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Perfex / Tasco 10TE / Astro Optical R-74 mount overhaul

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#151 Masvingo

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Posted 29 January 2020 - 04:05 PM

A very impressive rebuild, Graham, and thanks for the great write-up and pictures. I think these rebuild threads are my favourite threads.

 

Glad to hear that the optics are living up to the Royal reputation.


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#152 BlueGumby

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Posted 31 January 2020 - 01:52 AM

A very impressive rebuild, Graham, and thanks for the great write-up and pictures. I think these rebuild threads are my favourite threads.

 

Glad to hear that the optics are living up to the Royal reputation.

Hi Masvingo, thanks much. I was only spurred on by your own post which you have done so well, although you ran out of juice during the assembly.wink.gif

 

I always wanted a Unitron, and had not actually heard of Royal until I investigated this particular sale, and then decided I should not miss the opportunity. I'm so pleased I went ahead. I also think any old scope is going to need servicing to make the most of it. These threads share the love and help others to gain more from the hobby.

 

More power to you.

 

cheers ... Graham.P


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#153 Masvingo

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Posted 31 January 2020 - 02:07 AM

...although you ran out of juice during the assembly.wink.gif

Yes, I got too tied up with the excitement of getting the mount back together and forgot to take a lot of pictures! 


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#154 Senex Bibax

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Posted 02 February 2020 - 05:46 PM

This thread is so detailed and useful, I have printed and collated the entire thread into a PDF document suitable for printing. I'm happy to share it, as long as the individual contributors do not mind. It is large, over 150 pages and over 40 MB...


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#155 BlueGumby

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Posted 03 February 2020 - 08:59 PM

This thread is so detailed and useful, I have printed and collated the entire thread into a PDF document suitable for printing. I'm happy to share it, as long as the individual contributors do not mind. It is large, over 150 pages and over 40 MB...

Hi Senex, I am fine with that, but I caution you that I am still working on the mount and have a simple modification to add and a summary of my results. Working on it now and should be posted within 7 days.

 

cheers ... Graham.P



#156 Senex Bibax

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Posted 03 February 2020 - 09:37 PM

No problem, I can redo the PDF afterwards. I have my own mount overhaul about to begin, almost the same but the Sears 6339 version.

Hi Senex, I am fine with that, but I caution you that I am still working on the mount and have a simple modification to add and a summary of my results. Working on it now and should be posted within 7 days.

 

cheers ... Graham.P


Edited by Senex Bibax, 04 February 2020 - 07:41 AM.


#157 imgumby001

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Posted 03 February 2020 - 09:38 PM

Man! I know this is an older thread, but what a great write up!! Soo awesome! Great info and pics. Thanks so much!

#158 BlueGumby

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Posted 12 February 2020 - 06:16 AM

BALANCING THE TELESCOPE & MOUNT

 

Giving some sober thought to the performance of the R-74 mount after a very impressive first light I realized that there was an issue with RA fine adjustment. Freeing up the RA axis had revealed a significant imbalance which was producing axial load in the RA worm leading to undesirable friction when operating the slow motion control.

 

I realize most readers on CN would be familiar with balancing an equatorial telescope, however I am going to include the procedure here as it forms part of my refurbishment process.

 

Choose what is being balanced.

 

I wanted to balance the scope exactly as I might be using it, so I first chose what basic accessories should be included. In this case the bare minimum of a 0.965" star diagonal and a 0.965" K25 eyepiece (total weight 140g). I measured total rack & pinion focuser travel to be 38mm. I set the focusing barrel to the midpoint of its travel (19mm from fully in) and then extended the draw tube to focus on infinity (182mm from back of eyepiece socket to focuser body).

 

I locked the counterweight in position at the very end of the counterbalance bar.

 

Balance optical tube about declination axis (always first)

 

With my chosen accessories installed I unlocked the RA axis and rotated the mount until the declination axis was horizontal. Next I loosened the declination lock and gently rocked the scope on the declination axis to check balance. I adjusting the position of the telescope tube in the cradle for best balance. (I also recorded the position for future reference when setting up).

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#159 BlueGumby

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Posted 12 February 2020 - 06:18 AM

Balance optical tube & counterweight about RA axis

 

This procedure can be made more effective by setting latitude to 0°, however if you are not too far North or too far South then your regular latitude setting will most likely do.

 

I locked the declination axis then loosened the RA lock and gently rocked the scope on the RA axis to check balance. The head of the mount was heavy on the scope side meaning that the factory counterweight was undersize.

