Thoughts on backfocus/collimation - Forums

Topic starter

Nov 18, 2022 · Edited: Nov 18, 2022

Hi All,

I recently got a mono camera and after a whole 1.5 year of epsilon 130D ~~pain and suffering~~ ownership decided to finally tackle some of my image train/camera tilt (apparently it gets a lot easier to do when you actually mark which side of the sensor corresponds to which corner in your image).

I've dialed down most of the tilt (according to hocus focus and the remaining tilt is under 10 microns - hard to address with 1 mil copper shims) and the collimation is as good as I can get it aside from the secondary mark, that's not quite centered and contributing to the uneven light falloff.

Example frame (L, 30 seconds):

https://cdn.discordapp.com/attachments/247352166078087178/1043026671365804072/2022-11-17_19-49-56_0000_2.00_146_2430.jpg

Any thoughts as to if I should attempt to adjust the backfocus? It's at the standard 56.2 + 1mm for filter. This is an APS-C Sensor.

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ManuManu

Nov 18, 2022

Hello Shinpah,
I have this scope since 2 years and did "suffer" for good collim but I love the results of the Epsilon with my ASI6200. I advise you to:
1-Before collimation, rotate the focuser to the desired angle for shooting and don't move it anymore (better: mark the orientation) because the focuser himself is not perfect…
2- Carefully see with the chechire that the shape of the secondary is perfectly rounded and centered to the focuser before playing with primary mirror.
Good luck
Keep in touch!
CS

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Psion

Nov 18, 2022 · Edited: Nov 18, 2022

Hi,

According to the picture you show, the collimation is not good. I've tried collimating 130D with various collimating tools, unfortunately, it was never quite right on the FullFrame. I finally bought an OCAL collimator and found that this is the only way to properly collimate high speed systems.

https://www.youtube.com/watchv=BdLcaiVZB_4&t=3s&ab_channel=OCALWORLD

Good luck
Josef

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Shinpah

Topic starter

Nov 18, 2022

Hi,

According to the picture you show, the collimation is not good. I've tried collimating 130D with various collimating tools, unfortunately, it was never quite right on the FullFrame. I finally bought an OCAL collimator and found that this is the only way to properly collimate high speed systems.

https://www.youtube.com/watchv=BdLcaiVZB_4&t=3s&ab_channel=OCALWORLD

Good luck
Josef

I've thought about doing tool assisted collimation. But almost every image I've seen with the 130D has a similar boxy, rhombus shaped/astigmatic field at the edges and I'm more concerned about spending money toward managing tilt at this stage of the game. Do you have any single exposure examples of what an improved field would look like?

Well Written

Thierry Boufflet

Nov 18, 2022

Hello Shinpah,

I also have a E130D, I love this instrument, but after tons of tuning without real sucess, I decided to change the focuser to a Moonlight one. And I must say I'm really happy with it. It's a serious business to install it, but the first test results look neat.
https://www.flickr.com/photos/191503816@N08/52501436076/in/dateposted/

Psion

Nov 18, 2022

I've thought about doing tool assisted collimation. But almost every image I've seen with the 130D has a similar boxy, rhombus shaped/astigmatic field at the edges and I'm more concerned about spending money toward managing tilt at this stage of the game. Do you have any single exposure examples of what an improved field would look like?

I have also the Moonlite focuser. You can see these pictures:

https://www.astrobin.com/2xnbw2/
https://www.astrobin.com/lrifdy/

Josef

Shinpah

Topic starter

Nov 18, 2022

Hello Shinpah,

I also have a E130D, I love this instrument, but after tons of tuning without real sucess, I decided to change the focuser to a Moonlight one. And I must say I'm really happy with it. It's a serious business to install it, but the first test results look neat.
https://www.flickr.com/photos/191503816@N08/52501436076/in/dateposted/

Yeah, I've been running a moonlite for the last 9 months, haven't noticed any difference from the stock focuser other than the fact that the stock focuser dropped lots of little metal shavings off the CC threads down the tube and the moonlite has serious light leaks from the drawtube.

Your image is a bit more rounded in the corners, although processing/seeing can influence those a fair amount.

I'll probably just recollimate/actually center the secondary and check the primary for pinching.

andrea tasselli

Frequent contributor

Nov 18, 2022

It doesn't look like an issue with tilt but rather with collimation. And I don't think spacing is going to be helping a lot in this.

Shinpah

Topic starter

Nov 19, 2022 · Edited: Nov 19, 2022

I'd actually be pretty happy if this were a collimation issue - it would mean I couldn't reliably improve upon it.

