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Parallax Instruments HD-250C GE Mount
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Parallax Instruments HD-250C GE mount
My dream of owning a large APO had come true. A TMB Optical TMB-229 APO refractor was available in stock and I had sufficient funds for this undertaking, the dream of a lifetime. The next question was “What mount to choose?”
There are many excellent choices available. My main considerations were:
1. Reasonable delivery time
2. Easily handle the TMB-229
3. AP GoTo electronics was a bonus
I wanted the Astro Physics electronics so that I wouldn't have a learning curve to deal with in using the GoTo functions. This was not nearly as important as the first two items.
In researching the various mounts in this capacity class, there are quite a few manufacturers making excellent mounts. All have a good reputation and deservedly so. Luckily Jim Phillips (an experienced planetary observer) brought Parallax Instruments to my attention. Additionally at Black Forest Star Party, Russ had a TMB-175 mounted on a Parallax mount doing CCD work. I had spent some time watching him and could see in the auto guide graphs these were smooth mounts in terms of RA tracking.
I talked with Joe at Parallax Instruments about my requirements. He uses Byer's gears in the mounts and I knew right off this would be a big plus. Joe was working on a new mount to fit between the monster sized observatory mount (HD-300) and the smaller HD-200C. The HD-200C didn't seem as beefy as I wanted. [This was a poor conjecture on my part as the HD-200C would have been plenty beefy.] This new mount would have a steel polar housing made for the user’s latitude, a 15" Byer's gear for the RA axis and larger 18V-DC servo motors. I was impressed Parallax was coming out with the mount to fill the gap in their product line. Ordering a new model means it’s the first off the line, which was a bit of a concern. Joe’s confidence and answers to my questions alleviated any fears.
The pier was the next hurdle but as luck would have it, Particle Wave Technologies has the Monolith which would handle the proposed setup. It was available in a reasonable delivery time frame. Orders were placed for the TMB-229, PWT Monolith and Parallax mount. All I had to do now was wait.
About 2 months later all three items showed up on the same day. I had carefully scheduled everything so it would arrive on the same day.
The mount is shipped in a large wooden crate and the shipping company has no problems handling a large and heavy item like this. Once it hits your observatory that all changes. Now you have a large box and heavy mount to deal with. The mount disassembles at the Dec and RA axis. The Dec weights about 80 pounds and the RA weights about 120#. It takes two strong guys to lift the RA axis. In my case I used a 60” Monolith; therefore you have to lift the mount relatively high. I got a bit confused placing the RA axis and managed to put it on the pier 180º wrong. Luckily the full index adapter of the PWT Monolith allowed the RA to be rotated the 180º easily. After a couple hours and some hard, tense work, the mount was in place on the pier as was the scope.
This thing is big and built like a tank. The 15" Byer's RA gear is huge. For the most part the black glossy paint is very well done. Some of the areas could have used a little better prep work before painting but this is not an issue. The RA/DEC axis gloss finish is the smooth-est I have ever seen. The thick, rich gloss gives a luxurious finish to the mount. A coat of polish (Aeroshell Flight Jacket) makes it look even better. Pictures don't do justice to the mount; it has to be seen in person. The worm gear is about 1º” in dia-meter and held solidly in place by bearings on a thick metal block. There is a lot of contact area at the worm mesh that should lead to a long gear life as well as high torque cap-ability. The servo motor is large and uses a gear head for speed reduction. With all the gear reduc-tion, the servo must be capable of exerting high torque.
The learning curve on the AP GoTo electronics was zero since I have an AP-900. The tracking is a bit off due to polar alignment but with the full index adapter I am able to dial in the AZ no problem. The ALT axis is adjustable with four sets of push/pull screws. Once accurately polar aligned the GoTo is on the mark. This is swinging an 85" long OTA.
After some use, the RA axis was showing a strange problem. It would lag for about 10 arc-seconds then "bangs" and catches up. Joe (Parallax Instruments) and I discuss the problem. Parallax offers a warranty repair and walked me through the procedure of disassembling the mount to remove the part that needs machined. In my case, it's easier for me to take the piece to a local machine shop rather than send it back to Parallax. The local machinist has it finished up in a few minutes.
