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Old 06-09-2020, 08:48 AM
Rusty2
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Focusing issue

Howdy !
I'm having a focusing issue . I've been doing a lot of reading but don't have the experience to know which direction I should be looking to resolve the problem .

I bought a 12" LX200 . The documentation says "pinpoint stars" . But after using the scope for the last 2 or 3 months there's nothing "pinpoint" about it even when using a bhatinov mask . Using a camera or eyepiece , pinpoint is unacheivable , there is no pinpoint .

If you look at the "doughnut's" below you can see that the scope is ever so slightly out of collimation . If the documentation says pinpoint , surely collimation must be the problem , right ?

I'm assuming that a focal reducer may help but I'd rather get it working correctly without throwing unnecessary money in the wrong direction and reducing focal length .

I am using the "rough focus" knob but no matter how carefull I am there is no pinpoint . I understand that I will eventually need a collimation tool and a proper focuser but right now I'd like to resolve the problem before proceeding any further .

Any thoughts would be greatly appreciated . : )
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  #2  
Old 06-09-2020, 09:11 AM
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multiweb (Marc)
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I think the field is superb for that type of scope. What seems to be the problem?
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Old 06-09-2020, 10:04 AM
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Merlin66 (Ken)
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Rusty,
I used the 12" LX200 for many years, doing spectroscopy.
Yes, it needs good collimation but also needs a finer focus control.
I used the Meade zero shift Crayford to improve focusing.
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Old 06-09-2020, 10:47 AM
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mental4astro (Alexander)
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Rusty,

As Marc says, the images you've presented look pretty good for these sorts of scopes.

But if you are having doubts, there are two things that we as end user can tweak with mass produced SCTs. One of which though is a decision of conscience you need to make for yourself if you want to do it!

1, If your scope shows mirror shift when you focus (and the majority of SCT's do), AND you are only using the scope's native focus knob, you need to adjust your focus AND your collimation in one particular way ONLY or you will never be happy with the image.

Mirror shift comes out of what really is a poor focus mechanism design. It is possible to achieve a mechanism that achieves focus by moving the primary mirror that has zero mirror shift (and the Russian made Maksutovs have been using this for more than 20 years), but for whatever reason Meade, Celestron, Synta and even Vixen have refused to change or have done so stubbornly.

But it is possible to work with mirror shift, but you need to stick to this proceedure.

When focusing, you need to turn the focus knob counterclockwise a little past the focus and then slowly turn the knob clockwise to achieve focus. If you go past focus you need to again turn the knob counterclockwise past focus and then clockwise again to achieve focus. What this does is always sets the primary mirror into the same position and hence puts the object into the same position in the field of view.

When it comes to collimation, with an SCT (or other scope) that shows mirror shift, you can ONLY set the collimation in one position, either inside of outside of focus. You cannot achieve collimation in both positions because mirror shift will never allow the primary mirror to be in the same position both inside and outside of focus. Instead you need to tweak the collimation following the very same routine as I outlined above for focusing. It is only this way that you will be putting the primary mirror into exactly the same position each and every time for focus and hence collimation.

Collimation tip 1: the instructions that come with our scopes always shows a large "doughnut" of the defocused star. This is fine only for course collimation. Final adjustment of collimation needs to be done using a very small doughnut as any error that may be present won't be spread out and hence evened out when the doughnut is large. This goes for all scopes with a central obstruction.

Collimation tip 2: Don't do the collimation procedure with the scope horizontal. Carry out the collimation procedure with the scope at roughly 45 to 60 deg elevation. This is roughly a good average position that the scope would be at as the elevation the scope is at also affects the primary mirror's position!

2, I have come to find that the corrector plate of SCT's and Maks are EXTREMELY sensitive to the amount of pressure that is exerted by the retention ring that holds it in place! MANY times with mass scopes, the screws that hold the retention ring in place are wound in much too tight!!! It really is surprising just how little torque is needed to put on a screw for this pressure to distort the finely ground corrector plate. Remember we are talking about wavelengths of light, not millimeters. This includes the thick corrector plate of maksutovs, not just the thin corrector of SCTs. And if the corrector is being subjected to the strain induced by over-tightened screws the final image will never be as sharp as it could, and can also influence the shape of the doughnut.

