12 replies to this topic

[Goheleth]

Hello,

 

I think some subjects have already discussed this but maybe not for heavy instruments. I currently have a mount that has a useful load of 65kg and on which is mounted a newton weighing about 30kg.

 

I want to add a parallel refractor with a weight of around 15kg. I've already done the test by putting it on and as the mount is in direct drive I'm having problems balancing it in DEC. This is also where I realized that with the imaging train on the newton it's no longer totally symmetrical and the center of gravity is slightly off-center in relation to its axis. What's more, since the scope is so far from the mount, I have to go up to the maximum counterweight load (60 kg) to balance it in AR.

 

So I'm wondering whether it wouldn't be better to position them side by side by machining a plate and installing two dovetails on it. Of course, the plate would have to be stiff to avoid excessive bending under the weight of the instruments. The disadvantage I've noticed is that the center of gravity of the whole assembly is inevitably more difficult to adjust so that it's perfectly centered on the DEC axis.

 

Here's what I've got so far:

- over-and-under :

advantage: weight well centered on the DEC axis
disadvantage: very large counterweight to balance in RA. As a result, the counterweight bar may bend, as the largest counterweights are at the end of the bar. As the bar has a diameter of 60mm and is screwed in M54, I think this is negligible.
- side by side :

advantage: weight of the assembly much closer to the mount and therefore easier RA balancing with fewer counterweights
disadvantage: bending of the support plate for both instruments and more precise DEC balancing to position the assembly's center of gravity on the DEC axis.

 

Perhaps some of you have already experienced this situation and tested both cases... In any case, I'd be delighted to hear from you.

 

Thanks in advance.

[Bean614]

It's really quite difficult to offer any assistance to you here, since we don't know what mount you have, nor what scopes you have.  Different Mounts, from Different Manufacturers, can have vastly different capabilities, properties, tendencies, and well-known 'traits', in spite of the fact that they seem to have the same 'Specs'.  Same is true for Scopes, of any design.

What Equipment are you using???

[Goheleth]

Thank you for your answer, here are the details concerning the equipment :

- Mount: ASA DDM85 standard

- Reflector : Newton Skyvision 250mm (similar to this one: https://www.webastro...plet_47844.htm)

- Refractor : Takahashi TOA130-NFB

 

 

[bbasiaga]

Generally speaking your mount max payload is affected by the moment arm of the load. Mea ING the farther the center of gravity of your scope is away from the mount, the less payload the mount can handle.

So generally you'd be better off with a parallel plate keeping the two scopes as close to the mount as possible. If you are trying to guide this way, you may not find success as the side by sides have a lot of flex. It only takes a few arc seconds of flex to kill an image. If just visual than no worries.

Brian

[Goheleth]

Thank you for your reply and these clarifications. The equipment is intended for astrophotography only, so it's advisable to put them on over-and-under.

As the weight of the two instruments is around 45 kg and 60 kg of counterweight is enough to balance the RA axis, I think this should be good for the mount. The disadvantage is that the heaviest counterweights are located at the end of the shaft. I just need to find the right balance on the DEC axis, because when it moves towards a star, it may stop in order not to overheat the motor because it doesn't reach the set point in the allotted time.

[bbasiaga]

What mount do you have that can handle all that counterweight and payload? AP1100?

Brian

[Goheleth]

Hello Brian,

 

The mount is an ASA DDM85 standard with a payload of 65kg.

[MJB87]

I've been there...

 

Had a 12" Cassegrain and an 5" refractor on an AP1100. Tried the side-by-side arrangement.  The mount was loaded close to max weight, when you add in everything.  

 

 

Here is what I learned...

 

1. The SBS setup involved an 18" side-by-side dovetail plate and two saddles. Just that stuff alone added a lot of weight to the setup.

 

2. Balancing is complicated but there is a procedure published by Astro-Physics. It works. You can find it here.

 

3. The big problem was that in some mount positions one of the two telescopes would be obscured by the wall of the ROR. 

 

In the end, I decided I really needed two mounts.  I use two refracroras on one of them but in a top-bottom configuration.