EDIT: There was tilt though, hocus focus was reporting something like 160~ microns from the top right to bottom left. Before shimming I had seagulls in one side and donuts on the other. It was still reporting about 15-20 microns across the top left/bottom right, but that's trickier to address.

andrea tasselli

Frequent contributor

Nov 19, 2022

I'd actually be pretty happy if this were a collimation issue - it would mean I couldn't reliably improve upon it.

EDIT: There was tilt though, hocus focus was reporting something like 160~ microns from the top right to bottom left. Before shimming I had seagulls in one side and donuts on the other. It was still reporting about 15-20 microns across the top left/bottom right, but that's trickier to address.

I assume the image you posted is after the tilt correction. Given that the focus depth tolerance is 24 um for a f/3.3 system I don't see how Hocus Focus can detect that with any precision lower than that value.

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Dark Matters Astrophotography

Nov 19, 2022

Hi,

According to the picture you show, the collimation is not good. I've tried collimating 130D with various collimating tools, unfortunately, it was never quite right on the FullFrame. I finally bought an OCAL collimator and found that this is the only way to properly collimate high speed systems.

https://www.youtube.com/watchv=BdLcaiVZB_4&t=3s&ab_channel=OCALWORLD

Good luck
Josef

This should not be used with the Epsilon.

OP - Use the Tak tools w/ crosshair and the markings they place on the mirrors to collimate the scope. This can be completed 100% indoors and in the daytime without the use of stars.

Shinpah

Topic starter

Nov 19, 2022

Hi,

According to the picture you show, the collimation is not good. I've tried collimating 130D with various collimating tools, unfortunately, it was never quite right on the FullFrame. I finally bought an OCAL collimator and found that this is the only way to properly collimate high speed systems.

https://www.youtube.com/watchv=BdLcaiVZB_4&t=3s&ab_channel=OCALWORLD

Good luck
Josef

This should not be used with the Epsilon.

OP - Use the Tak tools w/ crosshair and the markings they place on the mirrors to collimate the scope. This can be completed 100% indoors and in the daytime without the use of stars.

Been doing this the whole time, collimation typically looks like

Primary: https://cdn.discordapp.com/attachments/822159945436233743/1043372686727970946/Primary.PNG

Secondary: https://cdn.discordapp.com/attachments/822159945436233743/1043372625373708348/image.png

The red line is where I think I should adjust the secondary, the green blot is my not centered secondary.

Before tilt adjustment: https://cdn.discordapp.com/attachments/822159945436233743/1043375324920021062/image.png

After: https://cdn.discordapp.com/attachments/822159945436233743/1043374372162900078/image.png

The funky left side is probably residual tilt or the off secondary, hard to tell.

andrea tasselli:
I'd actually be pretty happy if this were a collimation issue - it would mean I couldn't reliably improve upon it.

EDIT: There was tilt though, hocus focus was reporting something like 160~ microns from the top right to bottom left. Before shimming I had seagulls in one side and donuts on the other. It was still reporting about 15-20 microns across the top left/bottom right, but that's trickier to address.

I assume the image you posted is after the tilt correction. Given that the focus depth tolerance is 24 um for a f/3.3 system I don't see how Hocus Focus can detect that with any precision lower than that value.

It's after - I think it's interpolating tilt using corner hfr, it's probably not good enough for extremely small amounts of tilt, but I was working with a 100 microns+

Dark Matters Astrophotography

Nov 19, 2022

Hi,

According to the picture you show, the collimation is not good. I've tried collimating 130D with various collimating tools, unfortunately, it was never quite right on the FullFrame. I finally bought an OCAL collimator and found that this is the only way to properly collimate high speed systems.

https://www.youtube.com/watchv=BdLcaiVZB_4&t=3s&ab_channel=OCALWORLD

Good luck
Josef

This should not be used with the Epsilon.

OP - Use the Tak tools w/ crosshair and the markings they place on the mirrors to collimate the scope. This can be completed 100% indoors and in the daytime without the use of stars.

Been doing this the whole time, collimation typically looks like

Primary: https://cdn.discordapp.com/attachments/822159945436233743/1043372686727970946/Primary.PNG

Secondary: https://cdn.discordapp.com/attachments/822159945436233743/1043372625373708348/image.png

The red line is where I think I should adjust the secondary, the green blot is my not centered secondary.

Before tilt adjustment: https://cdn.discordapp.com/attachments/822159945436233743/1043375324920021062/image.png

After: https://cdn.discordapp.com/attachments/822159945436233743/1043374372162900078/image.png

The funky left side is probably residual tilt or the off secondary, hard to tell.
andrea tasselli:
I'd actually be pretty happy if this were a collimation issue - it would mean I couldn't reliably improve upon it.

