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Vixen VMC 110L with binned flip mirror and new Takahashi prism fitted

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#1 Magnetic Field

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Posted 18 August 2019 - 09:12 AM

I read here only sporadically anymore. Nothing has changed since the last 25 years and my absence from the amateur astronomy scene. It is just mind blowing the brainwashing that is going on in the refractor forum; I often wonder if the forum is full of flat earthers and moon landing conspiracists.

 

Although I don't contribute much anymore but I thought it is only fair I post some pictures of my Vixen VMC  110L modification.

 

Recap:  I made a big fuss about fitting a prism to the back port of the VMC 110L:

https://www.cloudyni...-prism-blunder/

 

And maybe the following is useful for other people who want to get rid if the flip mirror.

 

1. Don’t buy a Vixen VMC 110L. It is literally junk.
2. If pure insanity has overpowered you and you insist on buying one I would think it worth fitting a diagonal prism. If you don’t mind f-ratios you can also consider mounting a star diagonal.
3. All I can say the Takahashi prism really reduces glare around the planets and I have had moments of fantastic views of Jupiter recently.
I was able not just only to see the Great Red Spot but also to see it in pink colour. And this from my flat window in the UK.
4. I was able to split Gamma Cassiopeia, WDS BU 1028 (magnitude 2.20/10.90, separation:  2.1”, position angle 260 degrees) a double star in Cassiopeia. The dimmer companion with averted vision and moments of good seeing looked like the tip of  a needle right outside of the first diffraction ring of the main component. This was not in the UK but in my native country in a valley in the Alps.
5. I also had great moments with Saturn. 2 brown bands and some shading on the North pole.
6. There are some unresolved eyepiece issues though when observing the moon (which I will post it in the coming days in the eyepiece forum).

 

Removing and binning the made in China and designed in Japan junk Vixen flip mirror

 

My photos are crap but will convey the message. When I took the photos there was a bereavement in my family.

 

Why do you need to remove the flip mirror?

 

Firstly, I aver it is an engineering disgrace and secondly if you want to use the top side eyepiece holder (see Figure 1) you cannot fully screw it in because the railing part of our junk flip mirror comes into your way. Alternatively, if you don’t wanna trifle around you could use the back side adapter but this means you will fit your prism (and even more so diagonal mirror) a far distance out from the back port (see Figure 7) and your f-ratio and field of view will become even smaller than with the shorter top side eyepiece barrel holder.

 

Fig0.jpg

 

1. You can easily remove the main optical tube from the main mirror/baffle tube unit by unscrewing it (you unscrew it counter clockwise, right-hand screw). Figure 2 shows the unscrewed optical tube and the main mirror.

 

Fig1.jpg

 

2. In the next step remove the main mirror. Just unscrew the 6 screws with a PZ-1 screwdriver until the screws  fall out (see Figure 3). Also don’t forget to unscrew the main focusing knob otherwise you cannot remove the main mirror/baffle tube unit from the housing case. For this I think you need a 1.0mm allen key for the lock screw. If the key is still too big file it to size with a wee little needle file (I only had a 1.5mm allen key at the time).  Figure 3 shows the focusing knob with the rubber cup removed. The rubber cup is glued on but can easily be pulled off (they must have used something similar to Pattex ).

 

Fig2.1.jpg

 

3. Once the main mirror has been removed from the housing body, you can remove the rubbish flip mirror and throw it into the bin (or bring it to the recycling centre and pass the bill to Vixen). Use again a PZ-1 screwdriver for the two little screws (see Figure 4).

 

Fig32.jpg

 

4. I used blackboard black metal paint to paint the interior of the back side housing part. I guess it won’t make an iota of a difference and there are not many photons that have not been absorbed in the main tube. Use painter’s tape to mask off the edge (see Figure 5 and 6). Figure 1 shows an arrow that points to a dehumidifier (the little bags that we often find with made in China products). I do not know what happened but it took more than 2 months until the smell of paint went away (paint was dry within 24 hours though).

 

Fig52.jpg

 

Fig6.jpg

 

5. After assembly. I now use the top side eyepiece holder (which is 3cm long once fully screwed in, see Figure 7) at the back port. As Figure 7 shows this means the centre of the Takahashi prism is located 5cm from the back side port.