The mount is reassembled with the worms are left disengaged, i.e. the clutches no longer hold the scope back. The only friction is in the bearings. This is a good test of how smooth the mount is as this is the friction the motors work against. I must say this mount is super smooth! Even my AP-900 isn't as smooth as this mount. As little as 1 pound of imbalance is enough for the scope to start drifting. With the scope perfectly balanced one is moving a large mass ‘weightlessly’. Even my non-astro brother is impressed by the action of this mount. With the worms once again engaged, the clutches take hold and add more friction to the manual motion (but not to the motors).
Fix Is Good
The problem is solved! After several hours of testing the RA jumps are gone. This mount is now 100% perfect. This problem was, in a way, a blessing in disguise. I was motivated to take the mount apart and was able to see first hand what the build / design quality was like (excellent). Joe really cares about customer support. He called on a Saturday night to help guide me through the disassembly process. I was most impressed with his support and glad I had chosen Parallax Instruments. This was a new model and Joe has assured me this problem will not occur again.
The internals (bearings, gears etc.) are of high quality and huge. For example the bearings which hold the worm gear in place are not your ordinary radial bearings, but angular-contact / tapered bearings. This means as you manually move the mount about, these bearings ultimately stop the worm gear from moving laterally. An angular contact bearing is best in this application.
The Byer's worm is a work of art. It’s thick and looks well machined but more importantly, the run-out is nonexistent. The gear mesh does not change based on which quadrant of the worm is in contact. In other words eccentricity of the gear is minimal.
There are two tapered bearings for the RA axis; the one at the top of the axis is HUGE! It looks like a bearing from a rail car wheel. Way overkill - NICE! The bearing races have a superb polish which adds to the smoothness of the mount.
Where the RA worm housing attaches to the square section of the polar axis housing, a 2 inch thick interface plate is used. This is where the RA bearing race mounts and it is critical there’s no flexure in this area of the mount. The design is totally up to the task.
The RA Axis must be three inches in diameter. It’s made from a more expensive and stronger 7075 alloy of aluminum rather than the 6060 often used. There will be no torque or flexing of this axis. I’m not sure why Parallax designed it to be so strong, but I’m happy they did so, rather than the other way around.
Figure SEQ Figure * ARABIC 1 Parallax Instruments HD250C – Unguided Right Ascension Track
The question is: How good is this mount? Specifically, pointing accuracy, tracking and stability. An SBIG ST402-ME was used to characterize the periodic error of that 15" Byer's gear. REF _Ref106715850 h Figure 1 shows a complete RA-worm cycle. The seeing was good that night with stars moving about ±0.5 arc-seconds. If you remove a few of the outliers then the PE comes in around ±1.5 arc-seconds. In absolute terms this is a phenomenal figure. The main concern should be the rate of change in PE. It's slow enough to be easily guided out. The DEC axis is shown in REF _Ref106715959 h Figure 2. This is an indication of the seeing on this test night.
The ST402 was used once again, but this time to guide making full corrections without PEC being programmed. The results are shown in REF _Ref106715959 h Figure 2. Guided the mount stayed within +/- 0.5 arc-seconds, on a night when seeing wasn't as good as the previous. This was an astonishing result, much better than I had hoped for. Remember this is moving a TMB-229 which has a large moment arm.
The GoTo features works very well with targets hitting the small CCD chip no problem on both sides of the meridian.
Figure SEQ Figure * ARABIC 2 Parallax Instruments HD250C Declination Axis Unguided
Figure SEQ Figure * ARABIC 3 Parallax Instruments HD250C Right Ascension Axis Guided
Post First Light:
After using the mount for the past several months I continue to be impressed by not only its performance but its beauty. At first it seemed way to large but as time goes on the size impact has lessened. I’ve used the mount for both visual and CCD and have been pleased with the performance in both situations. An extra guide scope won’t add any strain to the mount.
The HD-250C a solid, heavy duty mount that delivers on the performance. Tracking is phenomenal as is the pointing accuracy. This is truly an observatory grade mount.
The deep glossy black acrylic finish gives it a luxurious look. If one studies the mount closely they’ll soon appreciate the elegant design and the heavy duty nature of this device. Customer support after the sale is excellent.
If you're looking for a high performance mount I suggest looking at the line up from Parallax Instruments.
Tim Povlick, has been observing for six years and lives in Southern California. The scope collection consists of 5 refractors and one Newtonian.