Advice on new telescope/accessories

[kkara4 (Krishan)]

1) Prime focus: Canon 5D --> CWT-2070 --> RAD-1074 --> Baader Visual Back 3.25 Clicklock
2) Powermate 4x: Canon 5D --> CWT-2070 --> RAD-1074 --> Powermate 4x 2" --> Baader Visual Back 3.25 Clicklock

These are Televue part numbers that John Rodes suggested from Televue. Anyone see any glaring issues with this on the Edge 9.25?

[Camelopardalis (Dunk)]

I'm pretty sure you're going to need a spacer for prime focus at least, as the back focus will be nowhere near optimal. Check out the Edge HD white paper, I'm pretty sure it lists 146mm for all except the 8" Edge.

[astro744]

Originally Posted by kkara4

1) Prime focus: Canon 5D --> CWT-2070 --> RAD-1074 --> Baader Visual Back 3.25 Clicklock
2) Powermate 4x: Canon 5D --> CWT-2070 --> RAD-1074 --> Powermate 4x 2" --> Baader Visual Back 3.25 Clicklock

These are Televue part numbers that John Rodes suggested from Televue. Anyone see any glaring issues with this on the Edge 9.25?

Interesting why John did not recommend the PTR-4201 adaptor as follows:

2a) Powermate 4x: Canon 5D --> CWT-2070 --> PTR-4201 --> Powermate 4x 2" --> Baader Visual Back 3.25 Clicklock

The only thing I can think of is the CTW-2070 does not fit the PTR-4201 in which case you would need a third party Canon T-Ring as follows:

2b) Powermate 4x: Canon 5D --> Canon T-Ring --> PTR-4201 --> Powermate 4x 2" --> Baader Visual Back 3.25 Clicklock

I'm sure both options work and maybe you get vignetting with the standard T-Ring option especially with a full frame 36 x 24mm camera.

Maybe ask John out of curiosity if you are likely to get vignetting with my option 2b) and also ask what is the magnification factor with his option 2), (not using PTR-4201). Is it still 4x?

Addendum:

Maybe ask John when is the PTR-4201 recommended?

[kkara4 (Krishan)]

Originally Posted by astro744

Interesting why John did not recommend the PTR-4201 adaptor as follows:

2a) Powermate 4x: Canon 5D --> CWT-2070 --> PTR-4201 --> Powermate 4x 2" --> Baader Visual Back 3.25 Clicklock

The only thing I can think of is the CTW-2070 does not fit the PTR-4201 in which case you would need a third party Canon T-Ring as follows:

2b) Powermate 4x: Canon 5D --> Canon T-Ring --> PTR-4201 --> Powermate 4x 2" --> Baader Visual Back 3.25 Clicklock

I'm sure both options work and maybe you get vignetting with the standard T-Ring option especially with a full frame 36 x 24mm camera.

Maybe ask John out of curiosity if you are likely to get vignetting with my option 2b) and also ask what is the magnification factor with his option 2), (not using PTR-4201). Is it still 4x?

Addendum:

Maybe ask John when is the PTR-4201 recommended?

Astro, in my haste to post this up while rushing to do other things, I forgot to mention he did in fact suggest the PTR-4201, in the configuration exactly as per your 2b). However, since you use a standard T ring in 2b) dont you get vignetting as you say?

He specifically suggests the RAD-1070 method for prime focus stating that it will help reduce vignetting, so I assumed this would also be best for the powermate added in as well!

I will ask him both of your questions Astro, will see what he says. He doesnt seem to want to reply anymore

[kkara4 (Krishan)]

Originally Posted by Camelopardalis

I'm pretty sure you're going to need a spacer for prime focus at least, as the back focus will be nowhere near optimal. Check out the Edge HD white paper, I'm pretty sure it lists 146mm for all except the 8" Edge.

Dunk, I have asked him this exact question in my latest reply to him, will see what he comes back with . It may well be the case that he suggests going the PTR-4201 route to add the effective extension length it provides

[killswitch (Edison)]

I didnt need a spacer to achieve prime focus with my DSLR and C8. The T-ring/adapter from my refractor just went straight into the Clicklock.

SCT's have plenty of focus travel.

[kkara4 (Krishan)]

thanks Edison, I was doing more reading this evening and a few people on CN have similar thoughts. I am really thinking of going for the 2.4" route as opposed to the standard T ring. I will give Televue a call directly in the morning to finalise things.

[Camelopardalis (Dunk)]

I'm just quoting their white paper, where they're specify a tolerance of 0.5mm, as hard as it is to believe.

