Help needed please!!

[catman161]

Hi Everyone, 

inam at the end of my tether with my OTA! 

So late last year I bought a Altair Astro Planeta 250 Truss-Cassegrain OTA from a really friendly and helpful member here on SGL. 

Mall was well, scope was performing well then muggins here decided to switch the feather touch focuser for a moonlite (I know I'm mental!) There was a method in the madness. I have been planetary imaging for about 3 years now and I was very familiar, and a big fan of moonlites DC motorfocuser. It was simple, tough, strong and functioned exactly as it should do. Also I was unable to find a branded motorfocuser that was built to fit the feather touch that wasn't ridiculous money (The starlight Boss II system basically). 

So I went ahead and switched the focusers. I then found due to the increased focuser body length of the moonlite that I was unable to reach anywhere near focus. So on the advice of Altair Astro I started adjusting the spacing between the secondary and primary by moving the secondary back and forth in its casing as Altair Astro informed me that the OTA was infinitely adjustable and should work with Amy focuser. Well I tried hundreds of different spacing options but still could not get it to work so eventually gave up, returned the moonlite focuser and DC motor (can I just say FLO were amazing in helping me throughout) and bought another starlight focuser (I rather hastily sold the one that came with the scope).

Since fitting the starlight feathertouch to the OTA I have been having trouble getting it all setup and performing as it was when I first got it. 

So I am pleading for help from one of the Astro community that has experience with this type of scope (truss tube/RC OTA's in general as I am told it is extremely similar I an RC OTA). 

I just don't have the technical knowledge required to be tinkering with it again and I really shouldn't have in the first place!

In retrospect (we are all so wise in retrospect!) what I should have done is just had a bracket bespoke made for the moonlight DC motor so that I could have attached it to the starlight focuser  

If any of you more experienced astronomers can help me I would be forever grateful if you could drop me a PM and we could possible arrange to meet or exchange numbers and have a chat. 

Thanks for reading my rather long winded story! 

Felix 

[SnakeyJ]

I can't offer any technical assistance, but lots of sympathy.    I've been very tempted towards getting a 12" RC in the past and thought they were pretty straightforward to collimate and that tilt would not be a major problem for small field long focal length planetary.    I'll sit in and see what the RC gurus can offer.

[DrRobin]

Hi Felix,

I might be able to offer some advice here.  I have an RC250 truss tube from Altair and although it is a fantastic scope it has taken me since October last year until this week to get it correctly collimated and aligned.  I have had quite a bit of experience getting collimation right on my C11 so was very frustrated with the RC250 until I worked out what was wrong and was going to write up my experiences in a post on here sometime this week.  I might say that I also use a Moonlite CS on my RC250, but the slight differences in design might make it less of a problem for an RC (more back focus).

Anyway here we go...

1) The distance between primary and secondary on an RC and to a slightly less extent on a CT is rather important (RC it is fairly critical) so I am a little surprised that you have been able to move the primary forwards to gain more back focus, but I am sure there will be an optics expert who will correct me if that statement is wrong.  I know with my C11 (the primary moves) there was an optimum distance between the mirrors and an optimum back focus distance.

2) The secondary and most likely the primary were not centred on each other.  If I slacken the adjusters both mirrors will move side to side or up and down a little.  I managed to work out that if the axis of the focus tube, primary and secondary were not all in line then using a laser to get the initial collimation was always going to be wrong. 

I ended up slackening the secondary and used a digital vernier gauge to centre it both horizontally and vertically, always assuming that the truss tubes were in the centre.  There was around 1.5mm movement from centre to each side, just the clearance around the centre securing bolt.

After centering the secondary and collimating with the laser, I used a sight tube to look at the primary, secondary and baffle tubes and noticed that the baffle tubes were not concentric.  This suggested that the primary was off centre as well.  I reckon there might have only been 0.5~1.0mm of movement in the primary and through trial and error and educated guess work I managed to get the whole lot looking right on the laser, the sight tube and when looking at the baffles, all in line.

3) A star test followed and because I find it difficult to look at picture of a star field and work out if the scope is collimated, tried Jupiter and it's moons.  I saw straight away that it still wasn't right (rather fuzzy detail on Jupiter and circular arcs from it's moons) and a further adjustment of the secondary on a bright star and another session on Jupiter and it now looks to be spot on.

 

So if I had to guess, I would say distance between primary and secondary might not be right and/or primary/secondary/focus tube horizontal/vertical offsets.  Testing all of these is not easy, I have come up with ways of checking for offsets, but have no idea what the spacing is supposed to be between primary and secondary or how to measure it accurately (without touching the mirrors). If you think the mirrors might be offset, I can outline how I found out they were wrong on my RC and depending on the equipment you have available you would be able to repeat what I did?