HERE is the decision of conscience you face - it is entirely up to you if you will loosen all the screws that hold the corrector's retention ring or not.

You do not need to remove the corrector! Just loosen the screws that hold the retention ring and then secure them in place again. A small lift of the corrector may also free it if it has been stuck in place over the years and dew seeped in under the retention ring. Freeing up the corrector will also release any remaining strain that the corrector may be experiencing.

The last SCT I bought I got it brand new. I struggled for over two years to get the bloody thing to behave and it just would not give me a good image even though I was carefully doing the focus routine for focus and collimation. Out of desperation one day I decided to pull the corrector out to check the secondary mirror. To my dismay I found ALL 6 screws that held the rention ring were all wound in very tight. Worse was to come when I lifted the retention ring and found that the ring was hiding two BIG chips that surrounded one of these screws! The bloody dope who assembled this scope had not only wound the screws too tight but heard the corrector ping when the chips happened, undid the screws, removed the chips and replaced the ring thanking their lucky stars that the chips were hidden by the retention ring! AND they again over tightened those screws. When I re-secured the retention ring, I wound the screws in to just finger tight firm, not wrist wrenching firm. And guess what? The image the scope through up improved out of sight!!!

I also experienced similar with my Russian made Maks, not from factory assembly but ME! I cleaned the corrector and over-tightened the retention clips! Not wrist wrenching tight, but still too tight! And the corrector of a 10" Mak is considerably thicker than the corrector of an SCT!

Alex.
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Old 06-09-2020, 12:19 PM
Rusty2
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Quote:
Originally Posted by multiweb View Post
I think the field is superb for that type of scope. What seems to be the problem?
Hi multiweb , I guess my idea of pinpoint is different to the manufacturer : )

Quote:
Originally Posted by Merlin66 View Post
Rusty,
Yes, it needs good collimation but also needs a finer focus control.
I used the Meade zero shift Crayford to improve focusing.

I appreciate the tip Merlin , thank you !

Thanks for the comprehensive reply Alex , I appreciate it and understand what your saying . Looks like I need to do some fine tuning . All good , I can do that .
 
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Old 17-01-2021, 08:09 AM
Rusty2
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Hello again !
Wow , what a frustrating hoby ! Can anyone suggest a pshychologist ?

So , I bought a hotech collimator , used it about 7 times now to get used to how it all works although It would appear from the screenshot and halo below that I haven't perfected the process , all good , I'll get there .
I also bought a moonlight dual focuser and finally (with mirror lock) the scope now stays in focus .

There does seem to be an improvement in sharpness while observing but judging from the screenshot below but I'll have to go through the process again .

I took the gear out last night and setup on the driveway , got polar alignment , messed around with backward tracking issues , swapped guide scopes , made sure I was in focus and started snapping away hopeing that I'd fixed all the previous issues .

Looking at the photos this morning I noticed elongated stars , varying in elogation between shots . I looked around for answers and found that it's probably me packing up the gear and the concrete slab is moving the telescope , I'll be more careful next time .

I did notice something odd in the screenshot below and is present in all light frames . Yes it's blown out to highlight 2 anomalies at the bottom of the screenshot . I looked on Stellarium and expected to see nebula or a galaxy in the vicinity but there isn't anything there . The anomalies aren't in the dark frames that I also took last night .

Any ideas ?
I appreciate this website and the help you've given me and many others , thank you !
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Old 18-01-2021, 11:02 AM
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mental4astro (Alexander)
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Did you confirm your collimation by star testing? You cannot rely solely on a laser to colliamate a scope by. With an SCT or Mak you don't need a laser either. Star Testing will do it all, and without the hassle of needing to figure out what is needed with a laser.

Relying only on a laser is saying "near enough is good enough", regardless of what scope you are using.

If your collimation processes, laser and star testing, still doesn't give you the required test image then there may be other issues at play with the corrector. As I mentioned in my last post, unless the corrector is totally PASSIVE in its cell, it will result in a distorted image when trying to collimate that just won't be eliminated.

I saw this very thing in an SCT over the weekend. The scope's owner is new to SCT's. He had removed the corrector to clean it inside and out. He had an engineer mate help him with this. When I looked through his scope at high magnification the image of the doughnut was VERY distorted - wee thin line on one side and flared out like a comet on the opposite site of the doughnut. Just before I took to the collimation screws it occurred to me to ask about the corrector and check the retention ring screws. It is amazing how the corrector plate did not crack with the screws wound in so tight! John had to use both hands to torque the screws loose!!!