 

Fig7.jpg

 

6. Collimation took me a few hours from my flat window in the UK. It was worth it because I ended up with textbook like airy rings. It also holds up after a British Airways flight from Heathrow to Europe (OTA in my cabin bag). There is no recipe for collimation. It is trial an error. The Cheshire collimation tool eyepiece with crosshair is pretty much useless except that you get a rough first ballpark collimation. Also the intra- and extra focal star patterns are pretty much useless (except for checking astigmatism). I don’t care how my textbook like air rings in-focus look like outside of focus. I don’t give a ****; all that counts is the in-focus star.

 

 

7. The prism has the effect that the main mirror has to move closer to the secondary mirror to compensate for the new location of the prism (in comparison to the flip mirror location). But this also has a consequence that the factory set focal length will increase and the focal ratio of f/9.4 will change. But by how much? Ideally we would need to get closer to the back port than the 5cm (see Figure 7). I bought a new 30mm Vixen NPL eyepiece. This again gives me roughly 1.25 degrees true field of view (compared to the 1.25 degrees with the 25mm Vixen NPL and 1030 mm focal length).

 

 

8. I did a star drift method (e.g. https://www.oreilly....04/ch04s15.html) with Jupiter to get an idea of the real new focal length. There are various reasons why I wasn’t able to use a star from my flat window. Also doing this method with an alt-az mount is a pain in the **** as you need to make sure the start drifts along the centre of the eyepiece from one side to the other.  The new focal length is 1150 to 1200 mm for the current prism  set-up (see Figure 7). I think it is closer to around 1150mm (probably need to repeat it a few times to get some ideas of the measurement error; also the method is better for zero declination stars at the celestial equator):

 

Jupiter with 30mm Vixen NPL:

 

Time to drift through field of view: (5*60 + 20) seconds. Jupiter’s declination approx. –(22 +11/60) degrees at the time of the test:

 

(5*60+20)seconds*0.2507*cos(-(22+11/60)*PI/180o)/60

 

This would give a field of view of 1.24o (60 in the above equation is the conversion factor from arcminutes to degrees).

 

For the 8mm  Vixen NPL and 90 seconds drift:

 

90seconds*0.2507*cos(-(22+11/60)*PI/180o)/60

 

This gives a field of view of approx. 0.34o

 

For the 6mm Vixen NPL and 65 seconds drift:

 

65seconds*0.2507*cos(-(22+11/60)*PI/180o)/60

 

This gives a field of view of approx. 0.25o

 

If we assume f=1150mm:

 

e.g. 48o  apparent field of view  of the NPL eyepiece:

 

• 48o/(1150mm/30mm)=1.25o
• 48o/(1150mm/8mm)=0.33o
• 48o/(1150mm/6mm)=0.25o

 

Or with field stops:

 

• NPL 30mm: 25mm field stop/1150mm*57.269=1.24o
• NPL 8mm: 6.5mm/1150mm*57.269=0.32o
• NPL 6mm: 5mm/1150mm*57.269=0.25o


Edited by Magnetic Field, 18 August 2019 - 09:15 AM.

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#2 nirvanix

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Posted 18 August 2019 - 05:36 PM

Very comprehensive, thanks for posting this. I agree the flip mirror in the VMCs is a poorly executed thing. As I mentioned before, to keep the mirror in the right orientation I've got it buttressed with small pieces of sponge underneath and on the right side. Nevertheless I'm intrigued by what you've done with yours, especially as you say it reduces some of the glare. Perhaps I'll try it sometime!

 

Many of the forums on CN reinforce various myths, perhaps unintentionally as they may have come about through misunderstanding. Some of the most annoying (to me) are that you can't get good planetary views from a large aperture reflector, or from telescopes with large COs. Those probably came about due to early large newts with poorly figured mirrors.


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#3 sneeds

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Posted 19 August 2019 - 07:52 PM

Great read and tests MF, thanks.



#4 nirvanix

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Posted 20 August 2019 - 11:48 AM

I read this again and noticed you swapped around the eyepiece holders - very clever! I measured the top eyepiece holder on my VMC95L and it's only 2.5 cm from where it seats in the scope to the top - you report the one on the VMC110 is 3cm? Swapping would bring the diagonal 2.5 cm closer on my VMC95L, but if the optical path of the diagonal is longer than that I wonder if it would reduce the effective aperture of my scope? Do you know the optical path of the Tak diagonal?

