Jump to content

1912116577_Solarchallenge2022banner.jpg.913a200cf105ddeae9f37765cb1d0c73.jpg

What's wrong with my telescope?


Recommended Posts

On 29/03/2022 at 17:09, paulastro said:

It's the reason I use refractors. 

I use those too 😉 I love my Esprit 150 for nebulae, but for smaller objects I want a longer FL (which the SCT provides)

Link to comment
Share on other sites

Ok, so I have fitted the coma corrector, but as it slides into the 2" focuser tube I am not quite sure of the correct position. Currently I have it inserted all the way in, but I am still seeing significant coma. How do I find the correct position for the corrector?

image.thumb.png.1cb26293911f905768ab3a825f41c54f.png 

Link to comment
Share on other sites

38 minutes ago, StuartT said:

How do I find the correct position for the corrector?

According to Starizona, sensor needs to be positioned at 90.3mm away from the start of the back thread of corrector.

It has very strict tolerances for larger sensors, so you need to get your spacing right.

ASI2600 has 17.5mm of back focus and therefore you need total of 72.8mm extension between corrector and camera.

I would personally look to get 70mm extension and then add 2.8mm of spacers using distancing rings.

Alternative is to get something like this to dial in correct distance (like use 40mm regular extension and add this variable one to dial in exact distance):

https://www.firstlightoptics.com/adapters/baader-varilock-46-lockable-t-2-extension-tube.html

 

 

  • Like 1
Link to comment
Share on other sites

45 minutes ago, StuartT said:

Ok, so I have fitted the coma corrector, but as it slides into the 2" focuser tube I am not quite sure of the correct position. Currently I have it inserted all the way in, but I am still seeing significant coma. How do I find the correct position for the corrector?

image.thumb.png.1cb26293911f905768ab3a825f41c54f.png 

It’s not a coma corrector you need, it’s a field flattener…coma you get in Newtonians, SCT suffer from a very small flat filed, so a flattener is needed. I have an 8” SCT and use a really good flattener and it’s pretty darn good, never heard of anyone using a coma corrector on an SCT….but maybe I am totally wrong….

Edited by Stuart1971
Link to comment
Share on other sites

2 minutes ago, Stuart1971 said:

It’s not a coma corrector you need, it’s a field flattener…coma you get in Newtonians, SCT suffer from a very small flat filed, so a flattener is needed. I have an 8” SCT and use a really good flattener and it’s pretty darn good, never heard of anyone using a coma corrector on an SCT….but maybe I am totally wrong….

It is Starizona SCT corrector - and it does correct for coma. Coma is present in SCT design.

You are right - you can't use coma corrector designed for newtonian on SCT - but this is proper corrector for SCT scope.

  • Like 1
Link to comment
Share on other sites

Just now, vlaiv said:

It is Starizona SCT corrector - and it does correct for coma. Coma is present in SCT design.

You are right - you can't use coma corrector designed for newtonian on SCT - but this is proper corrector for SCT scope.

Ah, ok…but looking at those stars, they look like mine when I don’t use the flattener…👍🏼

  • Like 1
Link to comment
Share on other sites

3 minutes ago, Stuart1971 said:

Ah, ok…but looking at those stars, they look like mine when I don’t use the flattener…👍🏼

According to this post:

9 hours ago, StuartT said:

Yes, you are correct @vlaiv
My Starizona SCT corrector has now arrived and I located a 90mm extension tube, so I hope to try this out soon. I'll let you know how I get on!

Problem might be only in extension tube used. 90mm one is too long - total distance needs to be 90.3mm and camera eats up 17.5mm of it.

There also might be a bit of collimation issue or tilt - not sure which one. Collimation is easier to check and fix.

Stars seem to be more affected with coma on the left side of frame than on right:

image.png.a91e71c56ed91d32e29b20e2be597577.png

image.png.8526e74fce6e6cc9f08f8089ebfad821.png

also, bottom right corner seems rather fine compared to others:

image.png.ea47480554cd6c4c1e07527b11831fd9.png

  • Thanks 1
Link to comment
Share on other sites

18 minutes ago, vlaiv said:

According to Starizona, sensor needs to be positioned at 90.3mm away from the start of the back thread of corrector.

It has very strict tolerances for larger sensors, so you need to get your spacing right.

ASI2600 has 17.5mm of back focus and therefore you need total of 72.8mm extension between corrector and camera.

I would personally look to get 70mm extension and then add 2.8mm of spacers using distancing rings.

Alternative is to get something like this to dial in correct distance (like use 40mm regular extension and add this variable one to dial in exact distance):

https://www.firstlightoptics.com/adapters/baader-varilock-46-lockable-t-2-extension-tube.html

 

 

Ah! I was forgetting the 17.5mm in the camera. I added 90mm of extension tube. So maybe this is too much and that's why I still have coma.