When all the screws were reset, this time to finger tight, we checked the image again. Two things to note here:
1) the new doughnut shape was nearly perfect, nothing like the highly skewed image we first saw
2) that it had nothing to do with the collimation in the first instance could be seen because the image of Rigel had not shifted in the FOV as would be the case with such a large change in the image.

All the scope needed to tweak its collimation was torqueing just one colli screw just a small amount, and BINGO! All good to go.

Your elongated stars suggest to me that you haven't done a star test. You will need to determine if the corrector plate is sitting passive first. Only then will you have the right conditions necessary for tweaking the collimation.

If your scope is still under warranty, you may want to have the store you bought it from to do this for you as you may otherwise void your warranty, or you may not be confident with doing this then the store you bought it from should be happy to check this.

Another two things to note about Hotech lasers:
1) They need to be tightly torqued into place. This is the only way to be certain that the laser is sitting square. The tight torque of the Hotech is important!!!!
2) Hotech lasers must be checked for collimation too. I have one and after 5 years of using it I found out mine was NOT properly collimated.

Regardless of what brand of laser you are using, it is wise to double check that it is collimated. Very easy to do. Some lasers are also easy to adjust, others more complicated (like the Hotech), and others only factory accessible. Dropping a laser is NEVER a good idea, and if has been dropped you MUST check it to see that it is still true. If you buy one second-hand, check it. Don't assume it is tuned.

Alex.
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Old 18-01-2021, 04:23 PM
Rusty2
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Hi Alexander !
No , I didn't confirm it by start testing . I wasn't aware that I had to do that , thank you !

I did try star testing the scope before buying the laser and found it very difficult to see what was going at the back of the scope while tinkering at the front of the scope . I guess I could use a monitor facing forward so I can watch what's happening with the donut while I adjust the screws .

I was very careful with the corrector plate as you mentioned here previously . I had the corrector plate gently packed into postion with slivers of business cards and the locking screws tightened just enough so that they were not over tight .

The telescope is very sensitive to any movement and the elongated stars were of different lengths in different shots which to me meant that it had been moving slighly more in some shots than others . I thought it may have been a slight wind but I did notice previously that walking on the concrete driveway was effecting the shots .You may be right though , I do remember being very still while watching some of the shots come through so yes I'll be doing a star test .

I was actually very fussy about setting up the hotec laser in my last session and all indicators said that it was very close . The only thing I didn't do was double check the laser dots on the mirror in the eyepiece before I packed up the gear . Yes I did read somewhere about the hotech lasers being out but the author suggested that a new laser should be okay but I will check it anyway , thanks for your helpful suggestions : )
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Old 18-01-2021, 07:00 PM
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mental4astro (Alexander)
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Rusty,

Remember this, many, most if not all professional observatories use lasers to collimate their instruments, but the final proof and tweaking is always done by start testing. It is the ultimate proof and validation. It is something that we can take from the Big Boys of astro and apply directly ourselves as amateurs with our scopes

Here's one tip to help collimating your SCT:
* Look at the secondary mirror, and with three little tabs of masking tape put one tab on the edge of the OTA to mark the corresponding location of each screw.
* Next put another three little tabs of masking tape down on the outer edge of the primary mirror cell to match the tabs on the top of the OTA.
* When looking through the high power eyepiece (it MUST be a high power eyepiece for collimation purposes), figure out a tab you would like to shift and then you can easily translate this to the secondary's colli screws AND keep track of which colli screw you just adjusted.

Alex.
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Old 19-01-2021, 04:05 PM
Rusty2
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Thanks Alex , I think I'm going to check the actual laser first , double check the collimation of the scope with the laser and then do the star test with the tabs .
I appreciate the information Alex !
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Old 19-01-2021, 05:10 PM
glend (Glen)
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Check out this new tool from Teleskop-Express:

https://www.teleskop-express.de/shop...elescopes.html

I bought one to collimate my Classical Cassegrain and it is wonderful.
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Old 21-01-2021, 03:53 PM
Rusty2
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Thanks Glen !
I'm pretty sure my laser is okay (brand new) and if there's a problem then it's more like user error but thanks for the link : )
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