 

Seems to me you're very satisfied, sans flip mirror, with the optical performance of your VMC110. My VMC95 is very,very sharp, but there is a bit of contrast reducing glare. You report that the Tak prism diagonal removed much of it. That's intriguing.

 

Enjoy your new rig.



#5 photoracer18

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Posted 20 August 2019 - 02:18 PM

Its things like this that show why my small Mak is the LOMO 95 instead of something from Meade, Celestron, or Vixen. A real SCT threaded VB on the back to take easily available parts and no kluge flip mirror.

 

Nice work. 


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#6 Magnetic Field

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Posted 21 August 2019 - 07:11 AM

Its things like this that show why my small Mak is the LOMO 95 instead of something from Meade, Celestron, or Vixen. A real SCT threaded VB on the back to take easily available parts and no kluge flip mirror.

 

Nice work. 

I pretty much doubt the 95mm LOMO has something to show for it when we talk about deep sky observing (due to the small aperture). I once owned a Meade ETX-90 and by all means it was completely useless for everything.

 

Even with the rubbish flip mirror the Vixen VMC 110L is an excellent deep sky performer (pin point stars at least for visual observing).


Edited by Magnetic Field, 21 August 2019 - 07:25 AM.


#7 Magnetic Field

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Posted 21 August 2019 - 07:23 AM

I read this again and noticed you swapped around the eyepiece holders - very clever! I measured the top eyepiece holder on my VMC95L and it's only 2.5 cm from where it seats in the scope to the top - you report the one on the VMC110 is 3cm? Swapping would bring the diagonal 2.5 cm closer on my VMC95L, but if the optical path of the diagonal is longer than that I wonder if it would reduce the effective aperture of my scope? Do you know the optical path of the Tak diagonal?

 

Seems to me you're very satisfied, sans flip mirror, with the optical performance of your VMC110. My VMC95 is very,very sharp, but there is a bit of contrast reducing glare. You report that the Tak prism diagonal removed much of it. That's intriguing.

 

Enjoy your new rig.

1. I am in a valley in the Alps. I cannot make a snapshot of the Vixen 1.25" eyepiece holder with 36.4mm thread. I post a link instead:

 

https://www.amazon.c...r/dp/B00140CEAE

 

Vixen completely lost the plot and vitiated their reputation. They are now incompatible with their own standards.

 

I ordered directly from Japan (£13 ebay UK)  the Vixen 36.4mm threaded eyepiece holder. The length is 27mm (3mm shorter than my top side eyepiece holder from the flip mirror).

 

 

2. It turned out the the pitch of the original Vixen 36.4mm thread is different than the pitch of the thread of the 36.4mm eyepiece holder of the current Vixen VMC 110L. The original labelled Vixen eyepiece holder (see link above) cannot be used on the VMC 110L (and I guess 95L).

 

3. By the way: can anyone have a go at Gamma Cassiopeia, WDS BU 1028 (magnitude 2.20/10.90, separation:  2.1”, position angle 260 degrees). Was it just a fluke and in my imagination that it can be split with small of a telescope as my Vixen VMC 110L? Edit: I posted the question also in the double star section: https://www.cloudyni...vixen-vmc-110l/

 

4. Optical path length: Maybe this if of help (Baader pinched the article from a cloudynights article posted some time ago):

 

https://www.baader-p...14-03-06_v2.pdf


Edited by Magnetic Field, 21 August 2019 - 07:51 AM.


#8 nirvanix

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Posted 21 August 2019 - 10:44 AM

Thanks for the reply MF. I did give it a try last night - splitting gamma Cass. with the VMC95 and my 4" refractor, but the seeing was like soup boiling on the stove, so no luck. I'll try again soon. In theory it should be within the limits of performance, but it will depend on keeping the shine from the primary well controlled. That means smooth optics and precise collimation.

 

I'm quite tempted to go for the prism diagonal - it will cost more than I paid for the VMC95L!

 

By the way, I had my VMC95 out yesterday afternoon for birding and that's when I appreciate it's weight and format. I put it and a photo tripod in a backpack and mountain biked to a nature preserve in my area.


Edited by nirvanix, 21 August 2019 - 10:56 AM.