What about the amount of insertion of the coma corrector into the focuser? Is that also critical?

SCT and coma corrector to 2600MC.jpg

Link to comment
Share on other sites

12 minutes ago, vlaiv said:

There also might be a bit of collimation issue or tilt - not sure which one. Collimation is easier to check and fix.

I think there may be tilt. The problem with this corrector is that it simply slides into the focuser, so not a very good mechanical arrangement for photography. Because the little screw which tightens the focuser around it only tightens on one side. This is presumably causing tilt. I prefer optical devices connecting via screw threads!

  • Like 1
Link to comment
Share on other sites

2 minutes ago, StuartT said:

What about the amount of insertion of the coma corrector into the focuser? Is that also critical?

Not sure, as I don't have SCT experience.

I do know that with moving mirror focusing - position of focus plane plays a part because it alters separation between primary and secondary mirror. Not sure how it affects correction (it does introduce small amount of spherical aberration) as it changes focal length of the system somewhat.

Given that you have regular focuser fitted at the back - that might not be an issue.

You probably need not worry about it at the moment. Focus on getting proper distance for now.

  • Thanks 1
Link to comment
Share on other sites

Posted (edited)
37 minutes ago, vlaiv said:

Stars seem to be more affected with coma on the left side of frame than on right:

@vlaiv - I agree that the distortion is much worse on the left of the image, so I think tilt is a problem. I am sure this is due to the insertion of the corrector into the focuser barrel and the weight of the ZWO camera. Is there a solution to this?

Edited by StuartT
Link to comment
Share on other sites

10 hours ago, StuartT said:

@vlaiv - I agree that the distortion is much worse on the left of the image, so I think tilt is a problem. I am sure this is due to the insertion of the corrector into the focuser barrel and the weight of the ZWO camera. Is there a solution to this?

I switched to threaded connection in my setups to avoid that, but here is what I would try if you can't manage threaded connection:

Get one of those distancing rings for T2, metal one, not plastic (it needs to be rigid) and put it between corrector and extension. This will create sort of stop / edge so when you are inserting corrector - it will only go inside up to this distancing ring as it is larger than focuser opening.

Something like this:

image.png.183b8f0b744e1722e9d01bb6d1a801b3.png

Then, when inserting corrector - turn scope so it's pointing up and slide it up the focuser tube until it reaches this distancer and then hold it flush against focuser tube. This ring is supposed to prevent tilt as it is perpendicular to optical axis. While holding assembly against focuser - tighten focuser compression ring.

Do this without camera attached so that you have less weight. After everything is firmly secured in focuser - screw on camera at the end of extension tube. You will probably need to mark things to get proper orientation (just insert with camera attached first and mark way corrector is oriented so you orient it the same when camera is not screwed on - when you screw on camera later it will be at wanted orientation).

Hope this helps.

  • Thanks 1
Link to comment
Share on other sites

I don’t think those coma correctors were designed to be used with crayfords on the back of SCT’s, I think the idea was they were to be put straight in the back of the SCT and then an extension and then the camera, this would make it rock solid, and you would just use the SCT native focuser….

Now you could do this but you would need to work out the focus point with the crayfors first, and see whether the length of the focuser would be too much, with the drawtube in the focussed position, ideally you want it to be less that 90mm so you can add an extension if needed….👍🏼

  • Like 1
Link to comment
Share on other sites

Posted (edited)
2 hours ago, vlaiv said:

I switched to threaded connection in my setups to avoid that, but here is what I would try if you can't manage threaded connection:

Get one of those distancing rings for T2, metal one, not plastic (it needs to be rigid) and put it between corrector and extension. This will create sort of stop / edge so when you are inserting corrector - it will only go inside up to this distancing ring as it is larger than focuser opening.

Something like this:

image.png.183b8f0b744e1722e9d01bb6d1a801b3.png

Then, when inserting corrector - turn scope so it's pointing up and slide it up the focuser tube until it reaches this distancer and then hold it flush against focuser tube. This ring is supposed to prevent tilt as it is perpendicular to optical axis. While holding assembly against focuser - tighten focuser compression ring.

Do this without camera attached so that you have less weight. After everything is firmly secured in focuser - screw on camera at the end of extension tube. You will probably need to mark things to get proper orientation (just insert with camera attached first and mark way corrector is oriented so you orient it the same when camera is not screwed on - when you screw on camera later it will be at wanted orientation).

Hope this helps.

Thanks @vlaiv this looks like a workable solution. 

2 hours ago, Stuart1971 said:

I don’t think those coma correctors were designed to be used with crayfords on the back of SCT’s, I think the idea was they were to be put straight in the back of the SCT and then an extension and then the camera, this would make it rock solid, and you would just use the SCT native focuser….

