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It is long time I wondered whether the mount polar scope could be used in a less tiring, but more effective way to achieve a convenient, very quick and fairly precise polar alignment (till better as 1 arcminute), than could be achieved usually, and without having to buy rather expensive devices made for this purpose.
The solution I found – easily achievable and doable for the most of Astro DIYers - is to mount a small video camera attached to the polar scope eyepiece. This will allow us to achieve an excellent alignment to the celestial pole.
I invented the acronym VAPA (Video Assisted Polar Alignment) to define my mount stationing method.
NCP on January, 2018 NCP on May, 2021 (Stellarium)
In addition to the Polaris, there are two stars of mag 6.5 just around the North Pole celestial field, in Ursa Minor constellation.
They are λ (lambda) and HIP 7283 (double star)
As you can see the position of two stars is very interesting, in fact their RA position differs by a value very close to 135°. In addition, it is possible to see in the same field two other fainter stars whose positions are very peculiar too.
If you draw a line from each tiny star to meet the NCP, you will see that these two lines define a right angle having its vertex at the NCP. (see pics below)
I made this modification on my standard HEQ5 (old black 'Heavy Duty' one), but the same arrangement can be applied to any other mount that has a polar scope with removable eyepiece and sliding ring with glass reticle.
Considering that the HEQ5 has a very small polar scope (its objective lens has a diameter of about 14 mm), the system will work even better on other mount models, equipped with polar scopes of much larger aperture.
Polar camera construction
- 1 IMX 225 (or IMX322) module with 6mm lens, equipped with Video out, OSD and power supply cables (from Aliexpress)
- 1 OSD menu pcb (optional)
- 1 small plastic box
- 1 film plastic can (135)
- 1 rca panel connector
- 1 coaxial power connector (3.5 mm)
- coaxial cable (conductor plus shield)
- some 2mm screws, spacers and bolts
- 1 epoxy resin
The small polar camera looks as you can see from the pictures below
Making the Reticle
Materials (see photos below):
an A4 paper sheet on which we have printed a circle graduated in degrees
0.030 mm (30 micron) fishing wire
3 small truncated-cone springs
- Unscrew the eyepiece and remove the three adjusting grubs of the reticle ring.
- Remove the reticle ring and unscrew the threaded flange which holds the glass reticle in place
- Remove the glass and reposition the flange.
- Attach the ring to the centre of the graduated circle with a very small amount of vinyl glue (flange down)
- Stretch (gently!) a piece of wire and fix its ends tightly (with small pieces of scotch tape) at 0° and 180°, in order to precisely bisect the circle and the ring.
- Do the same thing with another piece of wire, stretching it between 45° and 225°.
- Make sure that the two wires cross in the centre of the ring accurately (although extreme precision is not required). See photo below
- Using a very small amount of cyanocrilate, solder the four wire ends on the ring, just where they get each other into contact.
- Allow to dry.
At the end of this procedure we will have created a wire reticle delimiting two couples of angles, 45° and 135° wide respectively.
- From the inside of the eyepiece barrel, do insert the tiny truncated cone springs into the three grub threads, the smaller base pointing outwards from the barrel.
- Replace the ring in its place with the reticle towards the polar scope objective.
- Screw in the grubs until the ring is secured, but do not tighten them (see photos below)
Calibrating the reticle
- Adjust the eyepiece so that the reticle can get focused (use glasses if you wear them to see well at a distance!) then fix it with a drop of silicone just on the visible part of eyepiece thread.
- Unscrew the locking ring of the polar scope tube and adjust the distance between the reticle and the objective so that you can see a distant object (a bright star, or a detail on the roof of a building) well focused together with the reticle.
- Fix the tube ring.
If there are one or more grubs around the locking ring, they must be screwed in tightly.
Displaying the area of the celestial pole
- 1 7" 1024x600 HDMI screen
- 1 RCA -> HDMI video converter
- Connection cables
Insert the camera nose (135 film barrel) on the polar scope eyepiece and aim the polar axis at the sky area just around the Polaris
If all the connections are correct, we will see the Polaris on the screen (if we don't see it right away, we can easily find it by searching near around) together with a good number of other stars.
In a Bortle 6-7 sky (as it happens in many suburban areas) we can easily find out stars up to 10th magnitude (provided the atmosphere is transparent enough). The field of view will be about 3°x 5° (see photo).
Camera configuration by OSD menu
The camera menu will get elicitaded pressing central button and you can do your choices pushing up, down, left and right buttons
Fill in the follow parameter:
Push central button to enter Main menu. Push down button to enter submenus and right or Left one to select voice:
- Lens → Manual
- Exposure → Shutter → 15-20
→ AGC → 6
→ Brightness 1
→ Return → Main menu
- Day/night → B / W → Return → Main menu
- NR → High → Return → Main menu
- Special → Defect
- live DPC → On → AGC level 50-60
→ Level 0
- White DPC → On → -level 0
→ AGC 5-7
→ Sense-Up 30
→ Start → Follow indications
→ Return → Return → Return → Save & end.
