Reducer problem: increase or decrease distance?

[gorann]

I have attached a TS 3" 0,79 reducer (TSRED379) to my Esprit 150 and I am trying to get the right distance to the chip of my ASI071. TS gives a table but it stops at a FL >800 mm where the distance should be 55 mm. For shorter FLs the distances are longer (see attached table).

Since the Esprit 150 has a FL of 1050 mm, I first tried a slighty shorter distance: 54.3 mm. That gave me noticible (but not terrible) elongated stars in the four corners, drawn out in the direction of the corners (so tangental astigmatism) - first image below shows top right corner. I was then told by @wimvb that this showed that I should increase the distance so on Sunday night I tried 55 mm. The result was rather terrible tangental astigmantism - second image below shows top right corner (unfortunately I did not check the images carefully until next morning)

There is a 2013 thread on SGL saying that if a flattener gives tangental astigmatism the distance should be increased (like I Wim suggested and I did) but apparently it made my case worse:

 

And here is a very telling image from that thread:

But the advice in the SGL thread was given for a flattener. Could it be that with my reducer (rather flattener and reducer) I should instead shorten the distance when I get tangental astigmatism? Are flatteners and reducers different in this respect? Would be nice with some expert advice: maybe @vlaiv ?

Top right corner with 54.3 mm distance:

 

Top right corner with 55.0 mm distance

Edited October 29, 2019 by gorann

[david_taurus83]

Here's my findings. Is your 54mm spacing really that bad though..

https://stargazerslounge.com/topic/340444-flattener-spacing-does-it-work/?tab=comments#comment-3702062

[vlaiv]

Here is the best "expert" advice that I can offer:  Just do it!

I mean - try different spacing and select one that works the best.

I really don't know how it all works - never investigated what happens with field flatteners  / reducers - other than how to mount them and use them. My first suspicion was that maybe this corrector does not provide large enough corrected field as 071 sensor is large - but according to TS - it should provide 45mm diameter and that should offer plenty of correction for 071 even if corners at 45mm are not perfect (as it has diagonal of only 28.4mm).

Not even sure that one can specify FF/FR distance based solely on focal length. Focal length is related to field curvature / radius of curvature, but I think that lens design also plays a part in that - triplets having stronger field curvature than doublets for example (I might be wrong at this). For this reason I believe that TS table of distances is rough guideline and not general rule (maybe suitable for doublet line of their scopes, but deviates with triplets for example, or scope using different glass elements).

Best thing to do with any FF/FR is to experiment with distance unless you have FF/FR that is matched to particular scope - then distance specs should be precise.

[gorann]

1 hour ago, david_taurus83 said:

Here's my findings. Is your 54mm spacing really that bad though..

https://stargazerslounge.com/topic/340444-flattener-spacing-does-it-work/?tab=comments#comment-3702062

Interesting thread that shows I am not alone with my problem (and I kind of knew that). I was just too optimistic thinking I would not have to waste precious imaging time next time it clears on checking corner stars but I will obviously have to.....

I did email TS and ask for the correct distance for their reducer and my Esprit 150 (which they also sell) and their answer was that I need to try it out myself.

[gorann]

35 minutes ago, vlaiv said:

Here is the best "expert" advice that I can offer:  Just do it!

I mean - try different spacing and select one that works the best.

I really don't know how it all works - never investigated what happens with field flatteners  / reducers - other than how to mount them and use them. My first suspicion was that maybe this corrector does not provide large enough corrected field as 071 sensor is large - but according to TS - it should provide 45mm diameter and that should offer plenty of correction for 071 even if corners at 45mm are not perfect (as it has diagonal of only 28.4mm).

Not even sure that one can specify FF/FR distance based solely on focal length. Focal length is related to field curvature / radius of curvature, but I think that lens design also plays a part in that - triplets having stronger field curvature than doublets for example (I might be wrong at this). For this reason I believe that TS table of distances is rough guideline and not general rule (maybe suitable for doublet line of their scopes, but deviates with triplets for example, or scope using different glass elements).

Best thing to do with any FF/FR is to experiment with distance unless you have FF/FR that is matched to particular scope - then distance specs should be precise.

Thanks Vlad, if not even you know a shortcut without experimenting, then I will not feel too much of a fool experimenting! I will start ith 53 mm next time and see what that looks like.

Edited October 29, 2019 by gorann

[david_taurus83]

4 minutes ago, gorann said:

Interesting thread that shows I am not alone with my problem (and I kind of knew that). I was just too optimistic thinking I would not have to waste precious imaging time next time it clears on checking corner stars but I will obviously have to.....

