I have too many different telescopes and switching between them with that mount is a bit annoying. 

 

Should be interesting to see how this compares to my other scopes. I did love the view of Pleiades and the Double Cluster in it with my 34mm 68 degree. While it's a beast, It's also sort of small. So different from the 6 inch F8s I've seen in the past.  


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#9 GOLGO13

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Posted 19 February 2020 - 11:28 PM

C150R

 

Some interesting results for NV. HA using my 6nm astronomic was very good! I like it better than the 6 inch F4 newt for HA work. I used it prime focus (had to use a 1.25 prism diagonal). I used it afocal with the 40mm plossl. I also used it afocal with the 40mm plossl, but added the .7 reducer right in front of that. This combo did well on the Rosette.

 

Very happy with the results for HA.

 

For non-HA it was not as good. Using the scope without a filter wasn't horrible...but any semi-bright star showed some bloat. With the Astronmik UHC (which is basically a 642nm) it was not too bad. But with the 685 baader I actually couldn't get a great focus. Not sure why. So that filter didn't seem to match well. I didn't try my 612nm. So based on those quick views I'd say the 6 inch F4 or the 10 inch dob would be better for non-HA. But HA seemed really good.

 

Conventional eyepiece wise I really am enjoying the low power views. But I do want to compare the view to my 103s (which has near perfect optics). I personally think this one has only OK optics. The star test showed good collimation, but some sort of SA or overcorrection or both. I had to break out my Star Testing Astronomical Telescopes book. Either way, the in focus images so far looked quite good. So hard to say how much to put into the star test.

 

Cool scope and a bit beastly. However, I find it quite manageable. I plan to put it on a Unistar Deluxe. But it actually works OK on my Porta mount (wooden legs).


Edited by GOLGO13, 19 February 2020 - 11:32 PM.

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#10 bobhen

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Posted 20 February 2020 - 08:19 AM

If you haven't seen it, HERE is a comprehensive review of the 150 F5 from a few years ago that also includes a short paragraph about Intensifier Ha observing.

 

The author also liked the scope.

 

Bob



#11 GOLGO13

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Posted 20 February 2020 - 09:21 AM

If you haven't seen it, HERE is a comprehensive review of the 150 F5 from a few years ago that also includes a short paragraph about Intensifier Ha observing.

 

The author also liked the scope.

 

Bob

Thanks so much for this. Looks like he used a green filter when doing the star test. I'll give that a try. It was a somewhat odd star test, but I'm more used to ED doublets for testing. One side of focus looked really good. The other side had brighter concentric in the middle (like half the circle), but darker circles on the outside. 

 

The only other achromat I star tested was a lovely Tele Vue Pronto. That one star tested very well.

 

All of this being said, the image seemed pretty good when focused. And for night vision (why I got it mostly), it seemed very good for HA, but possibly not as good for other items. But I will need more time with it.



#12 GOLGO13

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Posted 20 February 2020 - 10:08 AM

I should try the semi-apo filter I got with the NV to see if that makes a difference for non-HA items. It did seem to work quite well for normal observing.



#13 simoneb

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Posted 21 February 2020 - 01:31 PM

I am very glad you started this thread, and please keep updating us with your findings about the usage of NV with your 6 inch F5 Achromat.

I have a 150mm F5 Achromat myself (Skywatcher Startravel) that was completely forgotten in the closet, but as I have recently joined the NV club (I purchased the TeleVue NV kit) I was thinking about resuscitating that scope for wide field low power viewing. I was thinking more about Ha work (as I thought that the narrowband filtering would cancel any aberration issue). I have also purchased a 685nm longpass filter that will hopefully help with stars, but to date I still haven't done anything with this scope, so unfortunately I do not have results to report. As I have a pvs-14, I am obviously only interested in afocal results (I can't remove the objective lens and therefore prime focus is not an option).

Keep updating us and thanks again for starting this thread!

Simone      



#14 GOLGO13

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Posted 21 February 2020 - 07:20 PM

Well, I was wrong about the optics. I did both a star test and a Ronchi test. Star test looked very normal today. No issues. Why different today? The sky is much calmer and clearer.

 

Ronchi test showed very slight spherical overcorrection. But very good from what I could tell. This is my first time with my Ronchi eyepiece...very cool.

 

My 81s showed perfect star test and Ronchi (I figured as much).

 

So it appears to have pretty darn good optics considering the big fast optics.

 

CA is certainly there. Again, this is not an issue for me for night vision. And at low power on many objects I think it will be good.


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#15 GOLGO13

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Posted 21 February 2020 - 07:21 PM

I am very glad you started this thread, and please keep updating us with your findings about the usage of NV with your 6 inch F5 Achromat.

I have a 150mm F5 Achromat myself (Skywatcher Startravel) that was completely forgotten in the closet, but as I have recently joined the NV club (I purchased the TeleVue NV kit) I was thinking about resuscitating that scope for wide field low power viewing. I was thinking more about Ha work (as I thought that the narrowband filtering would cancel any aberration issue). I have also purchased a 685nm longpass filter that will hopefully help with stars, but to date I still haven't done anything with this scope, so unfortunately I do not have results to report. As I have a pvs-14, I am obviously only interested in afocal results (I can't remove the objective lens and therefore prime focus is not an option).

Keep updating us and thanks again for starting this thread!

Simone      

Simone,

 

I think you will be quite pleased with the PVS-14 afocal on this scope. It's a nice combo.




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