With my refractor, incorrect spacing was really obvious in the captured image (once I got it on my computer screen). I use the Celestron T-adapter with my C8 so it's already at the right spacing...or so I'm led to believe

[Amaranthus (Barry)]

I don't use a spacer on my C8 with a DSLR either. I just go Visual Back --> T-Ring --> DSLR and it comes to focus with plenty to spare.

[kkara4 (Krishan)]

thanks Barry/Dunk.

So this morning I got on to Televue, didnt quite catch the fella's name, but he was more than happy to provide advice...ended up being a 25 minute conversation .

He brought up the EdgeHD white paper on his computer and we had a chat.

He advised that it is always better with an SCT to have the camera further back than the specified prime focus distance as opposed to too close to the rear cell, but otherwise there is plenty of range at which focus can be achieved.

Lets look at prime focus photography first:

As stated by the white paper (page 13), T mount to sensor distance for DSLRs is usually 55mm. This is the same for the CWT-2070. The RAD-1074 adds about 3mm to the overall length (it has a very thin shoulder). Now on page 13/14 of the white paper, it can be seen that the Large T Adapter adds 91mm of extension from the rear cell, so with the 55mm standard T-sensor distance you get guess what 146mm. The Baader visual back provides a lot less (not sure how much) extension, so the Televue gentleman recommended the use of a X3C-0009 3.5" (88.9mm) 2" extension tube, thus getting me very close to the 146mm mark.

It sounds to me like Celestron want you to get it to the 146mm mark to get you in the middle of the forward/back focus range of the mirror.

The gent also recommended the use of their ACC-0003 2" visual back, where apparently the ring from the celestron diagonal threads onto so that you dont need a 3.29" to 2" thread adapter. However this is 2.1/4" long, so i dont think the 3" extension would be required, he then suggested the X2C-0008 2" extension, which again gets me close but slight further back.

So for prime focus full frame DSLR, his recommendation and my preferred option is:

Baader 3.25 Visual Back --> X3C-0009 --> RAD-1074 -->CWT-2070

Now his comments about the powermate 2" sizes were interesting. Apparently there may be an issue with inserting the powermate in place of the X3C-0009 above (or in between the X3C and RAD), because the glass element of the powermate would be too far physically from the camera sensor. There may also possibly be a problem with internal reflection. The gentleman recommended this instead:

Baader 3.25 Visual Back --> X3C-0009 --> Powermate 4x --> PTR-4201 --> standard Canon T ring.

Although the T ring is 42mm, because it is much closer to the glass element of the powermate physically, apparently there will be minimal vignetting. If I try the CWT-2070 and RAD-1074 combo with the powermate, then I would not need the extension tube (and ill have the CWT-2070 and RAD-1074 anyway), so I have a number of options to test out. I still feel ill get vignetting with the PTR-4201 though

[kkara4 (Krishan)]

John Rodes reckons the Baader 3.25 visual Back --> Powermate --> Rad-1074 --> CWT-2070 would still work though (approx same total back focus distance). I will confirm this with him as soon as I can. Havent even got to the diagonal yet!

[astro744]

Originally Posted by kkara4

thanks Barry/Dunk.

So this morning I got on to Televue, didnt quite catch the fella's name, but he was more than happy to provide advice...ended up being a 25 minute conversation .

He brought up the EdgeHD white paper on his computer and we had a chat.

He advised that it is always better with an SCT to have the camera further back than the specified prime focus distance as opposed to too close to the rear cell, but otherwise there is plenty of range at which focus can be achieved.

Lets look at prime focus photography first:

As stated by the white paper (page 13), T mount to sensor distance for DSLRs is usually 55mm. This is the same for the CWT-2070. The RAD-1074 adds about 3mm to the overall length (it has a very thin shoulder). Now on page 13/14 of the white paper, it can be seen that the Large T Adapter adds 91mm of extension from the rear cell, so with the 55mm standard T-sensor distance you get guess what 146mm. The Baader visual back provides a lot less (not sure how much) extension, so the Televue gentleman recommended the use of a X3C-0009 3.5" (88.9mm) 2" extension tube, thus getting me very close to the 146mm mark.

It sounds to me like Celestron want you to get it to the 146mm mark to get you in the middle of the forward/back focus range of the mirror.

The gent also recommended the use of their ACC-0003 2" visual back, where apparently the ring from the celestron diagonal threads onto so that you dont need a 3.29" to 2" thread adapter. However this is 2.1/4" long, so i dont think the 3" extension would be required, he then suggested the X2C-0008 2" extension, which again gets me close but slight further back.