#9 Magnetic Field

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Posted 21 August 2019 - 11:38 AM

Thanks for the reply MF. I did give it a try last night - splitting gamma Cass. with the VMC95 and my 4" refractor, but the seeing was like soup boiling on the stove, so no luck. I'll try again soon. In theory it should be within the limits of performance, but it will depend on keeping the shine from the primary well controlled. That means smooth optics and precise collimation.

 

I'm quite tempted to go for the prism diagonal - it will cost more than I paid for the VMC95L!

 

By the way, I had my VMC95 out yesterday afternoon for birding and that's when I appreciate it's weight and format. I put it and a photo tripod in a backpack and mountain biked to a nature preserve in my area.

I used Loctite Threadlock glue for the little lock screw of the eyepiece holder (see Fig 7).

 

I am scared that my collimation (which gives me textbook like airy rings of the focused star) will hold up.

 

I think the best solution would be a Takahashi like clamp mechanism retaining ring (that is being used for the prism to hold the eyepiece) instead of the little lock screw.

 

I am also tempted to flock the main inner optical tube.

 

Btw: Knowing the following helped me a lot. I would be lying if I told you the companion just popped out.  Gamma Cassiopeia the position angle of the companion is 260 degrees (counted from celestial North -> Left -> South -> Right). That means no matter the direction of travel (--->) of the pair (O o) through the field of view of the eyepiece the dimmer companion  (o) is leading the main brighter star (O):

 

 O o ---> 

 

 

<--- o O

 

 

Edit Costs:

 

Last year I paid half price £199 for the Vixen VMC 110L OTA (it included a 25mm Vixen NPL eyepiece and Vixen red dot finder). It was a display item. I paid £90 for the Takahashi prism and another £50 for the Vixen 30mm NPL. Blackboard metal paint happened to be  lying around (otherwise it is cheap off ebay).


Edited by Magnetic Field, 21 August 2019 - 11:45 AM.

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#10 nirvanix

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Posted 21 August 2019 - 12:06 PM

I agree the brass compression ring is better than the small thumbscrew. Vixen ( and everyone else) should retire that format. If you have a dremel you could probably recess an area in that eyepiece holder to insert a strip of brass and make your own compression ring.

 

Regardless, your optical alignment will never be as bad as it was under the strictures of the higgledy-piggledy flip-flop mirror.

 

I flocked my VMC95L with Protostar and it was worth the doing. It got rid of some scatter from the urban glow I live under and I'm able to see dimmer stars.

 

You got a good price on the diagonal. It will cost me about $225 Cdn



#11 Magnetic Field

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Posted 21 August 2019 - 12:07 PM

I used Loctite Threadlock glue for the little lock screw of the eyepiece holder (see Fig 7).

 

I am scared that my collimation (which gives me textbook like airy rings of the focused star) will hold up.

 

I think the best solution would be a Takahashi like clamp mechanism retaining ring (that is being used for the prism to hold the eyepiece) instead of the little lock screw.

 

I am also tempted to flock the main inner optical tube.

 

Btw: Knowing the following helped me a lot. I would be lying if I told you the companion just popped out.  Gamma Cassiopeia the position angle of the companion is 260 degrees (counted from celestial North -> Left -> South -> Right). That means no matter the direction of travel (--->) of the pair (O o) through the field of view of the eyepiece the dimmer companion  (o) is leading the main brighter star (O):

 

 O o ---> 

 

 

<--- o O

 

 

Edit Costs:

 

Last year I paid half price £199 for the Vixen VMC 110L OTA (it included a 25mm Vixen NPL eyepiece and Vixen red dot finder). It was a display item. I paid £90 for the Takahashi prism and another £50 for the Vixen 30mm NPL. Blackboard metal paint happened to be  lying around (otherwise it is cheap off ebay).

The prism housing body of the Takahashi prism is smaller in size than the Baader 1.25" prism.

 

Maybe worth keeping in mind that a big prism body will obstruct the focusing knob.

 

Shown here the Takahashi prism in combination with the 30mm Vixen NPL ocular:

 

Fig8.jpg

 

]Fig9.jpg

 

Fig10.jpg


Edited by Magnetic Field, 21 August 2019 - 12:08 PM.


#12 Magnetic Field

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Posted 21 August 2019 - 12:14 PM

The prism housing body of the Takahashi prism is smaller in size than the Baader 1.25" prism.

 

Maybe worth keeping in mind that a big prism body will obstruct the focusing knob.

