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My 'modded' Meade ETX105 OTA - part 2


RT65CB-SWL

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Following on from Part 1...

A few years ago I decided to purchase an ETX105. It was just the OTA, without the mount/drive. I saw one for sale on eBay and won the bid. I had already had a small dovetail plate to attach it my TeleOptic Giro. So attaching it to my Giro was not an issue. I had used it to attach my TeleVue Ranger to the Giro until I purchased the TeleVue/Vixen mounting plate. The small dovetail plate was a white metal or aluminium casting. It was whilst I was attending a public starparty held by my local society a few weeks later that it snapped whilst I was tightening up on the spare Giro arm alongside the Ranger. Fortunately the ground surface was grass so it did help to cushion the impact. 1x ETX105 OTA falling rear end first + semi-hard surface = damage to plastic.

The following day, I had a good look at my ETX. The main tube appeared OK, (apart from minor paint chips), a bit of mud and grass here and there, and no serious damage to the meniscus cell, primary mirror which was a relief. After a careful clean, I removed the back, (why is it the the last screw/bolt you wish to undo is always seized). Upon closer examination and removal, it became evident the plastic back, (the section that houses the flip-mirror and counterweight, visual back), was damaged, (a large crack on one side, internal damage to two of the mounting/retention points ), and a cheap plastic star-diagonal.

I phoned my insurers to claim 'accidental damage'. They seemed reluctant to offer any help etc, as it was not 'at home' or 'in the home' when the incident took place. I phoned Telescope House to see if the back was available as a spare part. Telescope House were helpful and told to phone back in a few weeks as their 'parts' person was on leave. I phoned Telescope House a few weeks later as agreed and was put through to the 'parts' person. He said he would have a look and he would phone me back if they had one avaiable. He returned the phone call only to tell me me they had an ETX90 with a 'slightly damaged tube', but nothing for an ETX105. I then phoned Meade Instrument Corp who were unhelpful and was told, “The ETX105 is no longer made. We don't keep spares.” then hang-up.

A few weeks later, I placed a few classifieds on various amateur astronomy sites and forums, hoping someone may have one spare. Within a few weeks I received an email from someone saying they saw my classified and had one for sale. We agreed a price and shipping etc, and payment made. Despite numerous emails to and from the seller to this date, nothing has arrived. In the end I have decided to cut my losses. I wanted to use PayPal, but the seller did not.

During my frustration and anger, I had drawn a few plans for a new style back. I settled on the one that would be easy to turn on a lathe without being complicated. I visited a few local engineering workshops to see if they would take on the comission. Many declined, one accepted and I left them the 'old' back and the plan of it and paid a deposit. As I did know the measurements of the screw holes and the focusser hole, the 'old' back served as a template for them to work on. Four weeks later, I recevied a phonecall to say it was ready. To my surprise it was milled and bored from a single block/billet of aluminium. I imagined the protruding tube for the eyepiece holder to be threaded or welded, but no, it is solid, (the wall thickness is about 2mm - where it is coupled to the OTA is is about 6.5mm). The holes for the focusser shaft and securing to the OTA lined up. I paid the outstanding balance. While the 'new' back was being made, I stuck a piece of parcel tape over the exposed end/baffle tube to prevent dust and other contaminants from entering the tube and then packed it away in its storage case.

Here is the link to part 1: http://stargazerslounge.com/topic/200993-my-modded-meade-etx105-ota-part-1/?hl=%2Bmy+%2Bmodded+%2Betx105#entry2122570

The parts list for my 'modded' ETX105

1x ETX105 OTA (less rear plastic flip mirror assembly and counterweight housing)

1x medium (21cm) SkyWatcher dovetail plate

2x No6 Jubilee clips

1x visual back/1.25inch eyepiece adapter (made by Stephen Mogg)

1x 3/4” x 1/4” BSW nylon thumbscrew for above

3x 30mm x 6-32 UNC stainless socket cap screws

3x 1/2” tap washers

3x 3/4” tap washers

1x aluminium knob (12.5mm diameter)

