27 replies to this topic

[marcd]

Hi

Owner of a 20" f/4 Dobsonian, I happily observe with the 67 mm TV (with Paracoor 2) and my OVNI-B especially for nebulae...
I rather reserve the Prime for the galaxies.
The result is still fantastic!
I'm wondering about purchasing a Nexus corrector/reducer for f/3 with the Dobsonian.
In Prime, no particular problem (G = 55 with about 0°40') but I don't know what to expect with the 67mm TV in afocal ( G= 22, f/1.61 and about 1°48')!
What are your opinions and experiences of this montage? 

Thanks

marc

[sixela]

I've got an f/3.72, and I almost never use the TV67 with the Nexus. You just can't use a 2" reducer and expect something that uses almost 2" of field stop to have anything but a severely vignetted field stop. It works well with the 40mm Plössl and Pan 41, though (so it's an alternative for people who already have a Pan 41).

On a TV67 all it gives you is higher surface brightness compared to the TV67 without reducer, but not really more field. Less magnification and less usable AFOV. More aberrrations, too. It's not *totally* useless if you're hunting large and superfaint stuff but you're almost never going to use it.

What is useful on the TV67 is a Maxfield 0.95x from Teleskop Service.

Your math is strange, though. At f/4 native, a TV67 on a Nexus yields an f/ratio of f/3*27/67 = f/1.2.

The Nexus is worth it alone to get nebulae with a lightweight prime setup, though. That's a killer combination. And as I said a Pan 41 or even 40mm Plössl plus Nexus can replace a TV55 (removing the converter adds quite a few aberrations and is a pain to do in the field), although you can't expect really clean edge stars especially when not filtering aggressively.

On another note: if you have a 150/600 form Skywatcher the TV67 does work well with the 0.86x coma corrector/reducer bundled with the scope (which also flattens the field quite a bit). 0.75x, though, that's asking too much, I almost always prefer the 0.86x.

Edited by sixela, 15 March 2024 - 04:18 PM.

[a__l]

0.95x from Teleskop Service this is Chinese SharpStar.  For him movement  is necessary inside P2+ ~50 mm

This means that  if the telescope works with the P2,  will not work with the SharpStar. There is a high probability of this.

 

On the other hand, if you cut the truss tubes to work with the SharpStar, you will not be able to look through the glass eyepieces with P2. SharpStar is not suitable for use with glass eyepieces.

Edited by a__l, 15 March 2024 - 08:59 PM.

[sixela]

There is a high probability of this.

My scope is built to the P2 (with a focal plane just 21 mm above the racked in focuser), and I use a Maxfield.

The stop is removable, leaving only a 2” barrel and M48 threads with which you screw it onto a TV67 with 2" extenders as a 2” nose, and it disappears into the focuser, together with part of the 2” extenders on the TV67.

So no, I find your statement about “if your scope works with a P2 it won’t work with a Maxfield” to have a high probability of being inaccurate.

If the stop would not be removable you are correct (and for the Nexus the stop indeed needed to be machined off), but the stop is removable.

The Maxfield still fairly short and on my scope, despite being deep in the focuser, doesn’t even interfere on the inside with my filter slide, but of course how much the Maxfield sticks into the scope and light path at the telescope end would indeed depend on the scope. On a 16” I wouldn’t worry too much though.

Edited by sixela, 16 March 2024 - 02:18 AM.

[marcd]

Thank you Sixela for this information, always so precise and educational!
Sorry for the mistake with my mathematics, I put these numbers on a paper a few weeks ago (I don't remember how!!) and I copied them without thinking!!
So the Nexus alone on the Dob in Prime is worth it alone, I can try the assembly with the 67 mm but with a very poor expected result!

I'm not going to go for the Plossl 40 or Pan 41, the nights will already be quite full with the afocal, the Nexus and the Prime...

marc

[a__l]

My scope is built to the P2 (with a focal plane just 21 mm above the racked in focuser), and I use a Maxfield.

The stop is removable, leaving only a 2” barrel and M48 threads with which you screw it onto a TV67 with 2" extenders as a 2” nose, and it disappears into the focuser, together with part of the 2” extenders on the TV67.

So no, I find your statement about “if your scope works with a P2 it won’t work with a Maxfield” to have a high probability of being inaccurate.

If the stop would not be removable you are correct (and for the Nexus the stop indeed needed to be machined off), but the stop is removable.