 

Shown here the Takahashi prism in combination with the 30mm Vixen NPL ocular:

 

Fig8.jpg

 

]attachicon.gif Fig9.jpg

 

attachicon.gif Fig10.jpg

One more thing before I forget:

 

The field stop of the 30mm Vixen NPL ocular is 25mm.

 

The baffle tube diameter (from recollection) is about 22mm.

 

However, I cannot see or do not notice (visually) any vignetting or image degradation at the field edge of the Vixen NPL 30mm during daylight observing or night time sessions.



#13 fred1871

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Posted 21 August 2019 - 09:47 PM

Splitting a difficult - very large Delta-m and small separation - pair such as Gamma Cas with a small scope the OP says is "junk" does strain credibility somewhat. Next, what, Procyon perhaps? I'll wait to hear what other observers manage with somewhat more aperture. False positives can happen, and sometimes happen even with experienced observers. I'm too far south to try this one.

 

Burnham discovered this close companion (around 2.2") with the Lick 36-inch, though subsequently he saw it with Lick's 12-inch (after missing it with 18.5-inch).

 

Some observers (preferably experienced with close unequal doubles) could try it with various apertures, and perhaps report their findings in the Double Star forum where someone has already begun a thread on the subject.


Edited by fred1871, 21 August 2019 - 09:53 PM.


#14 John Fitzgerald

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Posted 22 August 2019 - 10:30 AM

Splitting a difficult - very large Delta-m and small separation - pair such as Gamma Cas with a small scope the OP says is "junk" does strain credibility somewhat. Next, what, Procyon perhaps? I'll wait to hear what other observers manage with somewhat more aperture. False positives can happen, and sometimes happen even with experienced observers. I'm too far south to try this one.

 

Burnham discovered this close companion (around 2.2") with the Lick 36-inch, though subsequently he saw it with Lick's 12-inch (after missing it with 18.5-inch).

 

Some observers (preferably experienced with close unequal doubles) could try it with various apertures, and perhaps report their findings in the Double Star forum where someone has already begun a thread on the subject.

Yes, this strains credibility......YMMV.



#15 Magnetic Field

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Posted 22 August 2019 - 01:52 PM

Yes, this strains credibility......YMMV.

Interesting will be the 6 stars in Trapezium with the prism.

 

I tried it many times (telescope with flip mirror) last winter but with no avail.

 

Btw: I actually never said the optics are rubbish although I have no evidence either way good and bad (I don't believe in the star test because it cannot be used to judge optical quality).

 

But the flip mirror is tat. Also, it took me many many hours to get collimation with textbook like airy rings of the focused star. In the end it was pure luck that I ended up that way. If it goes out of collimation I would not bet money on it I will succeed again with good collimation.

 

I cannot recommend that telescope.

 

Coming back to credibility: you mean the guy (it was not me) in deep sky forum who claimed to have seen the horse head nebula with his 7cm Pronto refractor.  I don't believe it.

 

At least I am realistic and inquried here (and in the double star forum) if people can split Gamma Cassiopeiae with their 4 or 5" (good optics essential) telescopes. By split I mean to look at 260 degrre position angle. I don't ask anyone here (I have never claimed it that I have split it with without knowing where to look at) to "discover" the double star  from "scratch" without knowing a-priori the position angle.

 

We should all relax.


Edited by Magnetic Field, 22 August 2019 - 01:56 PM.


#16 John Fitzgerald

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Posted 22 August 2019 - 02:26 PM

<snip>

 

Btw: I actually never said the optics are rubbish although I have no evidence either way good and bad (I don't believe in the star test because it cannot be used to judge optical quality).

 

<snip>.

What?  The star test is the classic textbook test of optical quality, although with a real star, it requires very good seeing.



#17 Magnetic Field

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Posted 22 August 2019 - 02:37 PM

What?  The star test is the classic textbook test of optical quality, although with a real star, it requires very good seeing.

There is no point quibbling over the star test.

 

It doesn't work.

 

If you think you can test your optical performance and attach numbers to it based on the star test.  This has as much credibility as my report of Gamma Cassiopeiae.

 

The star test is not the subject of this discussion though.

 

It will also be interesting to see how the prism is doing on the 4 cratelets in Plato. I have never been able to see more than 3 at the same time because wee little craterlets  D and C are close together and they always appear as 1 single craterlet in my Vixen. I also reckon this has a much to do with good seeing though.



