assembly from two telescopes

[FrenchyArnaud]

Hi all, 

First of all, a quick word of introduction : I am French (nobody's perfect), 40yo (+taxes) and I live in the UK (Bournemouth) with my delicious (but british) girlfriend.

I always had the mind of a stargazer, but never the time, the opportunity, the [feel free to fill this space with any other lame excuse you can think of.]

Anyways, here is the deal. We decided to get our first telescope and after X discussions we aquired last week a second hand Celestron 114 LCM (Yes it's far from perfect - but still I think it's great for a start). It's been dubbed "Edwin". Don't even comment on that...

Now, yesterday, browsing the web I come across this very old gentleman who gives away, basically for free, an unbranded scope : 150mm, 750mm focal, german eq. mount, printed "event horizon". So I could not resist and took it. First try : very impressive. The sturdiness and the stability of the thing is really something. So is the weight, by the way... Which makes it more or less unmovable (I need to walk carrying to the natural reserve nearby to find less polluted skies and there is no way on Earth I carry that thing for 2 miles) 

Then I thought : I could adapt a dovetail on the tube and use the Celestron computerised mount (upgrading the Celesstron 114 to a 150mm in the process, which should be a real improvement in this suburb environment)

Now here are my two questions : 

1) Is there any reason why I should not do it? Or stuff I should make sure of before I do it.

2) How comes this 3ft tube is 750mm focal while the 2ft Celestron is 1000? No matter how I turn the question it seems to me this Celestron 114 is actually a 500mm, not a 1000 as advertised...

Thanks for your lights ! (Pun intented)

 

 

 

 

 

[Dave Lloyd]

Hi Arnaud, welcome. I'm puzzled too. It certainly doesn't look like the f8+ it's advertised as. More knowledge people on here will no doubt enlighten us.

[triton1]

I think the Celestron will be a "bird Jones" design using a spherical mirror and a built in Barlow in the focuser which basically cuts the length of the tube by half.

[FrenchyArnaud]

I googled bird jones, then checked on the Celestron itself, and yes it definitely is. No big deal for the time being but I wish I had known before I made my mind on that model. That being said, I won't loose sleep over it, it gives great views of saturn - and that was the primary goal. 

However what I am really interested in now is to consider modifying the 150mm tube to fit on the lighter computerized mount.. I have looked carefully, thought deep about it and I can see no reasons why it wouldn't go as expected (mechanically). I just wonder if there is something I ignore or haven't thought of which would prove to be a problem...

 

[John]

Hello and welcome to the forum

The main problems that I forsee in putting the 150mm scope on the LCM 114 mount are:

1. The wider tube may not clear the mount when the scope is pointing above a certain angle.

2. The heavier tube of the 150mm will not be steady on the mount which is designed to carry lighter scopes.

3. The motors of the mount may not be powerful enough to move the larger, heavier tube of the 150mm scope around.

The 114mm scope is the largest one in the LCM range so the LCM mount will be at it's limit with that scope on board. I fear that the 150mm scope will be just too much for the mount.

[Peter Drew]

I'm giving a Celestron 114LCM an MOT at the moment. I can confirm that it is a Bird-Jones design. It is also mounted offset from the centre of the mount such that it clears it completely. Unfortunately this makes for a rather unbalanced arrangement and the rather flimsy mount components mean that it is barely adequate to carry the original OTA. The example I'm examining had loose secondary adjustment screws, overtight primary mirror and a sloppy focus tube out of the box. With everything now sorted out and the unusual Celestron RDF replaced by a standard SW type one it now works quite nicely.   

[FrenchyArnaud]

8 hours ago, Peter Drew said:

the unusual Celestron RDF replaced by a standard SW type one it now works quite nicely.   

I am absolutely sure this makes perfect sense - at least it would if I had any idea what RDF and SW type mean  Don't forget I am french and newbee, and acronyms you think are obvious... are not

 

9 hours ago, John said:

Hello and welcome to the forum

The main problems that I forsee in putting the 150mm scope on the LCM 114 mount are:

1. The wider tube may not clear the mount when the scope is pointing above a certain angle.

2. The heavier tube of the 150mm will not be steady on the mount which is designed to carry lighter scopes.

3. The motors of the mount may not be powerful enough to move the larger, heavier tube of the 150mm scope around.

The 114mm scope is the largest one in the LCM range so the LCM mount will be at it's limit with that scope on board. I fear that the 150mm scope will be just too much for the mount.

These are all relevant points and I had concerns about those.