I did email TS and ask for the correct distance for their reducer and my Esprit 150 (which they also sell) and their answer was that I need to try it out myself.

Tbh I've givin up with it now. I personally think there is too much stuff in between the flattener and camera. Adapters, spacers, filterwheel carousel with filters. If any of those aren't machines to tight tolerances then they are going to induce some sort of tilt. I have the same flattener for my 102. I've ordered a DSLR and it's getting stuck on there at 55/56mm and it is what it will be.

 

I'd be happy if my corners were all that good at your 54.3mm.

[Adam J]

I would try 55mm as from the stars you have it needs more spacing. 

[david_taurus83]

3 hours ago, Adam J said:

I would try 55mm as from the stars you have it needs more spacing. 

He's tried 55mm. Decreasing distance improves the corners.

[gorann]

4 hours ago, Adam J said:

I would try 55mm as from the stars you have it needs more spacing. 

 

47 minutes ago, david_taurus83 said:

He's tried 55mm. Decreasing distance improves the corners.

Yes, going from 54.3 to 55 mm made it worse. I start at 53 nest time, if it ever clears....

[newbie alert]

I had this problem, but found out my camera was playing up in the end, magnum on here kindly fixed it for me.. but as a crude way of finding out where the focus point was I removed everything down to the reducer, used a bright torch from the front a moved a box forward and backwards until I had a sharp focus point and measured that point to the focal reducer..in my case it was at 55mm..

[vlaiv]

3 minutes ago, newbie alert said:

I had this problem, but found out my camera was playing up in the end, magnum on here kindly fixed it for me.. but as a crude way of finding out where the focus point was I removed everything down to the reducer, used a bright torch from the front a moved a box forward and backwards until I had a sharp focus point and measured that point to the focal reducer..in my case it was at 55mm..

Not sure how this works?

You are saying that you flashed torch at the front of the scope with FF/FR mounted at the back (but no camera or eyepiece) and then you used projection screen to find focus position and hence expected sensor position in relation to FF/FR?

That is not good way to do it, because actual focal point of scope+FF/FR will depend on scope lens - FF/FR distance.

You can verify this by simply moving focuser in/out and distance between FF/FR and projection screen when in focus will change - it will not tell you anything about optimum distance for best correction.

[newbie alert]

7 minutes ago, vlaiv said:

Not sure how this works?

You are saying that you flashed torch at the front of the scope with FF/FR mounted at the back (but no camera or eyepiece) and then you used projection screen to find focus position and hence expected sensor position in relation to FF/FR?

That is not good way to do it, because actual focal point of scope+FF/FR will depend on scope lens - FF/FR distance.

You can verify this by simply moving focuser in/out and distance between FF/FR and projection screen when in focus will change - it will not tell you anything about optimum distance for best correction.

It was a crude method Vlaiv admittedly but surely it's the lense that brings the point of focus?  Focus was already set from a previous clear sky many weeks before!!

For me it verified what my spacing was.. and wasn't near where the previous owner said..10mm out

[vlaiv]

1 minute ago, newbie alert said:

It was a crude method Vlaiv admittedly but surely it's the lense that brings the point of focus?  Focus was already set from a previous clear sky many weeks before!!

For me it verified what my spacing was.. and wasn't near where the previous owner said..10mm out

With focal reducers distance between it and sensor determines factor of focal reduction, but also determines "in/out focus" travel to reach new focal point.

Take for example CCDT67 (AP focal reducer) - it has focal length of 305mm and is designed to operate on x0.67. When you place it like it is designed (101mm away from the sensor) it requires about 50mm of inward focuser travel - that is quite a bit of change between places of focal point with and without focal reducer.

Let's see how distance change of only 1mm between camera and focal reducer impacts focus position: If we place sensor at 100mm distance, focus point changes by:

optimum setting:

distance - (distance x fr_focal_length) / (fr_focal_length - distance) = 101 - (101 * 305) / (305-101) = 50mm

change of only one millimeter:

100 - (100 x 305) / (305 - 100) = 48.78

Focus point changed by 1.22mm

So there you go, even small change in focuser position will have same magnitude in change in FR/sensor distance.

If your focuser has been 2mm away from where it should be, you could have measured 54mm or 56mm as "optimum" spacing. Again it would be optimum spacing for focal reducer only at that configuration, but field flattening is sensitive to these small changes in distance so if we are talking about field flattening - only way to get that right is by examining actual point sources over surface of the sensor for different spacing.