#18 John Fitzgerald

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Posted 22 August 2019 - 03:09 PM

There is no point quibbling over the star test.

 

It doesn't work.

 

If you think you can test your optical performance and attach numbers to it based on the star test.  This has as much credibility as my report of Gamma Cassiopeiae.

 

The star test is not the subject of this discussion though.

 

It will also be interesting to see how the prism is doing on the 4 cratelets in Plato. I have never been able to see more than 3 at the same time because wee little craterlets  D and C are close together and they always appear as 1 single craterlet in my Vixen. I also reckon this has a much to do with good seeing though.

Star test works, according to Mel Bartels.  No, maybe you cannot attach a wavefront error or strehl number to it, but you can tell if it's better than 1/4 wave, and see other defects, with the out of focus tests.    https://www.bbastrod...tarTesting.html



#19 nirvanix

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Posted 23 August 2019 - 10:32 AM

I'd like to offer this possible point of clarification:

 

The Vixen VMC95 and 110 are such complicated optical systems that for collimation purposes it's much better to use an in-focus star to nail your collimation because it's so hard to read what's going on with a defocused star. People that don't own one of these beasts may not get that, but I totally understand where MF is coming from.

 

Having said that I have used conventional star testing on various mirrors and  lenses to diagnose significant optical issues. It will show things like TDE, astigmatism, gross spherical error, pinched optics, coma very well. In my dob I was able to figure out that I had a secondary causing astig and mirror clips causing pinched optics.

 

I'm going to aim my 10" dob with great optics at gamma Cass as soon as seeing permits to see if I can get it. This scope did split Sirius in Feb, 2010.

 

I've gotten 3 Plato craterlets in a 4" refractor. As MF says, 3 and 4 appear blended into one.


Edited by nirvanix, 23 August 2019 - 11:31 AM.

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#20 Magnetic Field

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Posted 25 August 2019 - 08:53 AM

I read here only sporadically anymore. Nothing has changed since the last 25 years and my absence from the amateur astronomy scene. It is just mind blowing the brainwashing that is going on in the refractor forum; I often wonder if the forum is full of flat earthers and moon landing conspiracists.

 

Although I don't contribute much anymore but I thought it is only fair I post some pictures of my Vixen VMC  110L modification.

 

Recap:  I made a big fuss about fitting a prism to the back port of the VMC 110L:

https://www.cloudyni...-prism-blunder/

 

And maybe the following is useful for other people who want to get rid if the flip mirror.

 

1. Don’t buy a Vixen VMC 110L. It is literally junk.
2. If pure insanity has overpowered you and you insist on buying one I would think it worth fitting a diagonal prism. If you don’t mind f-ratios you can also consider mounting a star diagonal.
3. All I can say the Takahashi prism really reduces glare around the planets and I have had moments of fantastic views of Jupiter recently.
I was able not just only to see the Great Red Spot but also to see it in pink colour. And this from my flat window in the UK.
4. I was able to split Gamma Cassiopeia, WDS BU 1028 (magnitude 2.20/10.90, separation:  2.1”, position angle 260 degrees) a double star in Cassiopeia. The dimmer companion with averted vision and moments of good seeing looked like the tip of  a needle right outside of the first diffraction ring of the main component. This was not in the UK but in my native country in a valley in the Alps.
5. I also had great moments with Saturn. 2 brown bands and some shading on the North pole.
6. There are some unresolved eyepiece issues though when observing the moon (which I will post it in the coming days in the eyepiece forum).

 

Removing and binning the made in China and designed in Japan junk Vixen flip mirror

 

My photos are crap but will convey the message. When I took the photos there was a bereavement in my family.

 

Why do you need to remove the flip mirror?

 

Firstly, I aver it is an engineering disgrace and secondly if you want to use the top side eyepiece holder (see Figure 1) you cannot fully screw it in because the railing part of our junk flip mirror comes into your way. Alternatively, if you don’t wanna trifle around you could use the back side adapter but this means you will fit your prism (and even more so diagonal mirror) a far distance out from the back port (see Figure 7) and your f-ratio and field of view will become even smaller than with the shorter top side eyepiece barrel holder.