[luxo II]

Thank you for your answer, here are the details concerning the equipment :

- Mount: ASA DDM85 standard

- Reflector : Newton Skyvision 250mm (similar to this one: https://www.webastro...plet_47844.htm)

- Refractor : Takahashi TOA130-NFB

I would put the refractor on top especially if the 10" has rings.

10" MCT (15kg) with 6" MCT (yes pushing the limits on an AZEQ6)

https://www.cloudyni...50630-img-0986/

https://www.cloudyni...50629-img-0987/

 

https://www.cloudyni...mct-apo-on-g11/

 

I've also had this on a G11 and recently acquired an CQ350 which seems quite up to imaging with both the 10" and the MN65 I have. The 10" has losmandy dovetails top and bottom of its rings. Whatever second scope rides on top is attached to the upper dovetail with a pair of Losmandy DVA clamps. Even the finderscope is in rings on DVA clamp.

Edited by luxo II, 04 October 2023 - 04:24 AM.

[Goheleth]

Thank you for your answers which confirms me that both configurations are possible. Side-by-side use requires an additional plate with a certain weight to hold the two instruments. As the maximum weight is not reached, I don't have too many problems, but as you point out, the balancing procedure is more complicated.

 

Of course, the best thing would be to have two mounts, but I want to concentrate everything on a single mount. So I'm thinking of the over-and-under option, even if I'll have to find a way of aligning the two instruments so that they're looking at the same object. This is because the polar alignment of the mount is carried out on the telescope and not on the refractor. 

By the way, I'd like to know how you did the polar alignment of your mount with the two instruments side by side. Since my mount doesn't have a polar finder or laser, how do I know if the telescope is looking at what the mount is "pointing" at?

 

Here's an actual photo of the configuration. Indeed, on the telescope I've added a plate to fix the double collar supplied by takahashi to fix the refractor in parallel.

 

In any case, thank you for your answers and your insights into this question.

 

Attached Thumbnails

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Edited by Goheleth, 04 October 2023 - 05:02 AM.

[MJB87]

Orthogonality error was an issue in the SBS setup.  Getting the mount polar aligned was pretty easy. Then adjusting the setup for the telescope was more difficult. In the end, I polar aligned and made sure the 12" Cassegrain would have good pointing and tracking.  The 130mm refractor was "good enough" given the wider FOV. However, I was never really able to "dial in" the two telescopes so they would be pointing exactly at the same spot.

[luxo II]

I have transverse slots in the dovetails on the 10" allowing its rings, and those of the scope on top, to be moved side-to-side to adjust the dec offset, and to correct cone error I shimmed one of the rings, For the 10" both are now under 10 arcmin.

 

For the second scope on top, I put a camera in both scopes and centre a bright target in the 10", then shim the second one (up/down) or use the slots in the dovetail (left/right). They're parallel to a few arc min and, using rings and clamps, it is reasonably repeatable from one setup to the next.

 

The other thing is the mount (Skywatcher CQ350), has its own "home" sensors and it aligns to these at power-on. It also allows both the cone error and dec offset to be configured, if you have a way of measuring them; I did it by setting both to zero and looking at what the GOTO errors are in dec as a fraction of the camera field of view for objects near dec 0 at the meridian.

Edited by luxo II, 05 October 2023 - 03:52 AM.

[Goheleth]

Thank you for your answers and explanations. I'm also thinking of setting up an adjustment system to try and get both instruments looking at the same thing.

 

Since the mount doesn't have a polar finder, the mount's software points to different stars and I refocus them on the telescope. In this way, it calculates the offsets for each one and then determines the center of the different stars. It can then calculate the global offset and apply it to point any object in the sky. The disadvantage is that this is done on the telescope, not the refractor. But that's just a point of detail.

 

The most important thing will be to balance the DEC mount, because with the Newton I'll have to balance the third axis because of the optical accessories. This imbalance causes the mount to crash when it moves to point at an object. As it takes longer to reach the object than it calculates, it goes into protection mode to prevent the motors from overheating. The disadvantage of direct-drive mounts...