Now you could do this but you would need to work out the focus point with the crayfors first, and see whether the length of the focuser would be too much, with the drawtube in the focussed position, ideally you want it to be less that 90mm so you can add an extension if needed….👍🏼

Well I didn't email Starizona before I bought it specifically on this question (I also sent them a photo of my scope) and they assured me that it would work perfectly with this type of focuser. I'm not sure how it could be fitted directly to a scope back anyway, since the front end is simply a 2" barrel with an internal female thread to take a filter (see photo). Problem with using the native SCT focuser is that it's not very practical for astrophotography (hence why I bought the Steeltrack and fitted a Pegasus motor focuser to it).

Coma corrector 2.jpg

Edited by StuartT
  • Like 1
Link to comment
Share on other sites

50 minutes ago, StuartT said:

Thanks @vlaiv this looks like a workable solution. 

Well I didn't email Starizona before I bought it specifically on this question (I also sent them a photo of my scope) and they assured me that it would work perfectly with this type of focuser. I'm not sure how it could be fitted directly to a scope back anyway, since the front end is simply a 2" barrel with an internal female thread to take a filter (see photo). Problem with using the native SCT focuser is that it's not very practical for astrophotography (hence why I bought the Steeltrack and fitted a Pegasus motor focuser to it).

Coma corrector 2.jpg

I thought it would just drop inside the focus tube on the back of the scope, and then the lip on the corrector would stop it going in too far…and then any attatchment would hold it in place….but again, I maybe completely wrong…

Link to comment
Share on other sites

2 hours ago, Stuart1971 said:

I thought it would just drop inside the focus tube on the back of the scope, and then the lip on the corrector would stop it going in too far…and then any attatchment would hold it in place….but again, I maybe completely wrong…

Well you would think so, yes. But they have designed this as a plain cylinder. No lip. So there is no way to know how far it should go in

 

  • Like 1
Link to comment
Share on other sites

1 minute ago, StuartT said:

Well you would think so, yes. But they have designed this as a plain cylinder. No lip. So there is no way to know how far it should go in

There might be another solution - threaded one.

Baader steel track has threaded connection, if I'm not mistaken

Probably M58, but I'll check. Actually - it is S58 (what ever that means), but there is this adapter:

https://www.teleskop-express.de/shop/product_info.php/info/p8220_Baader-M48-Adapter-for-2--Diamond-Steeltrack.html

Now you need M48 female thread that is a bit longer and you need T2/M48 conversion ring

https://www.teleskop-express.de/shop/product_info.php/info/p13710_TS-Optics-Adaptor-with-female-T2-thread-and-male-M48x0-75-filterthread.html

You also need long M48 female thread extender.

https://www.teleskop-express.de/shop/product_info.php/info/p4515_TS-Optics-Adapter---continuous-M48-female-thread--as-with-2--filters-.html

and simple T2 extension

Here is diagram of how it would all fit:

image.png.4af18286925476355d778ed0f45272ac.png

  • Thanks 1
Link to comment
Share on other sites

14 minutes ago, vlaiv said:

There might be another solution - threaded one.

Baader steel track has threaded connection, if I'm not mistaken

Probably M58, but I'll check. Actually - it is S58 (what ever that means), but there is this adapter:

https://www.teleskop-express.de/shop/product_info.php/info/p8220_Baader-M48-Adapter-for-2--Diamond-Steeltrack.html

Now you need M48 female thread that is a bit longer and you need T2/M48 conversion ring

https://www.teleskop-express.de/shop/product_info.php/info/p13710_TS-Optics-Adaptor-with-female-T2-thread-and-male-M48x0-75-filterthread.html

You also need long M48 female thread extender.

https://www.teleskop-express.de/shop/product_info.php/info/p4515_TS-Optics-Adapter---continuous-M48-female-thread--as-with-2--filters-.html

and simple T2 extension

Here is diagram of how it would all fit:

image.png.4af18286925476355d778ed0f45272ac.png

Thanks for this, but I don't think I quite understand what you are suggesting. Is the left hand side of your diagram the telescope end and the right hand side the camera end? So the steeltrack to M48 ring is helping to lock the T2 extension inside it somehow?
(so are you saying not to follow your idea above with the Baader distancing ring?)

Link to comment
Share on other sites

8 minutes ago, StuartT said:

(so are you saying not to follow your idea above with the Baader distancing ring?)

No, I would try distancing ring first as it is far easier and cheaper solution.

I wrote above as it occurred to me that I have similar setup on one of my scopes and that you can use combination of adapters to the same effect.

I have 2.5" focuser and on one of scopes I have x0.67 reducer (RC8" scope and CCD47 reducer). CCD47 has M48 thread and 2.5" focuser has M63 thread.

It is far better if I sink reducer in focuser tube then to simply screw it on with adapter as focal reducers move focus position forward.