So for prime focus full frame DSLR, his recommendation and my preferred option is:

Baader 3.25 Visual Back --> X3C-0009 --> RAD-1074 -->CWT-2070

Now his comments about the powermate 2" sizes were interesting. Apparently there may be an issue with inserting the powermate in place of the X3C-0009 above (or in between the X3C and RAD), because the glass element of the powermate would be too far physically from the camera sensor. There may also possibly be a problem with internal reflection. The gentleman recommended this instead:

Baader 3.25 Visual Back --> X3C-0009 --> Powermate 4x --> PTR-4201 --> standard Canon T ring.

Although the T ring is 42mm, because it is much closer to the glass element of the powermate physically, apparently there will be minimal vignetting. If I try the CWT-2070 and RAD-1074 combo with the powermate, then I would not need the extension tube (and ill have the CWT-2070 and RAD-1074 anyway), so I have a number of options to test out. I still feel ill get vignetting with the PTR-4201 though

Very informative! Thanks for filling in others on this forum with so much detail.

You probably spoke with David Nagler (Al's son) or the man himself, Al Nagler. They are full of knowledge and always ready to help sort out any issues and give good advice.

Don't be too surprised if in time you end up getting all the options and then you can tell use what works best. Extension tubes and adapters always come in handy and you can often use them on other telescopes. I've invested in multiple adapters, some I hardly ever use but they're there if I do need them.

Since you've done your homework and very well too, go with the recommendation and see what results you get.

[kkara4 (Krishan)]

Originally Posted by astro744

Very informative! Thanks for filling in others on this forum with so much detail.

You probably spoke with David Nagler (Al's son) or the man himself, Al Nagler. They are full of knowledge and always ready to help sort out any issues and give good advice.

Don't be too surprised if in time you end up getting all the options and then you can tell use what works best. Extension tubes and adapters always come in handy and you can often use them on other telescopes. I've invested in multiple adapters, some I hardly ever use but they're there if I do need them.

Since you've done your homework and very well too, go with the recommendation and see what results you get.

cheers Astro, I was thinking the same thing, just buy it all and sell what I don't find I need later!

Now I just need to confirm the visual back detail with Dunk, and then figure out what to do in terms of a Diagonal - the way I see it the Diagonal can be used in place of the extension tube since it is itself an extension of sorts, but then with a powermate hanging on the end it might want to always rotate like a big lever!

I have a feeling it was Al, because I heard the word "Al" in the earpiece and then there was a little break in the line, and maybe I was confused because I was expecting to hear more than just "Al"

[Camelopardalis (Dunk)]

Yeah it's interesting to see what sort of focal length the Powermate is introducing. The other thing to consider is that at the focal length you'll be at with the Powermate inline is such that the vignetting may be minimal, but also if you're imaging at f/40 then the chances are you're mostly interested in the bright planetary object in the middle of the field

Given what you're spending, chuck in the Celestron one for sh*ts and giggles...it's inexpensive and simple.

Don't get me wrong, the TV stuff is nice...really nice, they always make good stuff, but it 4 pieces when 2 would do?

Likewise, the Baader stuff is good quality stuff too. They really think about their target market. I use their visual backs on both my Edges.

[kkara4 (Krishan)]

Originally Posted by Camelopardalis

Yeah it's interesting to see what sort of focal length the Powermate is introducing. The other thing to consider is that at the focal length you'll be at with the Powermate inline is such that the vignetting may be minimal, but also if you're imaging at f/40 then the chances are you're mostly interested in the bright planetary object in the middle of the field

Given what you're spending, chuck in the Celestron one for sh*ts and giggles...it's inexpensive and simple.

Don't get me wrong, the TV stuff is nice...really nice, they always make good stuff, but it 4 pieces when 2 would do?

Likewise, the Baader stuff is good quality stuff too. They really think about their target market. I use their visual backs on both my Edges.

Thanks Dunk, and thanks for the PM!

Another interesting thing, apparently you can have insertion depth issues with visual backs on the 9.25 and 14 edge, because if you look at the white paper you can see the lens elements in the baffle tube are right near the threaded opening. For example, you cant use the Astro-Physics maxbright diagonal in their 3.25-2 visual back for this reason, it doesnt fully insert.

Not sure how much "taller" the baader is compared to the Astro-Physics though (I forget which edge you have Dunk!)

[Camelopardalis (Dunk)]

I use the Baader visual back and I've just got a tape to it and it's 54mm (give or take) long, from the base of the thread to the 2" aperture. So long as the nose of the diagonal isn't longer than that, you should be OK. Likewise the powermate.