Hope that helps

Robin

[DrRobin]

Hi,

I have just read Altair's product page for this scope.  If you think that the primary or the secondary is now in the wrong place I would contact them to see if they can put it back on a interferometer as it looks like mirror position might be critical with this scope.

Robin

[catman161]

Hi Robin 

thanks for your comments. The primary hasn't been moved, I have only made adjustments to the secondary. 

As for taking it back to Altair Astro I drove it all the way to their shop only to be told they couldn't do anything that day. Then when I asked if I could leave it with them to be sorted they refused and said "we can't be taking people's scopes home to check them, work has to stop some time". So as far as them being able to help I think I have exhausted that avenue. They seemed almost annoyed at my suggestion which left me a little disappointed as it took me over 3 hours to drive there. 

If there as any way I could perhaps bring my scope to you as I do not have the tools/knowledge nor skill set to be messing with it again, I fear I may make it worse. I would of course be willing to pay you for your time. I have really struggled to find any kind of service readily available for troubleshooting issues with these types of scopes. 

Im just desperate to get it back to full working order again as I think from the Jupiter images I took in less than favourable conditions before I started messing with it this OTA has great potential. 

Thanks again Robin, really appreciate you taking the time to respond. 

Felix 

[DrRobin]

Hi Felix,

You are around 4 hours from me and every time I fiddle with one of my scopes it seems to take me half a day or more so I am not sure a meet up would be really practical.  I think if I was you I would give it a couple of days on here there might be someone more knowledgeable than me and most likely closer who could help out.  What about a local club?

There are some checks you could make yourself, do you have a good laser?  A Howie Glatter with circular diffuser would be ideal?  And/or a collimation/sight tube? Both of these might give you insight in to what is wrong?

Otherwise, how do you do your collimation?

Robin

[catman161]

Hi Robin, 

ah yes I have just seen you're in Newcastle upon Tyne  

yes I use a howie glatter with pinpoint and concentric circle reticule. To be honest I am not totally sure what I am doing with the circle pattern reticule myself. 

I am in dis us soon with a club member from EMS who has kindly offered to help out too. So may be okay from a practical pint of view. Thanks Robin  

[DrRobin]

Yes, stuck in the frozen north up here.

A Howie Glatter with the concentric circle pattern will get you most of the way there, or at least in the right ball park, but I found that you could get the correct patterns with the laser and still be mis-aligned, especially if the axis of the focuser tube isn't aligned with the axis of the primary and secondary.  That is when the sight tube/cheshire comes in to it's own.  I bought a cheapo Skywatcher sight tube/cheshire at Astrofest (I think FLO sell them) and FLO also sell a rather nice sight tube/chechire, they are dead easy to use.  They confirm what the laser is telling you is correct.  I find that the laser is good for aligning the focuser (centre spot on the secondary) and for aligning the secondary to the primary, rings concentric on the primary, but not as good as the sight tube/cheshire for aligning the primary.  Additionally if the centre spot is wrong on the secondary or the secondary is in the wrong place I could not see how the laser could tell me this, but you could see it as an error in the sight tube/cheshire.

As for your Howie Glatter have you seen the You-tube video tutorials on aligning an RC with a Howie Glatter, they explain how to use the concentric circle patterns to get the scope spot on?  The link to the first video (of three) is here

The only think he doesn't mention is that if the axis of the focus tube is out of line with the axis of the primary and/or secondary then the laser will hit the centre spot of the secondary at an angle and will lead to a mis-alignment.  You should of course check and make final adjustment on a slightly defocused star and I was quite surprised how far out what I though was a well collimated scope on the laser was when using a star.  The other source of error is that the accuracy of the centre spot on the secondary mirror is sometimes not in the centre and if the secondary mirror is out to one side then again with just the laser you will get an error.

So star testing is essential.  I find star testing very subjective and not at all easy, is the shadow caused by the secondary really concentric, are the other rings which are visible all aligned so usually use a camera and then stack the image of the star and look at it on the screen, then go back to the scope and make another small adjustment.  My final test is on the fine detail on Jupiter, does it look good?  I have even made small adjustments whilst viewing Jupiter, a small turn, is the view getting better, if not turn the other way or try one of the other secondary adjustment screws until the view improves.  It is a bit of trial and error and if you end up miles away you have to go back to the star or laser but I have found this technique works for me.

Robin

[Astrobits]

Adjusting the spacing between the primary and secondary in a cassegrain system is not recommended. The figure on the mirrors ( secondary mainly) depends on the distance between it and the primary. See this site: http://bobmay.astronomy.net/CassNotes/cassformulas.htm

Schmidt Cass systems are more tolerant as the mirrors are spherical.

Nigel

[dweller25]

Hello Catman,

Who made the Planeta - could it be CFF telescopes - If so it may be worth talking to them ?

There could be something useful here ....

http://www.cloudynights.com/topic/479380-classic-cassegrain-popularity/page-7