| review: Fornax 50 german equatorial mount (appr. € 4400,-) |
The Fornax 50 is an hungarian constructed and built mount of high quality.
As you can easily see it does not have the neat finished look like a Losmandy for example.
It looks more like pure rigidity and rock solid construction.
Exactly what I do prefer.
I bought mine from Astro-Stumpp in Germany and they could deliver after one week.
It was one of the first mountings delivered to Germany.
Looks like I had good luck.
The Technical Specification:
- max. load of telescope 45 kg
- mount weight 21 kg
- both axis diameters are 60 mm
- cone bearings in x layout (no play!)
- both worm wheels diameter 217 mm, 432 teeth
- 5-phase stepper motors go directly to both axis, no additional gear boxes
- 3000 micro steps per turn of each stepper, 6 micros per full step
- GoTo control for Messier, NGC and IC objects and major stars or any RA/DEC entry
- maximum GoTo speed is 1 degree / sec.
- counter weight axis turns with RA, diameter 33.8 mm
- LX200 compatibel (from Version 2.0 on)
- auto guider input (12 Volt switches, SBIG ST-4 compatible, operates only with *normally open* mode)
- polar alignment scope and eye-piece included
- 12 Volt power supply included
- Controller includes motor focus control and red flash light
- 1 counter weight with 6 kg included
Unpacking and Mounting
OK, that thing is transportable, but I will not do it very often!
My permanent peer is 1 m high and with the two grips of the mount fixed on DEC-axis-housing I was able to lift it to there some how ;)
I fixed the 3 screws of the peer and pointed the mount roughly to the north.
After assembling my 10"/f4 Newton and balancing (with an additional 12kg counter weight from Astro-Stumpp) I was ready to polar align.
There are three screws in the base of the mount to unlock azimuth fine tuning and 2 more for altitude.
After unlocking I tested the polar finder scope.
It has clear markings where to point an arrow (while turning the eye-piece) to beta urs maj.
Then you have to bring Polaris to another marking by means of the mechanisms in the base plate.
They both work fine. Very smooth and precise.
And without the need to tighten the fixing screws the mount is rock solid while polar alignment.
But there is no built-in illumination and the markings of the eye-piece where soon invisible after sundown.
So I had to twiggle with my little red flash light and though I am not trained with polar alignment scopes it finally worked fine.
I improved everything with Scheiner's method because precise polar alignment is important for the accuracy of the GoTo control.
As you can see below the drift in DEC is appr. 1 arc sec in 15 minutes.
Pointing
The Fornax has permanent engaged friction clutches.
Therefore you can manually point the telescope at any time.
Just release the scope and it will move in RA instantly again.
There is no need to engage or disengage clutches manually and then wait several seconds until the mount tracks again.
I am a *trained* dobson pointer so this is a big advantage for me.
Because of the big diameter of the worm wheels the clutches can be very big too - and therfore they work extremely smooth.
How much force is needed to point the mount can be adjusted by 3 hand-wheel screws as shown in the photo.
When the scope is balanced well, the clutches can be set so that the friction is minimal.
This makes pointing the scope easy and precise at the same time.
Tracking Accuracy and Periodic Error
The periodic error trace was taken with my guide scope CCD camera:
The +/- 4.0 arc secs is better than I expected.
And even better: the error is very *reliable*.
This means it is predictable, smooth and there are no shakes or dicontinuities.
This is important because I use ccd-self-guiding for long exposures.
And the sporadic shakes reported by some Meade or Losmandy mount users cannot be compensated by self-guiding nor by periodic error correction.
I think the reliable tracking has its reason in the lack of additional gear-boxes for the steppers.
In combination with the fact that there is virtually no backlash in DEC and RA this is a perfect mount for deep sky photography.
The drift after 3 revolutions is due to differential flexure between the guide scope and the main scope.
I recently improved the stability of the guide scope by using solid Baader guide scope mounting rings but did not measure the flexure yet.
I hope the old theory comes true that the worm and wheel will be worn in after longer usage and that the periodic error will be reduced by that even more.
For testing I exposed the Orion Nebula.
