This does not match my first hand experience. My NP101is is very solidly mounted on a DM4 with 4" ADM Vixen dovetail. If I had the slightest doubt in the security of the interface I would have used one of several other longer Vixen or Losmandy dovetail bars I have in a storage closet. The DM4 carries the 2-way receiver that accepts either "V" or "D" dovetail. So it is no more than a matter of selecting what works that determines what I have chosen to use.
The scope also mounts securely to my UniStar Deluxe and Stellarvue M2C with the Vixen plate.
It is certainly a valid choice if a user opts for the wider "D" plate but it is very certainly not correct to say that the "V" plate is insufficient.
I think it’s possible for the same piece of gear to be sufficient for one person and not for another because we all have different needs and expectations for our gear. Based my experiences with Vixen plates, I am not comfortable using them with scopes above about 3 inches, so I wouldn’t recommend them to you for that purpose, but I hope that I didn’t say they were not sufficient for you.
Like you, I have the DM4 and the optional DM adapter that allows using Vixen Compatible rails with it. When the adapter is used, only one jaw of the DM4 saddle fully engages the Vixen Compatible rail. The other only makes contact along a thin ridge, which I believe is sufficient engagement to hold a 3” scope securely, but I wouldn’t hang a 4” scope on this kludge. Hopefully, your mount doesn’t have this problem.
To address concerns about engagement without resorting to the weight and bulk of a true D plate for which the DM4 was originally designed, DM sells a proprietary hybrid rail that combines the shorter height of a Vixen rail with the engagement depths and angles of a D plate. To use it, you must remove the Vixen adapter mentioned in the first paragraph, rotate it 180 degrees, and reinstall it, so the bottom jaw now has the correct angles for a D rail.
I am evaluating the DM hybrid rail now. It’s holds my 92mm scope very securely with full engagement, and it would probably hold a 4” scope like an NP101 well too. The DM hybrid rail is roughly the size and weight of a Vixen Rail, which is good, but the mount p doesn’t work with any Vixen Compatible rail now, including the vixen rail on my 3” scope.
So, for the moment, my 3” scope has a true Vixen Compatible rail on it, and it’s securely mounted on a true Vixen Compatible UA mount, and my 92mm scope has the proprietary DM hybrid rail on it, and it’s securely mounted to my DM4. Haven’t yet decided whether to stick with hybrid rail since it works well, or install a proper D rail on the 92mm, but I don’t consider a Vixen Compatible rail secure enough for me to want to use one, but if you think it’s sufficient for you, then it’s sufficient for you.
I think a 4” scope is the sweet spot for the DM4, and DM4 was originally designed to work with a D Plate, and I prefer the robust connection offered between a D plate and D jaws, and there many different sizes of D plates readily available, so I will probably want to set up both of my refractors with D plates, so both can be used with the DM4.
Edited by gwlee, 25 August 2020 - 02:10 AM.