Egg shape stars on top of refractor image

[Stefan87]

Hey everyone! I have recently received a second hand ES ed80 triplet refractor that I purchased in some classifieds. Tonight was my first light with it after months of cloud and I immediately found I had severely elongated stars at the top of the frame. I run it without a FF/FR as the bloke I purchased it off uses the same camera (Asi533mcpro) and he said it didn't need one due to the small chip size of the camera. 

This would lead me to assume he had not experienced the same problem as me. I have a long imaging train with a filter drawer and an OAG for guiding. I immediately thought this was flexing and took an exposure pointed at zenith only to find the same issue. My guiding was ok, sitting at 1" or 2". 

I'm concerned that I have stuffed the collimation by removing the lens to get some marks off the inside during some cloud induced boredom. I loosened some of the Alan bolts around the edge and pulled the entire assembly out. In hindsight I realise how stupid this was, but I was careful and I'm not sure if this is the issue or if I simply need a field flattener?

I've attached a fits file that I captured showing the elongation and a picture of my imaging train. I'm really hoping I haven't stuffed my collimation by removing the glass.

 

Thanks everyone

 

Stefan2022-02-09_21-45-05__-10.00_180.00s_0008.fits2022-02-09_21-45-05__-10.00_180.00s_0008.fits

 

Edited February 9, 2022 by Stefan87

[vlaiv]

I blame this part here:

These clamped connections can easily introduce tilt.

When assembling the setup - it is important that you hold firmly rest of the optical train against this clamping connection as you tighten compression ring screws (like "pushing in" motion with other hand).

If you don't do that - it will "sag" under its own weight and be clamped like that - with a tilt. That tilt then might stay as is when pointing to the zenith as it is held by clamps.

One way of checking if it is connection is to take two different exposures:

1. one as is

2. one where you rotate imaging setup by 180 degrees in that clamp

You should get image that is 180 degrees rotated - if star elongation rotated as well (stayed with the stars) - then there is tilt elsewhere in the train, but if star elongation "stayed with the frame" (like again in top part) - then it is this clamping connection.

[alacant]

Hi

You're gonna have to re-align the lens with the tube before you can diagnose tilt. Here is hands on with an es triplet...

So long as you haven't touched any of the lateral screws and only the three pairs of axial hex screws, you are fine, but you'll have to re-align the lens cell with the mechanical axis of the tube.

This is most easily done in a darkened room using a laser pointing centrally and axially into the telescope. Set the laser first by fixing it at the same height as the centre of the telescope on a table and projecting the beam onto a wall. The height must be the same at the table as at the wall. Mark the point on the wall  Now interpose the telescope aligned along the axis of the laser (you can do this by blowing cigarette smoke so you can see to align the tube with the beam). Adjust two of the three pairs of screws until the beam hits the same point on the wall. They are a simple push pull mechanism. Blu-Tak or WHY helps kee everything fixed relative to each other.

You can then work out how to minimise tilt in the rest of the setup having now eliminated the lens alignment.

HTH

Edited February 11, 2022 by alacant

[michael8554]

The middle and lower half of the image have good star shape.

Then from the middle up the shapes get worse.

Which I believe means it's not tilt, but perhaps misadjustment of some of those bolts.

Michael

[Stefan87]

6 hours ago, vlaiv said:

I blame this part here:

These clamped connections can easily introduce tilt.

When assembling the setup - it is important that you hold firmly rest of the optical train against this clamping connection as you tighten compression ring screws (like "pushing in" motion with other hand).

Good tip! I did not do this. I will try this and hope it Is something this simple! Thank you!

 

1 hour ago, alacant said:

 

So long as you haven't touched any of the lateral screws and only the three pairs of axial hex screws, you are fine, but you'll have to re-align the lens cell with the mechanical axis of the tube.

I'm relieved to hear that. So in future, I will be ok to remove the lens using those pairs of screws as pictured below? Can their adjustment affect how "flat" the lens will lie on the tube?

 

That sounds llik a complicated process. But I must try. Thank you for explaining it to me. Do you know of any resources or videos that could guide me through that to ensure I'm not stuffing it up? Thank you.

58 minutes ago, michael8554 said:

The middle and lower half of the image have good star shape.

Then from the middle up the shapes get worse.

Which I believe means it's not tilt, but perhaps misadjustment of some of those bolts.

Michael

Are you referring to the 3 pairs of bolts holding the lens in?  I attached a picture of one pair for clarification. Thank you!

 

[alacant]

10 minutes ago, Stefan87 said:

Can their adjustment affect how "flat" the lens will lie on the tube?

Yes! That's what they are for.

The do not simply hold the lens, they need to be adjusted so that the lens sits square to the tube. 