Each frame was 60s exposure time without any guiding.
All the 16 luminance and RGB frames where perfect and usable!
I would have taken more if it was possible that early in the year (September).
16 times exposure of 60 seconds *without* any guiding on 355 mm focal length and very small pixels (6.7 mym)
Despite the saturation of the ccd I think the mount could do it even longer.
Working like this is real fun!
GoTo Capabilities
After the mount was perfectly aligned again I tried the GoTo capabilities.
The procedure to tell the mount where it points to is easy:
- Point to any brighter Star (I use a Telrad finder)
- center the star on the ccd frame
- look-up the star's name in an atlas or planetarium software
- find the star in the list of stars of the mount (by name or by constellation)
- type in the number on the keypad
- press 2 which means *set*
That's it.
Now the mount knows where it is pointing to.
To GoTo an object is even easier:
- Type number for object catalogue (Messier, NGC, IC or star)
- type the number, e.g. 27 for Messier 27
- type 1 which means GoTo (you could also use 2 for a new *set* here, eg. when you pointed the mount manually meanwhile)
The mount now starts to move to the new location.
There are 8 different speeds to do so.
I always use the fasted speed without any problem but I think the mount should be balanced well for that.
If not, the steppers might loose a step from time to time and there is no angle encoder to compensate for that.
Remeber: the faster a stepper slews the weaker is its torque and the easier it can loose a step.
The mount is very quiet at full speed.
No need to worry about the sleeping neighbours.
In some slower speeds the mount is a bit louder because of resonances introduced by the steppers.
The accuracy of pointing in GoTo is simply perfect.
The object was always on my ccd frame which is appr. 15 mins of arc in diameter.
Most of the time it was dead center.
This is true for slewing half the sky (180 degrees) and for pointing very low to the horizon.
Remember that below 45 degrees from zenith the refraction of the atmosphere reaches values of several minutes of arc.
I do not know whether the controller is calculating this error like Vixen's SkySensor does.
The reason of the compensation might be that my method of aligning is performed with stars on the meridian.
This implies that my mount is not pointing to the real pole but to a *visual* pole including a bit of compensation to the error of atmosphere's refraction.
Anyhow - the GoTo works perfect.
High Resolution Work on Moon and Planets
While the lack of additional gear-boxes for the steppers is perfect for deep sky work there might be a problem for high resolution photography.
That's because the steppers have to turn very slow when the only gear is the worm and worm-wheel and therefore the total gear ratio is low.
And this means that one micro step gives a relatively big movement of the telescope in arc secs.
I calculated the arc secs. per micro step as follows:
degree per micro step = 360 degrees / number of teeth in worm-wheel / number of micro steps
degree per micro step = 360 degrees / 432 / 3000 = 0.000278
equals 1 arc sec per micro step
My 10"/f4 can resolve something like 0.5 arc sec.
To achieve this with the mount I theoretically should have about 0.25 arc secs per micro step or better less.
In praxis I only had the chance to test it with Mars which was very low at my location so that seeing was for sure worse than everything else.
I will have to wait for Jupiter and Saturn to be higher in the sky this autumn or catch the moon to give it a try.
What is clear so far is that the very fine tracking and low periodic error makes it easy to keep a planet in the view of the ccd.
I used Barlow lens projection 5x, so my field of view of the ccd was only about 3 minutes.
The red planet moved around a bit but stayed on the frame virtually forever.
There was no need to correct the position at all.
20.08.01 Saturn
Saturn now came into my view early in the morning.
Though it was still low I gave it a try with the Fornax and my 10" f/4.
I used Barlow Lens projection with amplification factor around 6x.
The result is very promising and I now have the good feeling that the Fornax is good with planets too.
I took a lot of shots and later picked the best 16 of them.
There are several realy blurred images among them.
I cannot say whether this is because of frozen *bad* seeing or it is because of the Fornax performing a microstep at the time of exposure.
But in my opinion you have to select the best pictures later anyway only because of seeing.
So there is no additional disadvantage.
Please use the search tool on this site to find more astro photos taken with that mount:
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