EdgeHD - not holding collimation? thermal effect? pinched optics?

[HenryNZ]

I have been struggling to get my 8" EdgeHD to perform properly.

The thing is I know my EdgeHD is capable of producing nice round stars across the field of a APS-C DSLR. I have seen good image from this scope before.

My problem is that the EdgeHD does not seem to perform consistently.

For example: 2 nights ago I did my last collimation using Metaguide with 2x barlow. The resultant image is quite acceptable to me.

However when I took it out again last night, all stars across the field are triangular. I have not touched a single collimation screw, have not knocked the scope or made any other change to the optical chain.

I can understand slight shift of collimation from time to time, but I am at loss to what causes such dramatic change in performance between two nights of similar conditions.

Is it because something is loose allowing the collimation to shift (I think the screws are quite tight)?

Is it because of different degree of thermal equilibrium?

Is it because of pinching?

I don't mind collimating once a while but if I have to collimate every time I take the scope out it will eat into my imaging time very significantly.

 

What have I missed here? 

 

 

 

[Peter Drew]

Clutching at straws a bit but if the secondary collimation screws are too tight they could print through to the mirror and give the distorted image displayed in your closeup. Might be worth backing them off equally a touch and see if it makes any difference.

[Highburymark]

What do the defocused stars look like? If you could describe their shape it may be easier to diagnose.

I have no experience of imaging, but I find C8s can be sensitive to moisture in the atmosphere. Two consecutive evenings might feel very similar weatherwise, but often I'll find my Edge performs very differently. What sometimes isn't accounted for is changing conditions as the observing session progresses - ie falling temperatures or increases in humidity over time - so the scope never performs to its potential.

[newbie alert]

Are you giving it enough cooldown/acclimatisation will be ove an hour on a 8 inch..i think the edge has fans is that right..

I have a avx 8 and that was inconsistant,i found 1 or 2  collimation screws were too tight

[ollypenrice]

Was the nght of triangular stars colder than the others?

Olly

[HenryNZ]

I have posted about my problem and some updates on cloudynights, but here is what I have found:

 

last night seeing was acceptable so I checked the collimation and found that it was off. I recollimated and tightened the screws at the same time, since it had obviously changed between the two nights. However that gave me trefoil pattern of diffraction ring in focus and stars were triangular. Obviously something was pinching. I loosened the collimation screws in steps until the trefoil pattern was almost gone and the stars became nice and round across the entire field. I took a bunch of exposures and the stars were best I have seen from this scope. I then slewed the scope some 45 degree away, and to my great disappointment the triangular stars returned in full force. I then remembered I had the mirror lock on so I unlock the mirror and refocused. After that most of the triangular deformities disappeared but a slight hint could still be discerned.

My conclusion:

the cause of the problem was collimation shift, exacerbated by a bit of pinching  of the secondary. Recollimation and loosening the screws fixed the problem at one particular location. However slewing and mirror lock both can introduce problem with star shapes . There was still some pinch at the second location after loosening the mirror lock. I don't know whether that was residual from the lock or residual from the secondary mirror.

I am surprised the collimation can change so much between two nights with minimal moving in between. If this is an ongoing problem I really don't know if I can keep this scope as collimation significantly eats into imaging time.

 

[HenryNZ]

The scope was out whole day under telegizmo cover on that first collimation atttempt, out for may be 2 hours when that image of the bad star was taken and out for may be three hours when I did the recollimation last night. The ambient temperature was very similar in all three nights. 

[Highburymark]

Is it still within warranty? The collimation on the scope should not be changing so easily - in fact many owners go for months without recollimating

[sharkmelley]

On ‎04‎/‎02‎/‎2017 at 17:26, Highburymark said:

What do the defocused stars look like? If you could describe their shape it may be easier to diagnose.

 

I agree. Get an image of a bright defocused star.

Mark

[Dave In Vermont]

Regardless of 'cool-down' or other terms, the word I keep thinking here is "temperature." Aggravated through 'ccol-down' or other potential causes, temperature causes things to expand (exothermic reaction) or contract (endothermic reaction). And that can create problems on all manner of delicate settings.