 

attachicon.gif Fig0.jpg

 

1. You can easily remove the main optical tube from the main mirror/baffle tube unit by unscrewing it (you unscrew it counter clockwise, right-hand screw). Figure 2 shows the unscrewed optical tube and the main mirror.

 

attachicon.gif Fig1.jpg

 

2. In the next step remove the main mirror. Just unscrew the 6 screws with a PZ-1 screwdriver until the screws  fall out (see Figure 3). Also don’t forget to unscrew the main focusing knob otherwise you cannot remove the main mirror/baffle tube unit from the housing case. For this I think you need a 1.0mm allen key for the lock screw. If the key is still too big file it to size with a wee little needle file (I only had a 1.5mm allen key at the time).  Figure 3 shows the focusing knob with the rubber cup removed. The rubber cup is glued on but can easily be pulled off (they must have used something similar to Pattex ).

 

attachicon.gif Fig2.1.jpg

 

3. Once the main mirror has been removed from the housing body, you can remove the rubbish flip mirror and throw it into the bin (or bring it to the recycling centre and pass the bill to Vixen). Use again a PZ-1 screwdriver for the two little screws (see Figure 4).

 

attachicon.gif Fig32.jpg

 

4. I used blackboard black metal paint to paint the interior of the back side housing part. I guess it won’t make an iota of a difference and there are not many photons that have not been absorbed in the main tube. Use painter’s tape to mask off the edge (see Figure 5 and 6). Figure 1 shows an arrow that points to a dehumidifier (the little bags that we often find with made in China products). I do not know what happened but it took more than 2 months until the smell of paint went away (paint was dry within 24 hours though).

 

attachicon.gif Fig52.jpg

 

attachicon.gif Fig6.jpg

 

5. After assembly. I now use the top side eyepiece holder (which is 3cm long once fully screwed in, see Figure 7) at the back port. As Figure 7 shows this means the centre of the Takahashi prism is located 5cm from the back side port.

 

attachicon.gif Fig7.jpg

 

6. Collimation took me a few hours from my flat window in the UK. It was worth it because I ended up with textbook like airy rings. It also holds up after a British Airways flight from Heathrow to Europe (OTA in my cabin bag). There is no recipe for collimation. It is trial an error. The Cheshire collimation tool eyepiece with crosshair is pretty much useless except that you get a rough first ballpark collimation. Also the intra- and extra focal star patterns are pretty much useless (except for checking astigmatism). I don’t care how my textbook like air rings in-focus look like outside of focus. I don’t give a ****; all that counts is the in-focus star.

 

 

7. The prism has the effect that the main mirror has to move closer to the secondary mirror to compensate for the new location of the prism (in comparison to the flip mirror location). But this also has a consequence that the factory set focal length will increase and the focal ratio of f/9.4 will change. But by how much? Ideally we would need to get closer to the back port than the 5cm (see Figure 7). I bought a new 30mm Vixen NPL eyepiece. This again gives me roughly 1.25 degrees true field of view (compared to the 1.25 degrees with the 25mm Vixen NPL and 1030 mm focal length).

 

 

8. I did a star drift method (e.g. https://www.oreilly....04/ch04s15.html) with Jupiter to get an idea of the real new focal length. There are various reasons why I wasn’t able to use a star from my flat window. Also doing this method with an alt-az mount is a pain in the **** as you need to make sure the start drifts along the centre of the eyepiece from one side to the other.  The new focal length is 1150 to 1200 mm for the current prism  set-up (see Figure 7). I think it is closer to around 1150mm (probably need to repeat it a few times to get some ideas of the measurement error; also the method is better for zero declination stars at the celestial equator):

 

Jupiter with 30mm Vixen NPL:

 

Time to drift through field of view: (5*60 + 20) seconds. Jupiter’s declination approx. –(22 +11/60) degrees at the time of the test:

 

(5*60+20)seconds*0.2507*cos(-(22+11/60)*PI/180o)/60

 

This would give a field of view of 1.24o (60 in the above equation is the conversion factor from arcminutes to degrees).