1x 3mm x 20mm stainless socket cap screws

1x 123mm vinyl end cap

misc items: self adhesive felt, epoxy adhesive & thread lock

The re-assembly

There was only one oversight by me at this stage. What thread were the mounting/retention bolts/screws? The original ones were short and two of the original screwheads were damaged trying to remove them as they were seized during removal and needed replacing. Another phone call to Meade. This time, the agent was understanding and very helpful. I managed to locate a company near to the engineering workshop, so I purchased ten stainless steel 30mm x 6-32 UNC socket cap screws. A single 3/4” tap washer was placed over each of the collimation screws. I forgot to allow for the slight protrution of the baffle tube locking ring in my sketch/plan. These made a suitable platform and spacer for the new back. A 1/2” tap washer was placed in-line with the mounting holes and the socket cap head bolts would pass through them and again made a suitable platform and spacer. Then I attached ETX visual back/1.25inch eyepiece adapter with some epoxy adhesive, even though the 'new' back was threaded to attach it to, I do not fancy it coming loose and damaging or loosing an eyepiece or any attached accessory.

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The 'business' end of my modded ETX105 visual back, c/w 1.25inch eyepiece adapter and star diagonal alongside my TeleVue Ranger mounted on a Tele-Optic Giro Mk ll.  The visual back is from one billet of aluminium.

Sourcing the tube rings

I had to source a set of tube rings to attach the ETX to the Giro. The external diameter of an ETX105 is 124mm. The nearest set I could find was 125mm. I ordered a set and dovetail plate from Vixen Optics. The rings were packed out with 0.5mm strip of rubber to take up the slack. I was not to happy with this

arrangement. After searching the internet for some bespoke tube rings, (most were/are and only available from the USA). I asked here on SGL if anyone knew of anyone who made bespoke rings. I got a reply from Ray Lamaq, and once I emailled him with the dimensions, payment cleared etc, I received them in about four to six weeks. Firstlight with them and I was not happy. I was getting a slight ghosting or flare when looking at Venus, Jupiter, Saturn and any bright star (ie >mag. +0.5 to +2.5). Further examination revealed it may need collimating. I may have slightly distorted the meniscus cell when tightening up the rings and this may have affected the alignment of the secondary in the light train/path. (The same was happening with the Vixen tube rings too). The bolts were slackened off, then the tube became loose and I did not fancy a repeat). I did not fancy collimating at this stage as it meant removal and attaching the back several times and after reading several posts on Mike Weasners Mighty ETX site it seemed a daunting task.

During the past few weeks, (months, if you include the time it has taken me to submit this post), I had a re-think. There had to be or a better way of attaching the dovetail plate without drilling and tapping the OTA? So, I visited the local ironmonger & tool specialist, purchased 2x No6 Jubilee clips/rings, then a local department store, (that has a haberdashery department), and purchased some self-adhesive felt. Ordered a medium SkyWatcher dovetail plate from FLO, (the old one was looking slightly worse for wear and tear). The self-adhesive felt was cut to two-thirds of the ETX diameter and then stuck and trimmed to the Jubilee clips/rings. So it was 'off' with the bespoke rings and 'on' with the Jubilee clips/rings and dovetail bar as in the photo and made tight and secure.

I found the original Meade focusser knob was small and fiddly to use. I decided to do away with it and use something bigger. I had two aluminum knobs from a scrapped non-astro project that was started but but never finished, so I used one of them. I had a few 3mm x 20mm stainless steel cap screws, so I have used one of them to attach it to the focusser shaft. Using a the cap screw rather than the supplied grub screw serves as a useful lever for achieving fine focus without causing any/or very little vibraton as you just need to push or pull with a tip of a finger or thumb.

Collimation & firstlight

As previously mentioned, collimation of my ETX105 was out of allignment. Mounted on the Giro, I checked the collimation with a laser collimator, a 'shorty' cheshire eyepiece and collimation cap. To my surprise it was an improvement on the previous tube ring sets that had been previously used. The following day, (30th Nov 2013), I removed the socket cap bolts and applied some thread-lock and re-tightened them one by one in a circular pattern until each became tighter than the last. I also checked and re-tightend the Jubilee clips/rings that hold the OTA directly to the dovetail plate for any movement. Collimation re-checked and was better than before. So the photo below shows what it the collimation looks like. Does it look OK to you guys and girls?

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Handheld photo through the ETX105 with a collimation cap in the eyepiece holder. Does this look OK?

The night of 30th Nov / 1st Dec 2013, the sky was clear. My 'modded' ETX105 had Its 'firstlight' with a Meade 20mm Plossl. Jupiter & M45 were sharp. I deliberately racked the focus in and out and getting perfect coincentric rings with both eyepieces on Jupiter & Galillean moons and stars in Gemini & Orion.