Have fun -

Dave

[HenryNZ]

Thanks for all the ideas. 

I have done quite extensive tests last few nights. I now believe the main problem is collimation change from significant mirror flop. It would appear that the bad star image I took was on the opposite side I collimated on. I have since reproduced the problem on multiple tests and every time the scope changed orientation the collimation change. So the collimation change has nothing to do with secondary mirror or temperature, or if it does the effect is small. There is probably something else contributing to the problem unrelated to collimation but it is difficult to pinpoint / decipher as the miscollimation is dominant and this is compound by the built in flattener mixing things up.

 

I do not have a defocused star image as I forgot to keep one, but on one side of meridian the defocused star is textbook and the other side is obviously skewed. 

There is very obviously a problem of mirror flop which surprisingly wasn't helped with mirror lock. I now wonder if there is an particular orientation I should collimate in such that the mirror flop effect is minimised. 

 

[Dave In Vermont]

Some of us have solved 'mirror-flop' by installing a Crayford-Focuser on the scope. I use a Crayford 10:1 Dual-Focuser on my SCT, as well as my Maksutov. Then I set the main-focus around the middle-point and then use the Crayford for fine-tuning.

Works fine now. No more problems.

Best wishes -

Dave

 

PS - This is what I installed on both:

 

 

[sharkmelley]

5 hours ago, Dave In Vermont said:

Some of us have solved 'mirror-flop' by installing a Crayford-Focuser on the scope. I use a Crayford 10:1 Dual-Focuser on my SCT, as well as my Maksutov. Then I set the main-focus around the middle-point and then use the Crayford for fine-tuning.

Works fine now. No more problems.

 

Presumably (though you haven't said) you are locking the mirror with the mirror locks and then the mirror cannot flop as you perform meridian flips etc?

That gives me an idea for the Celestron C11 I abandoned because I got quite fed up having to recollimate it before every single imaging session.  My C11 doesn't have mirror locks but if I could permanently modify it so the mirror is fixed solid (epoxy resin?) then I could use the same approach.  At the present time I consider it all but useless for serious imaging.

Mark

[alanjgreen]

It's only an 8", mirror flop should not be an issue!

talk to your supplier and arrange to send it back.

This is not a typical C8 by any stretch of the imagination, there must be something wrong with it.

 

[Dave In Vermont]

I've also used this on my LX90 12" which has no mirror-lock (my LX200 is living in a new home now). It still works fine. I don't believe a mirror-lock is available on the Maksutov scopes from Synta - which makes these for Skywatcher, Orion, Celestron - but their focuser built-in has been greatly improved recently, with no mirror-flop being noted on this. At least in the Skywatcher ones - which I opted for. But the GSO Crayfords were installed by yours' truly regardless. These Crayford's are excellent at giving one a scalpel-sharp view though.

I've not examined the Celestron SCT-type you have. So I don't know if this would/could be an issue with the 8" models.

Good luck!

Dave

[cfpendock]

On 2/5/2017 at 12:49, Highburymark said:

in fact many owners go for months without recollimating

Years.

Chris

[HenryNZ]

Yeah I figure there is something wrong with it. Unfortunately I bought it on a trip to the States so servicing will likely need to return to base which will be expensive, probably costs more than half what the scope is worth. As I have nothing to lose, I will likely open the OTA to try to fix the problem once the retailer confirms my suspicion (that I will need to send it back to the States to get servicing).

 

For those who are interested and for posterity, please see my test results here:

http://www.cloudynights.com/topic/565341-cant-get-consistent-round-stars-with-my-edgehd-at-native-f10/?p=7693688

 

Basically, the symptom is identical to a severe case of mirror flop that is bad enough to change collimation, however this is not helped a slightest bit by mirror lock indicating the flop is not the usual type but more likely indicates something loose inside the OTA.

[alanjgreen]

Have you removed the faststar secondary to make sure it is sitting nice and flat against the metal backing?

There is a lens inside the Edge version too, at the back of the baffle tube, maybe that is loose?