EDIT: There was tilt though, hocus focus was reporting something like 160~ microns from the top right to bottom left. Before shimming I had seagulls in one side and donuts on the other. It was still reporting about 15-20 microns across the top left/bottom right, but that's trickier to address.

I assume the image you posted is after the tilt correction. Given that the focus depth tolerance is 24 um for a f/3.3 system I don't see how Hocus Focus can detect that with any precision lower than that value.

It's after - I think it's interpolating tilt using corner hfr, it's probably not good enough for extremely small amounts of tilt, but I was working with a 100 microns+

There is error in both the primary and the secondary that needs to be resolved. The crosshair is also not centered. So, this is further off than you think it is.

Here is what you need:

1. Secondary blot centered.
2. Primary circle centered and the annulus on all sides is concentric.
3. Crosshair centered with 1 and 2 true.

None of these three are true, so I really do not care how you have been "doing it this whole time" it is wrong. I am not trying to me mean or rude, I am just here to help dispel the myths and misconceptions people have about collimating this scope. It should be easy and everyone makes it much harder than it needs to be.

The first frame, I would need to see the actual sub on, but I see the effects of coma and astigmatism throughout the entire frame. Regardless of that I do not see the center of the frame showing signs of proper collimation.

The second frame, does not have enough visible stars to make any comments on, other than it seems slightly improved over the original frame. Nonetheless, I still do not see anything that would lead me to conclude that the collimation is good.

If you truly want help, please post full subs and I am happy to look them over.

Also, I have posted a guide here: How To Guide for Collimating the E160ED - AstroBin which is about the E160ED, but applies to all of the Epsilon scopes. Please review this if you can, as it is very useful.

Thanks,
Bill

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Dark Matters Astrophotography

Nov 19, 2022

The Epsilon should produce gorgeous pinpoint stars, like this:

Shinpah

Topic starter

Nov 19, 2022

I suppose a 4 x 4 grid isn't useful to show the center.

I'll do some tests over the weekend to see if the secondary blot actually matters or not.

For the reflection centering, which direction do you think is required? Maybe too far on the bottom right?

Dark Matters Astrophotography

Nov 19, 2022

I suppose a 4 x 4 grid isn't useful to show the center.

I'll do some tests over the weekend to see if the secondary blot actually matters or not.

For the reflection centering, which direction do you think is required? Maybe too far on the bottom right?

You dont need to test if the blot matters. I already did this, over a year ago. The answer to that idea is yes.

For the second question, please read my lengthy post about this and ensure the outcome meets the image showed. If it does, you are fine, if it doesn't you arent fine. This isnt a F7 system, friend, you have to get it perfect.

Shinpah

Topic starter

Nov 19, 2022

"I think" is now "yes", got it!

I'll test regardless.

Psion

Nov 19, 2022

This should not be used with the Epsilon.

OP - Use the Tak tools w/ crosshair and the markings they place on the mirrors to collimate the scope. This can be completed 100% indoors and in the daytime without the use of stars.

*** For collimation, I used both the original tool with the wire cross and the Takahashi collimating eyepiece and in both cases, the collimation was wrong on FullFrame, it was only suitable for APS-C. The OCAL collimation camera revealed that the eyepiece could not be fully collimated when collimated with optical-mechanical tools. I am now satisfied, and it is up to each person which way they choose. ***

Paul Wilson

Nov 19, 2022

I was strongly considering an alternative scope for my RASA 8, I have had so many problems with unfixable tilt on it. It is currently on "vacation" at the Celestron factory in LA. Perhaps when it comes back, whenever that is, it will be better. But I don't know.

So I thought that the Epsilon 180, same mirror size, from a "Premium" manufacturer might be the way to go. Lately I have looked at users results and comments about these scopes. It hasn't been pretty.

Every collimation video I have found, and a lot of comments on Cloudy Nights have described getting this scope dialed in as a "nightmare" or some such. Comments like, "it's a real bear", or, "it's far and away the toughest Newt to collimate", or "many hours of frustration" are common.

Almost every thread I have found, such as this one, finds guys frustrated and wondering what they have done wrong. They have bad corner stars, they can't get the collimation right, they have tilt.

I swear you could paste these reactions over what people say about the RASA (but the RASA has fewer collimation needs but is similarly difficult to collimate).

Please tell me again why I should buy one of these at 3 times the price of the RASA and still have the same problems? If I am going to have the same problems, I might as well get a much cheaper Newt. Or just wait and cross my fingers on the RASA. Where is the big payoff? Plus, what, I have to replace and upgrade the focuser, and the scope doesn't come with retaining rings, and nothing short of laser collimation will suffice?