Your photo shows one such push-pull pair. You must loosen one and take up the slack with the other.

 

[Stefan87]

8 hours ago, alacant said:

Your photo shows one such push-pull pair. You must loosen one and take up the slack with the other.

 

I think you have have hit the nail on the head good sir. I have just run out an purchased a Cheshire eyepiece as per the video. Wasn't sure if I needed a short or long one so I went short, assuming the longer ones would be for larger scopes/newts. 

 

Thank you, I will let you know how I go.

[Stefan87]

On a different note and while I have you, I have taken my first set of flats on this scope and noticed a weird gradient. I am unsure if this is being caused by my oag prism or not. There doesn't appear to be any visible blockage in my light frames as shared in the OP. What do you guys think?

[alacant]

6 hours ago, Stefan87 said:

taken my first set of flats

Hi

Get the lens fixed first, then upload a flat frame showing all the frame. Anyway...

You have only shown part of the flat frame so we're guessing. That isn't the shadow of the prism; set it along the long edge of the camera frame. That always appears as a curve in the opposite direction if set as described. 

To set the prism, back it out fully then gradually push it centre frame a mm at a time. Each push, take a flat frame. Continue until you see the shadow of it. Then back off a mm or so. That's it.

Even easier, lose the OAG and use a lightweight guide telescope.

Cheers

Edited February 10, 2022 by alacant
translation

[alacant]

2 hours ago, Stefan87 said:

just run out an purchased a Cheshire eyepiece

Ah, OK but you didn't need one. The method I outlined uses the green one of these. Around €2.

Apologies. Should have made it clearer.

Edited February 10, 2022 by alacant

[vlaiv]

5 hours ago, Stefan87 said:

On a different note and while I have you, I have taken my first set of flats on this scope and noticed a weird gradient. I am unsure if this is being caused by my oag prism or not. There doesn't appear to be any visible blockage in my light frames as shared in the OP. What do you guys think?

To me that does look like prism shadow. Take a look at actual prism/sensor and see if it is in that position relative to sensor. If it is - that will be confirmation.

Make sure you move it slightly away from center of the sensor

[Stefan87]

3 hours ago, alacant said:

Ah, OK but you didn't need one. The method I outlined uses the green one of these. Around €2.

Apologies. Should have made it clearer.

Thats ok! The link didn't seem to work. Was that a laser pointer? Have you got another link? Thank you!

[Stefan87]

16 minutes ago, vlaiv said:

To me that does look like prism shadow. Take a look at actual prism/sensor and see if it is in that position relative to sensor. If it is - that will be confirmation.

Make sure you move it slightly away from center of the sensor

I shall give it a go. Thanks so much mate!

[johnturley]

21 hours ago, Stefan87 said:

Hey everyone! I have recently received a second hand ES ed80 triplet refractor that I purchased in some classifieds. Tonight was my first light with it after months of cloud and I immediately found I had severely elongated stars at the top of the frame. I run it without a FF/FR as the bloke I purchased it off uses the same camera (Asi533mcpro) and he said it didn't need one due to the small chip size of the camera. 

This would lead me to assume he had not experienced the same problem as me. I have a long imaging train with a filter drawer and an OAG for guiding. I immediately thought this was flexing and took an exposure pointed at zenith only to find the same issue. My guiding was ok, sitting at 1" or 2". 

I'm concerned that I have stuffed the collimation by removing the lens to get some marks off the inside during some cloud induced boredom. I loosened some of the Alan bolts around the edge and pulled the entire assembly out. In hindsight I realise how stupid this was, but I was careful and I'm not sure if this is the issue or if I simply need a field flattener?

I've attached a fits file that I captured showing the elongation and a picture of my imaging train. I'm really hoping I haven't stuffed my collimation by removing the glass.

 

Thanks everyone

 

Stefan2022-02-09_21-45-05__-10.00_180.00s_0008.fits2022-02-09_21-45-05__-10.00_180.00s_0008.fits

 

 

Explore Scientific do specifically state in their manuals NOT to remove the lens assembly from the tube, as it cannot accurately replaced other than by an expert.  

[alacant]

47 minutes ago, Stefan87 said:

another link

[Stefan87]

16 hours ago, alacant said:

Are you sure this laser is collimated? I am not sure if this will cause more problems than it solves? Did you fabricate a holder to insert it into the focuser?

[AstroNebulee]

I bought this one from RVO a few years ago but others are available on amazon and look same

 

https://www.rothervalleyoptics.co.uk/ovl-laser-collimator-1252.html

 

 

Edited February 11, 2022 by AstroNebulee

[alacant]

5 hours ago, Stefan87 said:

Did you fabricate a holder

There is no need. The laser is not inserted into the telescope. Neither does the laser need to be collimated. Please see the explanation here.

