Mak owners weigh in please!

[Sunshine]

Hello all! today i had a very brief moment outside with my 150 Mak before the clouds rolled in, between the time i put it out under clear skies and the two hours i gave it to equalize the clouds had rolled in but with some breaks here and there.

so during a short break in clouds i picked the brightest star and quickly checked collimation with my 14mm eyepiece, my question is if i should bother to adjust collimation because while im getting a beautifully symmetrical disc

with sharp rings on one end of focus, on the other end im getting a disc thats not so symmetrical with a bit of doppler effect going on where concentric rings are compressed a bit more on one end, dealt with this before on my big dob

but not sure to what degree it will affect performance in my mak. I remember some saying here that Mak's don't ever need collimation and hold collimation short of a hammer blow to the tube, should bother opening the pandora's box that 

collimating can be in worst cases? (star was dead centre in FOV of course)

Thanks everyone!!

[nightfisher]

Okay, there has been a few cases where people have tinkered with mak collimation and made things a heck of a lot worse, making the scope unusable so my first bit of advice is proceed with extreme caution, do some more observing and then decide if this is a task you want or really need to perform

[mikeDnight]

It's probably best to wait until you get a clear night where you can give the scope a thorough workout. Use stars at or close to the zenith for testing, as they are less effected by atmospheric turbulance. Also, its worth bearing in mind that with a Mak  Cass, the secondary mirror amplifies the image. A Cassegrain catadioptric system can be 25 times more sensitive to internal heat than a Newtonian or refractor of the same aperture, due to the square of the amplification factor. My guess is that you could well be noticing some residual heat from the baffle tube which causes a blurring at one side of the diffraction rings. You mentioned that the rings were symetrical, which they would not be if the system was out of collimation. Also, the disturbance caused by residual heat may be more evident on one side of focus than the other. Observing at the zenith should reduce the problem if it is due to heat, and if it does, then you know not to tamper with the collimation.

In winter when the temperature gradient from indoors to outdoors is steep, residual heat in any telescope becomes more of a problem, which demands a scope be given time to stabilize.  With a MCT of SCT it is best to keep the scope cool at all times if possible, or on cold nights, give it upto three hours to reach thermal equalibrium before critically testing.

[Chris]

I would leave it well alone unless clearly out of collimation. The concentric star one side of focus suggests that it must be in collimation, so maybe look for other reasons for the effect you're seeing the other side of focus. 

Does the scope give good sharp views when in focus and cooled?

[paulastro]

I agree with previous contributors, try the scope out on a good night when it's had plenty of time to cool down before attempting to collimate it.  If you try to collimate it when you don't need to then you could get into all sorts of difficulties - depending on your experience of course.

You don't mention which make your scope is, though the aperture might indicate it's a SW?  Also, if it's new then its most likely to be in good collimation, at least going on the ones I have seen.  If it's used then it is possible someone's been playing with the collimation.

I would have thought the chances are, particularly if new, your scope is fine and it's more likely heat in the tube, seeing conditions or some other factor is responsible for what you're seeing.

[John]

Might still be some warm air clinging to the inside of the tube ?. That can cause a distortion in the star test pattern similar to what you are describing.

[Peter Drew]

It takes quite a while to get to know your telescope well enough to make a definitive judgement unless there is something seriously untoward. I would never have bought some of the telescopes that I own based on the images they sometimes give! 

[Stu]

In my experience maks give quite odd out of focus diffraction patterns. I would mainly concentrate on making sure they are concentric. Do you get mirror shift when changing direction of focus travel?

[Sunshine]

Thanks for all the great tips, the scope is brand new, i will not make any adjustments until i have had a good while using it and have done more tests under better conditions in more stable temperatures.

[RT65CB-SWL]

Have you seen this? ---> https://www.youtube.com/watch?v=vMkURZdJ84Q

[Mr niall]

I would say that the symmetry is the main thing, my Mak 90 is quite a bit off on the star test and still gives 99 percent perfect views. I actually took the whole front off, drilled and tapped four holes for a new dovetail and put it together again and it was still pretty much bang on!

its the views that count, I got a bit obsessed with diffraction patterns for a while when I couldn’t get a sharp view, in the end it was just a slightly grubby diagonal.... unless it’s clearly miles off I’d leave it (they do seem to be a thing the mercy of atmospheric conditions more than shorter focal length scopes).

edit; in fact that sounds more like a fractional imperfection in the primary (like mirror pinching) or more likely the aspheric secondary. I’d be stunned if it made any difference though.

[Sunshine]

Perfect thank everyone! i won't loose any sleep over it, i haven't really had a  chance to use it short of a few glances at stars anyway so my judgement may well be premature, i can't wait for some warmer weather and clear skies.

[spaceman_spiff]

Hi Sunshine,

One extra to mention, the diffraction pattern will appear non-symmetrical at the edges of the FoV so make sure that your star tests are done in the middle of the FoV.

HTH Dan.

[RT65CB-SWL]

On 22/01/2018 at 07:58, spaceman_spiff said:

Hi Sunshine,

One extra to mention, the diffraction pattern will appear non-symmetrical at the edges of the FoV so make sure that your star tests are done in the middle of the FoV.

HTH Dan.

That's interesting. Thank you for the update. I was going to give a 'phone call to SCTelescopes. 

@Sunshine Just to let you know, sometimes the locking ring that holds the primary in place can work loose. If this is the case return it to your reseller as it is still warranty/guarantee. My ETX105 did that a few years ago between the 're-mods' and is now OK. 

 

<--- the 1st 're mod' of my ETX105.

<--- the protrusion as per 1st 're-mod' photo has been cut off and threaded for an ETX to SCT adaptor ring to be fitted.

<--- the 2nd 're-mod' complete with an ETX to SCT adaptor ring & 1.25" visual back fitted.

[Sunshine]

Ok, so in order to avoid any speculation regarding exactly what i am seeing i decided to draw it exactly as i saw it, my memory serves me very well, ive included a photo of my drawing, im sure it will be far easier for all the keen minds here to diagnose than my vague descriptions lol thank you all!on the lower of the two the circles continue around of course, i just got lazy and only drew partial circles to demonstrate they are tighter together than on opposite end. 

[billyharris72]

I have to say that looks like it could be miscollimation to me. Seeing (and tube currents) can be a big issue with collimating on the Airy disk, but should be less of an issue when looking at a more defocused image like this, where the circles should be perfectly symetrical. If you are seeing the above and it is stable in the eyepiece then there is an issue.

It's not strictly true that Maks never need collimation, though they don't often need it and they do hold collimation well. This one looks like it could do with a tweak.

That said, I've never actualy tried to collimate a Mak before and would be wary of making it worse. To start with, how good is the image you are getting when using the scope at focus, or when looking at a slightly defocused (max 1/20 turn of the focuser away from focus) star at high magnification under good seeing? If it's doing the job to the required standard I would be inclined to leave it.

There are some materials on YouTube that might be useful if it comes to it.

Hope this helps.

Billy.

 

 

 

[RT65CB-SWL]

Mainly targeted for the Meade ETX90/105/125 user, you could try here ---> http://www.weasner.com/etx/menu.html  

[spaceman_spiff]

Is it possible that the axis the primary mirror is being moved along is not aligned with the optical axis? This would explain why you see perfect collimation at one focus position but not at another.

One test would be to look at the Airy disk patterns when you move the eyepiece in and out of the draw tube but not moving the primary mirror (keep it fixed at the position you see good collimation). If the collimation remains good then that is evidence of this theory...assuming the draw-tube is in alignment!

Dan