Leave all other voices at default position
Notice: DPC is the dead (hot) pixel control
Video assisted reticle alignment
This is done by grubs provided just for this task.
Truncated cone springs make the operation very easy and confortable. As you find Polaris in your screen, put it at the center of crosshair.
Rotating the polar axis, you can see the star will move from its initial position, so, you should screw the three grubs in and out until Polaris will stay ever at the crosshair center in any direction you can rotate the polar axis.
Grubs should be secured, but they should no be tighted.
Video assisted Polar alignment procedure
By adjustement of Alt-Az knobs, you should:
1) put the reticle center on the right angle vertex (NCP position on current date) made by tracing two virtual lines, starting from the two faintest stars, as indicated in previous image.
If you are very accurate, you can reach the NCP within a maximum error of 1 arcminute.
2) rotate the polar axis till the two brighter stars get both hidden by two crossed hairs of reticle (the pair one crossing until each other at 135°). If it doesn’t happen, it mean your mount is too
much far from NCP, so you must repeat the step 1).
Note: performing step 2) doesn’t be mandatory, but it will enhance alignment precision.
Photos above are recorded by SharpCap (stacking), thus they don’t display the correct image aspect ratio (12:7) as you can see visually, watching your ‘on the mount’ lcd screen.
In fact, SharpCap do not change aspect ratio (720x576 lines for PAL system) of native analog image captured by a video grabber (Easy Cap or similia).
That is why the above image appears a bit higher as it should be. Obviously, you can align your mount with your notebook screen rather as with lcd display on the mount.
To do that, you must use a video grabber device and OBS Studio to record movies or snap shot.
OBS Studio is a big free software allowing make all necessary image adjustement to reach the correct aspect ratio.
Below I posted an OBS Studio clip (sorry for the big amount of dinamic noise due the sudden ‘défaillance’ of my chinese video grabber … )
2021-05-07 22-41-02.mp4 Beppe
Last night I could do my first test with a Raspberry controlled HEQ5 mount. Worked much better now compare with all problem I had one year ago. I succeeded to: Plate solve, auto focus, take images. What's left to fix, auto guiding, GPS module to communicate.
Tonight it looks to be a clear night too. I will do a new test run, concentrate me on the auto guider, now in the beginning I use the internal guider, later maybe I change over to PHD2. Very exiting.
My setup of HEQ5 and a 300 mm Pentax 645 lens:
Hi to all.
The most popular mounts - even those that are included in the small-medium range group, do drive the axes rotation via worm wheel coupling .
This one are plagued by the drawback that I have always considered very unpleasant: the mechanical play is not properly controlled and it can’t be always limited within acceptable tolerance limits.
The adjustment of Back Lash (BL) is entrusted to two screws (more often they are grains) usually positioned at 180 ° from each other. However - as we all know, even a wheel very slight eccentric creates
an inconstancy of contact with the worm: when the play is adjusted for an initial position of the axis, it is not in other positions.
In other words, there are positions where the worm rotates with adequate friction, others in which the same friction can increase significantly, and still others where it is so reduced to the point
that it causes excessive play.
So, the tuning screws are sometimes too tight and sometimes too loose … mission impossible to find the right compromise!
This behavior is already annoying in manual movement because the task of pointing at an object in a steady way is not feasible because it ever 'jumps'.
In photography it's even worse.
Not only does an unavoidable backlash often appears in Dec – and it is very badly correctable also by the autoguider software, but- and it is dirty worse, the tracking in RA does not perform evenly
and smoothly. The typical ‘micro’ irregularities in star images is what we see in our shots.
Many high range mounts adopt different methods to contain lashes, and the best ones adopt mechanical systems with which that error is reduced to zero.
With this long introduction, I will tell you what very simple modification I thought of making on my HEQ5.
Please, stay tuned 🙂
Please forgive me in advance for my newbie and potentially frustrating question here!
I have got an HEQ5 mount for my skymax 180 that I was given for Christmas. We have tried following many YouTube videos and reading what we can to get this set up and polar aligned and feel like we are missing something fundamental as when we go to then star align with the GOTO remote, it points at the floor instead of the moon or the star we are trying to align to. So far we have used 5 freezing nights of trying to do these things to no avail and it is rather frustrating and extremely disappointing!
So I am wondering please, what is the best way for me to actually set this thing up so we can finally enjoy using the scope? Do I need to have Home position sorted before I've aligned so once we have (hopefully) done that then we can navigate everything easier? Is that potentially what we are missing?
With lockdown i dont have a handy shop to visit or anyone else that knows how to do this and while I am sure patience is a virtue, I feel so overwhelmed by it all! Any advice or recommendations on what to watch or do to help us are gratefully received!
Again, I am sorry for being such a newb, I am hopeful once we crack this it will become second nature over time! Thank you!