The Maxfield still fairly short and on my scope, despite being deep in the focuser, doesn’t even interfere on the inside with my filter slide, but of course how much the Maxfield sticks into the scope and light path at the telescope end would indeed depend on the scope. On a 16” I wouldn’t worry too much though.

In the first photo. P2+0.95x SharpStar
Second photo. P2+0.95x SharpStar+40 mm M48.
If P2 is within the distance of the red arrows from the filter slide, the light cone for Nagler 31 (for example) will certainly be cut off. When using eyepieces without NV

If there is no filter slide and the end of p2 is near the end of the 1.5" travel focuser (this is usually the wall of the telescope tube), the SharpStar body will fall into the light path of the telescope.

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Edited by a__l, 16 March 2024 - 04:40 AM.

[sixela]

It will indeed be in the light path more than a P2 (and the distance you indicate is indeed correct).

Whether that matters enough to make its use “impossible” is something I’d leave everyone to judge personally.

I’m not going to discuss focal plane positioning and its effects, that would put us on a long winded off-topic tangent about “proper scope design”, the particulars of the P2 and what an “acceptable” fully illuminated field is, something more appropriate to the ATM section (not that it hasn’t been discussed before).

Especially not since I suspect the original poster isn’t building a scope but trying to use his. With your picture people can measure things on their own scope and draw whatever conclusion is warranted.

Indeed the Maxfield will stick out slightly more on the telescope side than a Paracorr, by roughly 35 mm. Despite the placement closer to the secondary, on an f/4 it provides 100% ilumination up to 35 mm diameter and 95% illumination up to 43 mm diameter, and the lower effective focal length will show more field than using a Paracorr and will yield higher surface brightness for extended objects.

Edited by sixela, 16 March 2024 - 04:43 AM.

[marcd]

Especially not since I suspect the original poster isn’t building a scope but trying to use his.
 

Yes, I always try...
Some nights, I get there!!

[DeepSky Di]

Hi all - please keep this discussion on topic and try to help marcd with the original question.

[sixela]

Just to summarise the differences between coma correctors:
-The 2" Paracorr type 2 has a barrel that is roughly 75 mm long and wants to have the focal plane 14mm above its shoulder. That means the scope end is just 89 mm from the focal plane. The price you pay for that is a 1.15x focal length extension, so less field and a higher effective f/ratio.

-The Nexus has an 80mm barrel but wants the original focal plane 57 mm from its shoulder. If your scope’s focal plane is not 57 mm above the fully racked in focuser, you need to order (from Yoko or Starizona) a version with the stop machined off. And its telescope end will be 48 mm closer to the scope than a P2, at 137 mm from the focal plane. But your focal length change factor will be 0.75x instead of 1.15x.

-The Maxfield’s factor is in between, 0.95x, the stop is removable without having to machine anything off, and the difference in the position of the telecope end is roughly 35 mm with respect to that of the Paracorr, so a smaller difference than a Nexus.

Just as a data point: suppose that you put the focal plane of a scope 21 mm from a racked in Baader Diamond Steeltrack, so that you can still use a Nikon Nav-HW 17 and have some wiggle room around focus.

The focuser’s scope end to focal plane distance is 83 mm in this configuration (in which you usually use the focuser racked out just 7 mm when using a Paracorr).

The Paracorr type 2 sticks out 6 mm on the telescope end. A Maxfield 41 mm (likely not sticking out of the focuser mounting plate that much on a large dobson). A Nexus with stop removed 54 mm (possibly intruding slightly into the light path).

On smaller imaging scopes it’s rarely an issue: they tend to have at least 60 mm from the racked in focuser to the focal plane, so you gain roughly 40 mm in the coma corrector telescope end’s position. Indeed all of these coma correctors intrude a lot less into the light path than the popular GPU imaging coma corrector. My 150/600 from Skywatcher comes with a 0.86x and it doesn’t stick out into the tube at all, nor do the Nexus, Maxfield and Paracorr.

Even some large dobsons have a focal plane set quite high: @Gavster’s 16” scope has the focal plane more than 57 mm from the racked in focuser's top, which allows him to use a standard Nexus. Many larger commercial dobsons also support focus for a DSLR with a T-ring and a nosepiece, and have the focal plane position set in a similar way (usually with a 2" extension or extra drawtube extender to add when you want to use eyepieces).

Edited by sixela, 17 March 2024 - 01:12 PM.

[a__l]

NV requires various filters. Which is very inconvenient without the slide filter.
Above in #10 (summarise) there is nothing about this..