In order to make it all work, I'm using this adapter:

https://www.teleskop-express.de/shop/product_info.php/info/p9781_TS-Optics-360--Rotation---Thread-Adapter---M63-to-M68--M54-and-2-.html

https://www.teleskop-express.de/shop/product_info.php/info/p5144_TS-Optics-Adapter-from-M68-and-M63-to-M48---Riccardi-Connection-Adapter.html

First is actually rotator and second is the adapter.

It has larger M68/M63 thread that is used to attach it to focuser and it has M48 on both sides

image.png.f52b10ce73901ded68f203f72ac1872c.png

I use this telescope facing M48 to screw in reducer.

That gave me idea - in order to sink your corrector in focuser and still have threaded connection - you need something that will have M48 and T2 female threads facing telescope side and M48 o T2 thread facing camera side.

There is simple way to do that and it looks a bit like this:

image.png.b862bd92e27ac8a904bd9fce76ed03a9.png

This is 1.25" filter adapter for T2 thread and it is used like this:

image.png.53c20fdf77d84db0ba29bc1afa837074.png

You screw it into T2 thread and then you screw filter into it.

I linked above M48 / T2 version that you would need.

I'm going to give you just diagram of that part being used:

image.png.99bf31af7e84370a14f6e482e57c451f.png

You can screw things together and it will hold - in the same way this holds:

image.png.543d39ae960effe73de4d3c848269c0a.png

Nut in above image is tube with female M48 thread and two nuts that push against each other is M48 Steeltrack adapter and M48 / T2 adapter - once you screw them together - they will act as one unit.

You then have T2 thread facing telescope side (you'll need T2 extension to bring it out of assembly - and you have female M48 facing the camera.

In any case - this just shows that with proper combination of adapters - you can both sink your corrector inside focuser and have threaded connection - but it is somewhat complex and expensive as you need to purchase 4-5 separate bits and cost adds up.

Go with distancing ring as a lip that will lean against focuser to keep everything straight and if you really want threaded solution - well, above is one version of it.

Ideally - you would want baader steeltrack adapter that has T2 thread facing the scope and camera side - but I don't think they make such thing.

  • Thanks 1
Link to comment
Share on other sites

7 minutes ago, vlaiv said:

No, I would try distancing ring first as it is far easier and cheaper solution.

 

Thanks. I still don't think I really understand tour more elaborate solution (I would probably need to see it to really understand). But I will try the distancing ring anyway.

I guess my main concern is that even with a distancing ring, I am still replying on a really poor mechanical system in the focuser tube (1 screw and a brass ring which doesn't exactly spread the gripping force evenly around 260 degrees).

Maybe I should just buy a properly corrected scope with a real flat field... <sigh>

Anyway @vlaiv you have been super helpful as usual. Thank you

Link to comment
Share on other sites

4 minutes ago, StuartT said:

I guess my main concern is that even with a distancing ring, I am still replying on a really poor mechanical system in the focuser tube (1 screw and a brass ring which doesn't exactly spread the gripping force evenly around 260 degrees).

Maybe alternative would be to get Baader twist / click lock thing for their focuser?

That is supposed to keep things well clamped and centered on optical axis?

https://www.firstlightoptics.com/adapters/baader-2-s58-clicklock-clamp-for-diamond-steeltrack.html

image.png.9dc4692e8d79831d6042b95f2e6c269f.png

  • Thanks 1
Link to comment
Share on other sites

Ok, so I now have the Starizona coma corrector fitted and with the correct backspacing to the sensor using the hardware @vlaiv helpfully recommended (a 40mm tube and then the adjustable Baader gizmo to dial in another 32.8mm.

Problem is, the defocused star image now looks even worse than the one I posted at the start of this thread!!

 I'm almost at the point of throwing away this telescope!! (I'd need to sell the Starizona corrector too). SO frustrating 😭

Link to comment
Share on other sites

1 hour ago, StuartT said:

Ok, so I now have the Starizona coma corrector fitted and with the correct backspacing to the sensor using the hardware @vlaiv helpfully recommended (a 40mm tube and then the adjustable Baader gizmo to dial in another 32.8mm.

Problem is, the defocused star image now looks even worse than the one I posted at the start of this thread!!

 I'm almost at the point of throwing away this telescope!! (I'd need to sell the Starizona corrector too). SO frustrating 😭

Now collimate the scope to see if there will be any difference.

  • Thanks 1
Link to comment
Share on other sites

Posted (edited)
On 08/04/2022 at 13:50, vlaiv said:

Now collimate the scope to see if there will be any difference.

thanks. I have never had a good experience trying to collimate. I always make things much worse. It's supposed to be simple, but it really isn't. Maybe I should stick to refractors. Only problem is, I need the long focal length for galaxies and planets. 

I would happily pay someone to fix this telescope!

Edited by StuartT
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.