 

The weight of the OEM supplied counterweight was 1.8kg (4lb).

 

I used a small bar and tube to extend the counterbalance bar and then moved the counterweight further from the RA axis until the mount head was balanced. In doing so, the counterbalance arm was increased from 245mm to 300mm. The same increase in weight moment can be achieved by increasing the counterbalance weight without changing the arm. This would correspond to an increase in the counterbalance weight to (300/245)*1.8 = 2.12kg (4.67lb).

 

I confirmed this by taping a couple of steel cutoffs to the counterbalance weight and tested balance about the RA axis. Seemed pretty good.

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#160 BlueGumby

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Posted 12 February 2020 - 06:28 AM

MAKING A REPLACEMENT COUNTERWEIGHT

 

Next I modeled the OEM counterbalance in SolidWorks which confirmed that the counterbalance material is cast iron, and guess what?... if the material is changed to brass, the weight increases to 2.12kg! Bingo!

 

So it would be possible to balance the telescope about the RA axis by replacing the cast iron counterweight with a like sized one made from brass. The size of the weight would look authentic and it would still fit inside the original packaging. This also corrects the balance issue without unnecessarily increasing the load on the mount head (+0.32kg).

 

See drawing below:-

 

Because I had it in mind to be able to use a slightly bigger eyepiece, let's say up to a 1-1/4" Nagler with 0.965" adapter, I elected to make my replacement counterweight 6mm longer than the standard counterweight with a weight of 2.33kg which adds a further 0.21kg to the mount head.

 

Cost of the round brass bar offcut was A$42.90.

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#161 BlueGumby

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Posted 12 February 2020 - 06:30 AM

Note regarding loading of the mount head

 

The maximum arm of the weight is 245mm, whilst the arm of the telescope tube is about 150mm. This means that the additional 0.21kg can balance 0.21x245/150 = 0.34kg at the telescope tube.

 

With additional payload on the scope the total increased load on the mount head would be:-

 

  • swap cast iron to brass = 0.32kg
  • increased size of weight = 0.21kg
  • increased payload = 0.34kg
  • total increase = 0.87kg maximum

 

This is probably more than I would like to add, but I will see how it goes.

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#162 BlueGumby

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Posted 12 February 2020 - 06:45 AM

FIRST LIGHT WITH NEW COUNTERWEIGHT

 

So with the ability to correctly balance RA I expected the telescope to be far more user friendly, and it was!

 

I set it up early tonight (12th Feb 2020) and carefully balanced the declination and RA axes. With axes locked use of the slow motion controls resulted in only a light resistance which seemed quite natural. There was a small amount of backlash present on both axes but this did not detract overly from the viewing experience.

 

I pointed it to a rather low Venus and was pleasantly surprised that I could easily see the well defined gibbous phase in spite of considerable atmospheric disturbance. I ran it up to 240x with an Or5mm (Mizar) and 300x with a Japanese SR4mm which is challenging for lack of field of view and lack of eye relief however the mount worked with me to keep Venus near centre. That felt like a pass to me.

 

Unlocked motion of the mount was good, slow motion controls were good, however it could be improved further if the backlash could be reduced. The actual design of the drive on each axis makes it difficult to eliminate backlash but therein lies a challenge for someone. I also felt that the new counterbalance weight was working well and I would not personally want to make it any smaller.

 

Thanks for looking.

 

Cheers ..... Graham.P

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#163 Garyth64

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Posted 12 February 2020 - 08:32 AM

Good explanations and photos.

 

I had noticed too that I had to have the CW all the way down to get the scope to balance.  If I added anything else, I don't think it would balance on the original mount.

 

10TE on Oak tripod.jpg


Edited by Garyth64, 12 February 2020 - 08:34 AM.

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#164 Masvingo

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Posted 15 February 2020 - 01:40 PM

This thread is so detailed and useful, I have printed and collated the entire thread into a PDF document suitable for printing. I'm happy to share it, as long as the individual contributors do not mind. It is large, over 150 pages and over 40 MB...

I've no objections. goodjob.gif


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#165 Senex Bibax

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Posted 15 February 2020 - 03:03 PM

Thank you! I'm about to start overhauling mine, and I don't want to get grease all over my keyboard or tablet while I'm following this thread as I do so.

 

I need to redo the PDF as my first attempt didn't turn out so well on the printer.

I've no objections. goodjob.gif

 


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