Edited February 11, 2022 by alacant

[Stefan87]

On 11/02/2022 at 16:24, AstroNebulee said:

I bought this one from RVO a few years ago but others are available on amazon and look same

 

https://www.rothervalleyoptics.co.uk/ovl-laser-collimator-1252.html

 

 

I'm in Australia, and the ones available here state they are only good for relfectors. They do appear that be exactly the same. Do you think something like this will work? 

SVBONY Red Laser Collimator 1.25'' Battery 7 Bright Levels Triple Cemented Lens Metal with 2" Adapter for Reflector Telescope Collimation https://www.amazon.com.au/dp/B00LX34ZH2/ref=cm_sw_r_apan_glt_i_WF66K5RWNZ64Q331Y7GE

 

[Stefan87]

On 11/02/2022 at 19:32, alacant said:

There is no need. The laser is not inserted into the telescope. Neither does the laser need to be collimated. Please see the explanation here.

Silly me. Thank you very much! I'll grab an el cheapo laser and  and try it out!

[Adam J]

On 09/02/2022 at 13:23, Stefan87 said:

Hey everyone! I have recently received a second hand ES ed80 triplet refractor that I purchased in some classifieds. Tonight was my first light with it after months of cloud and I immediately found I had severely elongated stars at the top of the frame. I run it without a FF/FR as the bloke I purchased it off uses the same camera (Asi533mcpro) and he said it didn't need one due to the small chip size of the camera. 

This would lead me to assume he had not experienced the same problem as me. I have a long imaging train with a filter drawer and an OAG for guiding. I immediately thought this was flexing and took an exposure pointed at zenith only to find the same issue. My guiding was ok, sitting at 1" or 2". 

I'm concerned that I have stuffed the collimation by removing the lens to get some marks off the inside during some cloud induced boredom. I loosened some of the Alan bolts around the edge and pulled the entire assembly out. In hindsight I realise how stupid this was, but I was careful and I'm not sure if this is the issue or if I simply need a field flattener?

I've attached a fits file that I captured showing the elongation and a picture of my imaging train. I'm really hoping I haven't stuffed my collimation by removing the glass.

 

Thanks everyone

 

Stefan2022-02-09_21-45-05__-10.00_180.00s_0008.fits2022-02-09_21-45-05__-10.00_180.00s_0008.fits

 

You would expect to need a corrector even for a 533 maybe something the size of a 178 / 485 you could get away without but not a 533.

However, you would still expect the stars to be eggy on all sides not just one, so you may also have a colimation issue as others suggest.

I find the best way to colimate a refractor is with a star test, looking for concentric diffration rings.

 

Adam

[AstroNebulee]

6 hours ago, Stefan87 said:

I'm in Australia, and the ones available here state they are only good for relfectors. They do appear that be exactly the same. Do you think something like this will work? 

SVBONY Red Laser Collimator 1.25'' Battery 7 Bright Levels Triple Cemented Lens Metal with 2" Adapter for Reflector Telescope Collimation https://www.amazon.com.au/dp/B00LX34ZH2/ref=cm_sw_r_apan_glt_i_WF66K5RWNZ64Q331Y7GE

 

Yes it's exactly the same as what i used on my 72ed, like Adam says above you should need a corrector field flattener really too as when the scope is collimated you may see elongated stars in all corners. Did the chap who sold it to you show you any full frame images he took with the scope? 

Edited February 15, 2022 by AstroNebulee

[PadrePeace]

On 09/02/2022 at 14:02, vlaiv said:

I blame this part here:

These clamped connections can easily introduce tilt.

When assembling the setup - it is important that you hold firmly rest of the optical train against this clamping connection as you tighten compression ring screws (like "pushing in" motion with other hand).

If you don't do that - it will "sag" under its own weight and be clamped like that - with a tilt. That tilt then might stay as is when pointing to the zenith as it is held by clamps.

One way of checking if it is connection is to take two different exposures:

1. one as is

2. one where you rotate imaging setup by 180 degrees in that clamp

You should get image that is 180 degrees rotated - if star elongation rotated as well (stayed with the stars) - then there is tilt elsewhere in the train, but if star elongation "stayed with the frame" (like again in top part) - then it is this clamping connection.

I’m with Vlaiv on this. There will be no point adjusting the lens collimation before you irradiate any likely joint issues at the camera end. Those thumb screw compression joints are notorious for allowing tilt especially if you build your rig with the scope horizontal.

I’ve had this myself and now mount the camera and associated gear into the focuser thumb screw holder with the telescope in a vertical position with the camera dropping in from the top. In this way you are using gravity to seat the joint in the vertical. You will also be better able to ‘feel’ when the two flat faces mate up before you tighten the thumb screws. I finally position the torch on my mobile phone level with the joint on the opposite side to my eye but just out of sight so as not to blind yourself to see if light is passing through the joint which indicates a gap. Do this around the whole joint to be certain the joint is fully closed. I hope that’s clear enough.