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andrea tasselli

Frequent contributor

Nov 19, 2022

Paul Wilson:
Please tell me again why I should buy one of these at 3 times the price of the RASA and still have the same problems? If I am going to have the same problems, I might as well get a much cheaper Newt. Or just wait and cross my fingers on the RASA. Where is the big payoff? Plus, what, I have to replace and upgrade the focuser, and the scope doesn't come with retaining rings, and nothing short of laser collimation will suffice?

I'd never understand why one would have to put up with all this hassle, be it a RASA or an Epsilon, when much easier solutions do exist and at much lower price but there you go...

Arun H

Nov 19, 2022

I used to own a 130D. The scope could never give me circular stars no matter what Takahashi or I did. I later found out my issue was with a knurled mirror edge. Fortunately, Tak took it back for a refund. I have since moved to a TS optics f/4 Newtonian which is much more forgiving and fun. These Epsilons are capable of great results - but the buyer needs to know that they need to put a ton of work and money to make them work. Just read some of the threads here. You need to go into this purchase with your eyes open.

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Chris White- Overcast Observatory

Nov 19, 2022

Arun H:
I used to own a 130D. The scope could never give me circular stars no matter what Takahashi or I did. I later found out my issue was with a knurled mirror edge. Fortunately, Tak took it back for a refund. I have since moved to a TS optics f/4 Newtonian which is much more forgiving and fun. These Epsilons are capable of great results - but the buyer needs to know that they need to put a ton of work and money to make them work. Just read some of the threads here. You need to go into this purchase with your eyes open.

I wonder why some people have no trouble at all with epsilon and some people have endless struggle. I have found that the scope itself has been easy as cake for collimation. I replaced the stock focuser, collimated it in about 10 minutes and havent had to touch it in months. It was trickier and more tedious to dial in backspacing and tilt but once done, again, I havent touched it. I get nice stars to the corners of IMX455. I used to own a TS ONTC newt that I also found quite user friendly. I used catseye collimation tools for that. So going into the epsilon I wasnt new to newts (it's my 4th) but still, I found that with the tak tools it was insanely easy to collimate. In my experience collimation ease or difficulty is directly related to the quality of the primary cell, and secondary assembly. The epsilon build quality is superb.

I suspect that people who have problems are just playing a guessing game without trying to understand how a newtonian works... or in your case has a frustrating experience due to QC issues from the manufacturer.

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Paul Wilson

Nov 19, 2022

Well if people who have not previously had newts, or have had them, but say that the epsilons are “in a difficulty league” of their own, why are they struggling.
I would need perfectly clear and detailed instructions on each step and need to know exactly what “tools” are needed and see how to use them.
People say there are no instructions for collimation of the epsilon that they can understand.
I know from trying to correct a rasa, if you are twisting a corrector screw you don’t know if you have the right screw, which way to go, or when to twist a different one.
Many of us are not stupid but we are not engineers either.

Shinpah

Topic starter

Nov 19, 2022

The epsilon manually is pretty clear on the procedure, figuring out the secondary multi-axis movements can be unintuitive to figure out and physically making the fine adjustments I find non-trivial.

Primary is easy.

That said, mine doesn't hold collimation and never has.

Anyone know what the smaller hex screw for the 130D primary is? I just stripped one of mine.

Chris White- Overcast Observatory

Nov 19, 2022

Paul Wilson:
Well if people who have not previously had newts, or have had them, but say that the epsilons are “in a difficulty league” of their own, why are they struggling.
I would need perfectly clear and detailed instructions on each step and need to know exactly what “tools” are needed and see how to use them.
People say there are no instructions for collimation of the epsilon that they can understand.
I know from trying to correct a rasa, if you are twisting a corrector screw you don’t know if you have the right screw, which way to go, or when to twist a different one.
Many of us are not stupid but we are not engineers either.

I'm not an engineer at all. Went to school for economics and ended up in agriculture (maple syrup). I have zero credentials when it comes to this stuff other than a lot of hands on experience with machines and tools. That said, I found the included instructions that Tak provided to be pretty straightforward. They definitely could have done a better job but for the record they do walk you through it. I had never used the Tak tools before and the instructions were sufficient.

I do try to understand the "why" with these kinds of things. I've read Vic Menard's book on newtonian telescopes and have spent hundreds of hours testing, researching, reading and discussing these topics with people, so I'm sure that has helped me to be successful with it. I'm sure that I have been annoying at times with emails to Roland Christen and John Hayes and others trying to understand optics. I will admit, I only know enough now to get me in trouble...

My main point is, before twisting a screw... take a moment to understand what that screw is doing.

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