 

For the 8mm  Vixen NPL and 90 seconds drift:

 

90seconds*0.2507*cos(-(22+11/60)*PI/180o)/60

 

This gives a field of view of approx. 0.34o

 

For the 6mm Vixen NPL and 65 seconds drift:

 

65seconds*0.2507*cos(-(22+11/60)*PI/180o)/60

 

This gives a field of view of approx. 0.25o

 

If we assume f=1150mm:

 

e.g. 48o  apparent field of view  of the NPL eyepiece:

 

• 48o/(1150mm/30mm)=1.25o
• 48o/(1150mm/8mm)=0.33o
• 48o/(1150mm/6mm)=0.25o

 

Or with field stops:

 

• NPL 30mm: 25mm field stop/1150mm*57.269=1.24o
• NPL 8mm: 6.5mm/1150mm*57.269=0.32o
• NPL 6mm: 5mm/1150mm*57.269=0.25o

 

New thread about one outstanding issue:

 

https://www.cloudyni...-75mm-eyepiece/



#21 Magnetic Field

Magnetic Field

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Posted 03 September 2019 - 03:08 AM

I'd like to offer this possible point of clarification:

 

The Vixen VMC95 and 110 are such complicated optical systems that for collimation purposes it's much better to use an in-focus star to nail your collimation because it's so hard to read what's going on with a defocused star. People that don't own one of these beasts may not get that, but I totally understand where MF is coming from.

 

Having said that I have used conventional star testing on various mirrors and  lenses to diagnose significant optical issues. It will show things like TDE, astigmatism, gross spherical error, pinched optics, coma very well. In my dob I was able to figure out that I had a secondary causing astig and mirror clips causing pinched optics.

 

I'm going to aim my 10" dob with great optics at gamma Cass as soon as seeing permits to see if I can get it. This scope did split Sirius in Feb, 2010.

 

I've gotten 3 Plato craterlets in a 4" refractor. As MF says, 3 and 4 appear blended into one.

A classical example is the following: https://www.cloudyni...-goodbad-is-it/

 

Someone posts a picture of the intra- and extra focal airy pattern.

 

And now?

 

No one knows how good the telescope actually is based on those pictures. People then are arguing and speculating but no one would bet money on their own conclusions.

 

And this from our warm living rooms were we have all the time to judge the airy patterns.

 

And then there are the disillusioned people who think they can infer the quality of their telescopes and attach numbers to it based on this kind of airy patterns in the open field.

 

We cannot even do it from our living rooms.

 

Btw: When Suiter wrote his book there was no internet (not as we know it today) and people really believed they can use the airy patterns to judge the quality of the telescope (beyond astigmatism, collimation, coma, etc). Then people started to share their intra- and extrafocal patterns on the internet and people quickly realised: hey we cannot do it and the Suiter book can only serve as an educational read but won't help us.

When professional astronomers build big telescopes of course will they use air patterns to judge the quality of their telescopes but and this is the but they will spend 10 years from planning and execution to analyse those patterns.


Edited by Magnetic Field, 03 September 2019 - 03:12 AM.


#22 nirvanix

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Posted 03 September 2019 - 11:53 AM

I should of had the scopes out last night looking at gamma Cass. but couldn't get my self outta bed.  I also want to try some straight through viewing with my refractor and VMC (no diagonal) to see how much better the view is, if at all.



#23 Magnetic Field

Magnetic Field

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Posted Today, 12:50 PM

How do you do my friends?

 

Flocking the Vixen VMC 110L.

 

1. Buy black D-C-Fix flocking velour material (it is allegedly made in Germany). I Paid £8 for a 1m roll on ebay UK. The same D-C-Fix is also sold through various telescope dealers (see Fig 1:):

 

Fig0.jpg

 

2. Remove the Optical Tube (OT) (see Fig 2):

 

Fig1.jpg

 

3. Before you start (see Fig 3). I learned it the hard way (for your info: removing the self-adhesive  velour from the inside of OT is easy and you don't need special tools):

 

Fig4.jpg

 

4. Cut the material to size (see Figure 4):

 

Fig5.jpg

 

5. Remove the protective foil and "coil" the material and let it drop into the OT (see Figure 5):

 

Fig6.jpg

 

6. Start affixing it. Work your way around the inside of the OT; use your balm and fingers (see Figure 6):

 

Fig7.jpg

 

7. I let the two ends overlap (if you are mental you can probably exactly cut it to size). Again stop at the recessed line otherwise you will experience severe image shift (Jupiter whill dance in an out of your field of view while focusing at high power because the edge of the mirror will jam) (see Figure 7):

 

Fig8.jpg  


Edited by Magnetic Field, Today, 12:53 PM.



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