 

What I want to add for the OP is that with the Nexus you will have even more problems than with the SharpStar if you want to use your telescope for glass eyepieces as well. If you want a universal telescope, think about this. It is not always possible to use NV and glass will continue to be a priority. Just from personal experience.

[sixela]

We've posted the dimensional data. Whether it is "a problem" or not depends entirely on the scope, and "just from personal experience" it's very possible to use an NVD and glass eyepieces on a single scope (indeed on many commercial scopes that's just a nothingburger; e.g. on a Skywatcher you just remove or add the 45 mm 2" extension tube they supply for using with 2" glass eyepieces).

Perhaps not if you tune a self-built scope to the bone for glass eyepieces, a specific type of coma corrector (the Paracorr Type2, aka P2) and an aggressively maximized fully illuminated field. In that case you might indeed paint yourself in a corner where only a P2 will work or you'll have to accept compromises (like having a filter slide or even a coma corrector slightly in the light path, not using a filter slide when you use a Nexus, or moving the focal plane slightly out temporarily with other trusses or different main mirror collimation adjustments, at the expense of a slightly smaller fully illuminated field...the list is long). But then, using a P2 is in itself a sort of compromise (given its 1.15x barlow factor has side effects not everyone wants all the time).

But many of us are willing to live with such compromises when necessary, and compromises are not "problems".

Anyone using a commercial scope doesn't even have to compromise, the manufacturer will often have made compromises to maximize flexibility at the expense of an aggressive but one-sided optimisation that some others might have preferred.

I'll reiterate: I think pointing out what can be important to check and is relevant to the original poster's question (will my scope work with a Nexus? Does it need the lip removed? Will it conflict with my filter slide if I have one and the lip was removed? How much will the Nexus intrude in the light path?) are fair and on-topic. And your posts that helped answer those are still in the thread.

Discussions about which scope designs or coma correctors are "correct" and which are "wrong" or "a problem" are not (though the design criteria are regularly discussed, but on the ATM forum).

And we should not go around saying problems "are likely" without enough context, certainly given there are no problems on the vast majority of scopes that I've seen; I tried a stopless Nexus+NVD at a star party on more than ten scopes last weekend, unusable on zero scopes -- I confess I am not counting the scopes with a 1.25" focuser ;-).

A Nexus indeed has the potential for introducing issues because of a telescope end that protrudes into the tube/UTA that a Sharpstar/Maxfield won't, and that one has the potential to introduce an issue that a P2 won't.

But if there are no problems, it's not "more problems than a Sharpstar", because 0 > 0 is a false statement.

Edited by sixela, 17 March 2024 - 06:32 PM.

[marcd]

Thank you for this information even if I find myself very stupid to understand some of your conclusions!!
With my Sumerian 20" f/4 Dobsonian, I have always observed with glass (and Paracorr 2) with great pleasure, since purchasing the OVNI-B, I admit to having only used this one ( afocal 67 and Prime), the pleasure is so great and the observations rich!
I am not an optical specialist/engineer just a visual astronomy enthusiast. Your information helps me progress and I thank you for it!
For the Nexus, I sent an email to Joko (OVNI dealer) to order one (delivered without a lip) hoping for a positive result based on the pleasure of my observations...

marc

[sixela]

Don't know what focuser yours has, but on my 20" f/3.72 Sumerian no issues (the truss poles were cut to make the Nexus just stay clear of the filter slide, but you don't have one; Michael these days usually puts the focal plane so that a Paracorr gets in the middle of the focuser travel and so that an ES HR CC or 1.7x Baader coma corrector/GPK can still just be used, so that would suggest even less potential for interference, but what he picked for the truss length may have depended on what you told him).

If you have a Paracorr it's of course easy to predict (cfr. above) where the end of the Nexus will end up on your scope (just shy of 5 cm closer to the secondary than the end of your Paracorr).

Edited by sixela, 18 March 2024 - 05:44 AM.

[marcd]

The focuser and the filter slide are from Moonlite.

[sixela]

You'll see. Might need to put the filter on the Nexus and use the top hole, time will tell; it's not that much of a fuss if you use the Nexus with the same filter for prolonged periods of time (e.g. H-alpha for nebulae). From memory it's going to be close (you'll usually have about 25 mm from the focal plane to the Moonlite, the Moonlite will be roughly 60 mm high, the ring width is usually roughly 30 mm, and the Moonlite filter slide riser adds roughly 20 mm. That's 135 mm and you need 137 mm.)