Overall summary

My modified ETX105 performs exceptionally with my other eyepieces too, and other accessories that I have aquired over the years. I did have an issue with my 6mm Radian and having a small blind spot in the centre of the field of view. I am pleased to say that issue no longer exists. The Moon is a joy to observe. I have not yet observed Venus & Mars or even Saturn due to work commitments, weather, etc), but I am sure that they too will be a joy to observe, as they have been in the past with my other scopes that I own and use.

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Alongside my Ranger...

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...and C6/SCT

I did have other photos of the rebuild etc, but have I mislaid them. It would have made more sense to this post to see what was being done and achieved. I also have mislaid the original threaded end cap for the objective cell cap during a temporary house move. so I now use a 123mm vinyl end cap (as shown in the photo below). Overall, I am very impressed with what I have achieved. The overall length is about 4mm longer than the original Meade ETX105.

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Star diagonal, aluminium eyepice projection unit*, an old brass threaded e/p inserted into an aluminium 1.25inch adaptor* & 123mm vinyl end cap for the objective cell in front of my ETX105.

* these were made for me by 'Beacon Hill Telescopes'.

So what's next?

The next project for the ETX is attaching a finderscope and/or RDF. I think adding a Telrad or Rigel finder maybe slightly OTT!

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Hello Phillip,  

That is an excellent project, I am very impressed,  I think you have ended up with a better OTA than the original "plastic" version!

Several people have dismounted their ETX's, this shows that it is an excellent scope for the size but the original mount is not quite what it should be.

There may be some ETX owners out there that would consider doing the same mod to a serviceable OTA, I am certainly interested if the cost is not prohibitive.

I dismounted my ETX a few years ago and now it is fitted on a dual plate with Baader Solar Film filter (made from a DVDR Tub) next to a Lunt LS60THa for Solar work on a Celestron EQ4 mount.

However, I travel a lot with my work and have experimented with fitting the EXT105 to an old Celestron C80 GT goto mount that I own, just t see how "portable" it would be.

I will post some pictures when I have time, maybe you would be interested in going "Solar"

Regds, David B.

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HI Phillip,

Interesting project, mounted along side a short refractor I bet you've got a huge range of targets covered with just a couple of eyepieces, and good portability.

Do you know if the scope would reach focus without the extension tube bit on the visual back? 

Regarding mounting, do you think the scope would be strong enough to mount purely from the back plate? I'm thinking you could bolt a piece of heavy gauge aluminium angle onto your custom back so that it protrudes forwards and then bolt a dovetail to the angle, this could also give you somewhere to mount a finder scope(although it might be a bit redundant on a two scope setup?)

James.

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quote JED-E3 / James: "Do you know if the scope would reach focus without the extension tube bit on the visual back?"

answer: If you are referring to 'the extension tube bit...'; which is on the back plate, then the answer would be "No! the focal length of an ETX105 is 1470mm." I had taken my measurements from the original ETX back. I got the OTA length almost correct, (about 4mm longer :grin:, excluding the visual back/1.25inch eyepiece adapter). The original ETX back houses the flip-mirror and counterweight and had a threaded adaptor port/ring for attaching the Meade 1.25inch eyepiece adapter and many other Meade accessories as well third-party accessories :smiley:. I think Maks & SCT's are extremely critical about focal length :undecided:.

JED-E3 / James again: "...Regarding mounting, do you think the scope would be strong enough to mount purely from the back plate? I'm thinking you could bolt a piece of heavy gauge aluminium angle onto your custom back so that it protrudes forwards and then bolt a dovetail to the angle,..."

answer: I think the scope backplate would be strong enough. I designed it so it did add undue strain and stress on the three socket cap screws holding/retaining the back plate to the OTA :shocked:.

Hope this answers your questions. :icon_salut:

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Quote: "...I designed it so it did add undue strain and stress on the three socket cap screws holding/retaining the back plate to the OTA :shocked:."                      :iamwithstupid:

It should read: "...I designed it so it did not strain and stress the three socket cap screws holding/retaining the back plate to the OTA :shocked:."                                     :icon_salut:

Quote MoonNut / David B: "...That is an excellent project, I am very impressed,  I think you have ended up with a better OTA than the original "plastic" version!"

Answer: ...or a poor man's Questar!!

PS: I need more caffine :coffee2: :coffee2: and Jaffa Cakes!!

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