I've just measured the protrusions on the parts I use, and the Everbrite nose is 35mm, the Baader diagonal 25mm, and the 2.5x Powermate 37mm (no idea what it would be for the 4x though ). Unscrewing the visual back, the thread is 13mm long (give or take)...

So for me, I get... 54mm - 13mm - 35mm = 6mm ... so the nose of the Everbrite diagonal is 6mm away from the outside end of the thread.

Obviously, this doesn't cause me any concern with the 8" or 11", but the 9.25" does seem to have the correcting lenses much closer to the end of any of the scopes but so long as the thread is the same length/depth, and the Powermate insertion depth isn't more than 40mm, you should be OK. This ignores any additional spacer you might use of course.

[killswitch (Edison)]

Originally Posted by kkara4

He advised that it is always better with an SCT to have the camera further back than the specified prime focus distance as opposed to too close to the rear cell, but otherwise there is plenty of range at which focus can be achieved.

Did he happen to mention the downside of having it so close to the cell?

[kkara4 (Krishan)]

Originally Posted by Camelopardalis

I use the Baader visual back and I've just got a tape to it and it's 54mm (give or take) long, from the base of the thread to the 2" aperture. So long as the nose of the diagonal isn't longer than that, you should be OK. Likewise the powermate.

I've just measured the protrusions on the parts I use, and the Everbrite nose is 35mm, the Baader diagonal 25mm, and the 2.5x Powermate 37mm (no idea what it would be for the 4x though ). Unscrewing the visual back, the thread is 13mm long (give or take)...

So for me, I get... 54mm - 13mm - 35mm = 6mm ... so the nose of the Everbrite diagonal is 6mm away from the outside end of the thread.

Obviously, this doesn't cause me any concern with the 8" or 11", but the 9.25" does seem to have the correcting lenses much closer to the end of any of the scopes but so long as the thread is the same length/depth, and the Powermate insertion depth isn't more than 40mm, you should be OK. This ignores any additional spacer you might use of course.

Thanks a lot Dunk! What is the length on the baader visual back from the opening to the top of the thread shoulder then? This measurement i know ill be good with. I am 99% sure the 2x powermate is exactly the same insertion depth as the 4x.

Also here is a reply I just received from John Rhodes:

Krishan,
The SCT can focus from with-in itself to 3 feet behind it. you won’t need an extension tube of any sort.
You would on other scopes though…

Well there is less chance of vignetting wit the CWT and you need it anyway for Prime focus… Why have two connectors.

The light that leaves the PM leaves it as Parallel light rays this is the beauty of the PM’s here is the approximate change for every inch
if you were to add any.

http://www.televue.com/engine/TV3b_p...id=53&Tab=_app

You see the 2X is negligible and the 4 has a very small increase of less than IX and all this is at 4 inches of extension.

The CWT and RAD1074 is just as short as the Powermate T ring adapters and a standard T ring. Or very close.



John Rhodes

Technical Sales Rep.
Tele Vue Optics Inc.


So looks like I wont be bothering with the PTR-4201 and associated T ring, and this will simplify swapping between setups (prime focus, powermate, visual and visual powermate) greatly. Also looks like I wont be bothering with the extension tube from John's reply.

Dunk do you find you can get focus just fine on your 11" no matter what combination of tubes/diagonal/powermate you use?

[kkara4 (Krishan)]

Originally Posted by killswitch

Did he happen to mention the downside of having it so close to the cell?

He did not, but looks like John has cleared up all the issue anyway! But then I come across things like this:

http://www.celestron.com/c3/support3...articleid=2427

so in the end I think I will just get the extension tube as Astro said, and sell it later if needed!

[Camelopardalis (Dunk)]

Originally Posted by kkara4

Thanks a lot Dunk! What is the length on the baader visual back from the opening to the top of the thread shoulder then? This measurement i know ill be good with. I am 99% sure the 2x powermate is exactly the same insertion depth as the 4x.

...

Dunk do you find you can get focus just fine on your 11" no matter what combination of tubes/diagonal/powermate you use?

The thread is 13mm depth, and the visual back threads on all of it.

I've not used my C11 for DSO imaging yet, only planetary...focus is easy with a bahtinov mask, although to be fair it's fairly straightforward with the big rubber knob as it allows relatively fine focus movements.

The issue as I understand it with the back focus is because the field of the Edge HD isn't perfectly flat, Celestron only specify that it's 3x flatter than a conventional SCT. This means that away from the optimal back focus distance, focus across the whole field is harder to achieve. I'm less likely to notice this with my aps-c sensor, but as you're full-frame you're more likely to find it an issue.