1) Careful measurements show that the tube itself has plenty of room, because the mount is on the outer side of the axis - it's only an inch or so extra outwards as for the center of mass, so mechanically the mount will accept the tube (I think) - the Celestron is 60cm long, the unamed one is about 90cm, so it's only about 15 extra and I don't see that it will touch the legs.

2) Yes that is a real concern. The tripod is flimsy at best. Even if the tube is only 50% heavier I guess it will make a difference. I saw some pads you can set underneath the feet to limit vibrations but I don't really know if this has any efficiency at all ?

3) Yes that crossed my mind. Both tubes are pretty light but there is a difference, clearly. I will weight both of them tonight to have a clearer idea. That being said, I was thinking, if I balance the bigger tube properly, it seems to me the extra force needed from the engine should not be that much. The problem is, I would need a short dovetail so that, once installed, I still could put the 150 on the eq. mount ; and I am not sure if this will allow to place the dovetail on the axis of balance (the rings on the eq mount are really close together) Alternately maybe I can DYI some temporary dovetail attachment to just test it and see what happens. Not sure how to do that yet.

 

This all makes me think that before I do anything, I will probably spend a few nights with both scopes out, to see if the 150mm really makes a difference on the 114mm - I guess it should, but well, it probably needs to be confirmed. Because if it does not make much of a difference I will just stay with both tubes as they are. 

 

 

 

[Stu]

Just to clarify:

RDF means Red Dot Finder, an example of a cheaper version shown in the image attached

SW means Skywatcher, they have a standard fitting for finders which many manufacturers share. 

 

[Peter Drew]

Apologies "Frenchy" for assuming you would be familiar with the accronyms, blame your excellent English! Welcome to this forum.

[Cornelius Varley]

10 hours ago, FrenchyArnaud said:

I googled bird jones, then checked on the Celestron itself, and yes it definitely is. No big deal for the time being but I wish I had known before I made my mind on that model. That being said, I won't loose sleep over it, it gives great views of saturn - and that was the primary goal. 

However what I am really interested in now is to consider modifying the 150mm tube to fit on the lighter computerized mount.. I have looked carefully, thought deep about it and I can see no reasons why it wouldn't go as expected (mechanically). I just wonder if there is something I ignore or haven't thought of which would prove to be a problem...

 

 The key thing here is the mount. LCM stands for Lightweight Computerised Mount. The 114 tube weighs about 4 lbs  (1.8kg) and the 150 about 9lbs (4kgs) , both weights derived from similar sized telescopes so might not be precise but will give you an indication of the weight differences. The LCM mount was designed for lightweight telescopes, the 114 being the biggest telescope sold on this mount, so might not be adequate for the larger and heavier 150. Even if the 150 can be made to fit the mount you might find the motors might not cope with extra weight or ther tripod stable enough for the extra load.

[FrenchyArnaud]

@Stu : Thanks for that. Stupid me, RDF, I should have figured it out on my own... Well now Peter's statement makes perfect sense (as expected), and I agree, this RDF gizmo is close to useless, the small scope on the 150mm is a zillion times easier to use.

@Peter : thanks for the compliment but my English is definitely a WIP. Wait until you hear me speak   

Anyways, tonight, I disassemble everything and weight the parts. Then I will carry the whole damn thing ( Probably StarGazing should be an official Olympic Game )  and try and see if the modification is really worth it. My secret hope is that with the 150mm I should be able to make a few DSOs... So decision-making is postponed to at least tomorrow - or later. 

 

[FrenchyArnaud]

6 minutes ago, Cornelius Varley said:

 The key thing here is the mount. LCM stands for Lightweight Computerised Mount. The 114 tube weighs about 4 lbs  (1.8kg) and the 150 about 9lbs (4kgs)

I did not realize that there is such a difference. Basically it means that if there is an optical gain in switching to the 150, I need to figure a way to try the 150 on the LCM without permanently modifying the tube, in order to see if the engines cope or not.  If there is too much torque asked on the axis I would see noticeable backlash and I guess it is very likely what would happen. 

[Cornelius Varley]

The 150 will be a definite improvement over the 114. The problem will be that the mount will not support the heavier telescope, simply touching the telescope to adjust the focus might result in the telescope vibrating to such an extent that it becomes unusable.

[FrenchyArnaud]

Well, thanks to all of you for your insights. 

I have finally come to the conclusion that the german equatorial is a lot of (frustrating) fun, so I will keep both tubes as they are and will use either according to my mood. 

Maybe later I will motorise the eq. when the time will have come to consider imaging.