Clear skies

  

[Stefan87]

On 10/02/2022 at 04:57, alacant said:

This is most easily done in a darkened room using a laser pointing centrally and axially into the telescope. Set the laser first by fixing it at the same height as the centre of the telescope on a table and projecting the beam onto a wall. The height must be the same at the table as at the wall. Mark the point on the wall  Now interpose the telescope aligned along the axis of the laser (you can do this by blowing cigarette smoke so you can see to align the tube with the beam). Adjust two of the three pairs of screws until the beam hits the same point on the wall. They are a simple push pull mechanism. Blu-Tak or WHY helps kee everything fixed relative to each other.

You can then work out how to minimise tilt in the rest of the setup having now eliminated the lens alignment.

HTH

Sorry for the noobiness, but to clarify, this sounds like it is to align the lens cell, but can it be used for focuser alignment? Also, does the laser enter via the focuser or the lens? I'm so sorry, this is all very new to me. 

 

 

On 15/02/2022 at 11:30, Adam J said:

However, you would still expect the stars to be eggy on all sides not just one, so you may also have a colimation issue as others suggest.

I find the best way to colimate a refractor is with a star test, looking for concentric diffration rings.

 

Adam

I ended up purchasing a Cheshire eyepiece, and sure enough the circles were not concentric. This has been resolved now and I'm looking forward to my next cloud free night to verify if this has helped. So if there is an even field of egg shaped stars around the edges, a field flattener is required. Got it. $500 aud is a big dent in the wallet, but love makes you do crazy things right? 

 

 

On 15/02/2022 at 17:00, AstroNebulee said:

 Did the chap who sold it to you show you any full frame images he took with the scope? 

I didn't ask. He had a good reputation on Iceinspace, so I trusted that. I can be quite naive hahaha. I'm not too upset about the purchase, even with an alignment problem I am very pleased with the outcome. 

 

21 hours ago, PadrePeace said:

I’ve had this myself and now mount the camera and associated gear into the focuser thumb screw holder with the telescope in a vertical position with the camera dropping in from the top. In this way you are using gravity to seat the joint in the vertical. You will also be better able to ‘feel’ when the two flat faces mate up before you tighten the thumb screws. I finally position the torch on my mobile phone level with the joint on the opposite side to my eye but just out of sight so as not to blind yourself to see if light is passing through the joint which indicates a gap. Do this around the whole joint to be certain the joint is fully closed. I hope that’s clear enough.

Clear skies

Fantastic advice. I did manage to get another night and follow Vlavs advice to ensure the imaging train is correctly seated. The problem persisted unfortunately. But I love this method, it really eliminates this as a possibility. I will be sure to do this next time. Thank you for sharing it!


So as I alluded to before, using the Cheshire eyepiece I discovered that there was an alignment issue due to the circles not being concentric. I have solved this issue now and I am pleased with the alignment as per the Cheshire. I'm just waiting for another opportunity to test it, and from what I have been hearing on here I should be hoping for either perfect stars, or evenly egg shaped stars on each corner so a field flattener can correct it. Is this correct?

This leaves me with one more question. Focuser alignment. I don't have a collimated laser, and I read on some other forum posts that one way to get a rough idea of alignment is also using a Cheshire eyepiece, but observing the edge to see if it is even. Does anybody have any quick and easy ways to check focuser alignment without shelling for a collimated laser? Does using the Cheshire eyepiece to make sure the circles are concentric mean that I have also corrected for any focuser alignment issues?

Thanks for your help guys. I purchased a refractor thinking it would be an easy way to continue this hobby and ended up making it a bit more difficult for myself than it needed to be. I appreciate everybodies input! 

Here is the finished product of the first light with this scope, even with the egg shaped stars it is quite obvious that the optics of this OTA are fantastic.

Clear skies!

 

[alacant]

45 minutes ago, Stefan87 said:

does the laser enter via the focuser or the lens?

The laser enters via the lens. Have the lens about 50cm from a wall.

45 minutes ago, Stefan87 said:

but can it be used for focuser alignment?

No. But anyway, the focuser should be OK as it is; we've yet to see a misligned ES focuser. The main issue with the ES's 80 is the backfocus; the camera needs to be held so far from the tube. Why the tube couldn't be made a few cm longer, we've no idea. 

45 minutes ago, Stefan87 said:

ensure the imaging train is correctly seated. The problem persisted unfortunately.

You must first set the lens square to the tube. Only then can you assess tilt. 

Cheers

 

Edited February 17, 2022 by alacant