Unfortunately the Canopus has a sling and really wants the sling positioned on the proper height with respect to the mirror, so it's not easy to throw the focal plane up through collimation using the main mirror cell's collimation bolts (at least not if you don't want either astigmatism or messing with the sling). Mind you, I don't know if the sling lies in the correct plane now as I'm probably the only Canopus owner whom I've seen that was ever obsessing over it ;-).

If you do want to continue to use the filter slide, slightly shorter trusses will always work. Of course throwing the focal plane out (either by tweaking the sling's position in the mirror box or by shortening the trusses) to make a Nexus stop interfering with the slide (if necessary) will reduce the size of your fully illuminated field (but no idea what it is now, that depends on diagonal sizing since diagonals come in discrete sizes).

Adding a raiser to the filter slide base would also work (if it's just a couple of mm a few washers will do), but at the expense of perhaps making it intrude into the light path when it wasn't yet intruding. But perhaps it's already intruding in the light path and then it wouldn't make that much of a difference.

Finally one last thing if it's really close: you can put the Nexus slightly closer to the Ovni-B, but you'll have imperfect coma correction (not that much of a problem when using 40° of FOV on an f/4), a tiny bit of extra astigmatism and a tiny bit of field curvature (although the latter may combine favourably with that of the scope). It's worth a shot to see what it would do. You don't need to shave a lot off: reducing the distance also reduces the focal reduction somewhat and so you gain more distance than what you shaved off. I used a distance that was 5 mm off for months until I bit the bullet and really made a spacer tuned to the P2 (nailing the distance by measuring where the focal planes were laid out) and reused that on the Nexus. There are less aberrations now, but prime+Nexus with the wrong spacing was still beating all the afocal stacks with respect to edge of field aberrations.

I'd experiment and see what you like best (or in the words of someone else "dislike the least").

Edited by sixela, 18 March 2024 - 06:41 AM.

[marcd]

Thanks Sixela!

I think I understand it now!
If this should not work, the solutions seem simple (filter on the Nexus or a few washers at the base of the filter holder...)

[sixela]

Yeah, you can make it work. What is most convenient/practical, that's something the future will tell (depending on how much interference there would be between the Nexus and the other slots of the filter slide.

[a__l]

Moonlite has quite strict restrictions on the use of the slide. Therefore, the Nexus will not fit on your telescope if you use a slide. 50 mm is a lot. You have the option of using a large hole in the slide (which I wrote about earlier in a deleted post).
Therefore, you and the Nexus will have two problems, this is the inability to use the slide and the Nexus body will be in the light path.
You can cut the truss tubes, then you will have problems using glass eyepieces and P2. I don't recommend it.

[a__l]

Yes, you can make a UTA up/down switch. This is what one telescope manufacturer does. But it will probably be difficult for you.

[Joko]

Moonlite has quite strict restrictions on the use of the slide. Therefore, the Nexus will not fit on your telescope if you use a slide. 50 mm is a lot. You have the option of using a large hole in the slide (which I wrote about earlier in a deleted post).
Therefore, you and the Nexus will have two problems, this is the inability to use the slide and the Nexus body will be in the light path.
You can cut the truss tubes, then you will have problems using glass eyepieces and P2. I don't recommend it.

Several of our NV customers have the Nexus and it works extremely well with our NV eyepieces. Please note that we sell a modified version of the Nexus.

You're not an OVNI Night Vision customer and we never sold any product to you. If your telescope or NVD has manufacturing problems to reach focus with the Nexus, I recommend that you contact the manufacturer.

Edited by Joko, 23 March 2024 - 09:22 AM.

[a__l]

Several of our NV customers have the Nexus and none of them never experimented what you describe. Please note that we sell a modified version of the Nexus without the flange.

We never sold you any product so if your telescope has manufacturing problems to reach focus, I recommend that you contact the manufacturer.

This is exactly what I am writing about this version of Nexus. See my comparison photo with the P2 above.
Contacting the manufacturer will not help. Lots of changes for a telescope like this. Including automatic balancing.
Alternatively, buy a second set of truss tubes. And think about changing the balance.

[marcd]

I have already ordered the Nexus anyway and I could test it soon...
I will let you know the results.
Thanks

marc

[nicknacknock]

All,

 

Several off topic posts have been removed. Please stay on topic.

 

---

 

Mark,

 

Let us know how it goes with the Nexus once received

[marcd]

Thank you for that!

I will give you news as soon as possible.

marc