You can do that and still not waste imaging time / clear sky at night by use of artificial star - just place it far enough. That way you can change the spacing and examine each corner by moving scope so that artificial star gets in each corner before taking exposure - you can check for tilt and collimation this way as well.

[newbie alert]

Yes my scope has only got a flattener so whether I just got lucky or not..

Stars look good in all corners..

[Adam J]

15 hours ago, gorann said:

 

Yes, going from 54.3 to 55 mm made it worse. I start at 53 nest time, if it ever clears....

I plotted the TS optics data on spacing vs focal length into excel and derived an equation to fit the last three values to the line:

y = -0.0103x + 63.699

Spacing = -0.0103 * 1050 + 63.699 = 52.9mm 

Of course there are other factors beyond just focal length of the scope that effect spherical aberration so that can only ever be a starting point. 

 

Adam. 

 

[ollypenrice]

18 hours ago, gorann said:

 

Yes, going from 54.3 to 55 mm made it worse. I start at 53 nest time, if it ever clears....

A little voice in my head whispers the suspicion that this is a remarkably small distance change to make a perceptible difference. I wonder if it really was the distance in play here or might it simply have been a side effect of assembly/disassembly?

Olly

[Adam J]

4 minutes ago, ollypenrice said:

A little voice in my head whispers the suspicion that this is a remarkably small distance change to make a perceptible difference.

Voices only you can hear whispering paranoid thoughts to you. Maybe you should see a doctor about that? 

Edited October 31, 2019 by Adam J

[ollypenrice]

1 minute ago, Adam J said:

Voices only you can hear whispering paranoid thoughts to you. Maybe you should see a doctor about that? 

No, I'm advocating good scientific practice! The fact of removal-refitment has has, potentially, introduced a new variable. Lots of little spacers, possibility of tilt, rotation, etc.

Olly

[Adam J]

2 minutes ago, ollypenrice said:

No, I'm advocating good scientific practice! The fact of removal-refitment has has, potentially, introduced a new variable. Lots of little spacers, possibility of tilt, rotation, etc.

Olly

I feel you missed my sarcasm

[ollypenrice]

28 minutes ago, Adam J said:

I feel you missed my sarcasm

Irony rather then sarcasm, I think? But maybe you missed mine...   We English drive the rest of the world mad with our unfathomable ironies. Why can't those darned English just say what they mean? 

Olly

[gorann]

8 hours ago, Adam J said:

I plotted the TS optics data on spacing vs focal length into excel and derived an equation to fit the last three values to the line:

y = -0.0103x + 63.699

Spacing = -0.0103 * 1050 + 63.699 = 52.9mm 

Of course there are other factors beyond just focal length of the scope that effect spherical aberration so that can only ever be a starting point. 

 

Adam. 

 

Sorry for getting late back into my thread that apparently got a life of its own😉. I assume we all are or have been fighting this spacing issue quite a few times. I have to say that the dedicated SW flatteners for my two  Esprits have worked perfectly at the distances given, but that TS reduser provides a challenge.

So, Adam, you are my hope in the astrodarkness (if it arrives - only clouds now), and I will try 52.9 mm, or as close to that as possible. By the way, I, and Wim @wimvb, and my mathematically talented wife, all tried to make a curve fit to that TS table and were all a bit uncertain about the result.

Edited October 31, 2019 by gorann

[JamesF]

On 29/10/2019 at 19:07, gorann said:

I did email TS and ask for the correct distance for their reducer and my Esprit 150 (which they also sell) and their answer was that I need to try it out myself.

LOL.  Helpful   I got a fairly similar answer when asking about my TSRED279 and Photoline 72.

James

[JamesF]

56 minutes ago, gorann said:

So, Adam, you are my hope in the astrodarkness (if it arrives - only clouds now), and I will try 52.9 mm, or as close to that as possible. By the way, I, and Wim @wimvb, and my mathematically talented wife, all tried to make a curve fit to that TS table and were all a bit uncertain about the result.

I used a spreadsheet to try to draw a graph from the data TS give for the TSRED72.  I'm not really convinced there are enough sufficiently-accurate datapoints though, and I think the result I have is probably as much down to luck as accurate measurement of the spacing.

James

[wimvb]

1 hour ago, JamesF said:

I used a spreadsheet to try to draw a graph from the data TS give for the TSRED72.  I'm not really convinced there are enough sufficiently-accurate datapoints though, and I think the result I have is probably as much down to luck as accurate measurement of the spacing.

James

Same here. If I subtract 54.5 mm from the TS table values, I can fit a straight line in a log-log lin-log diagram. This means that for infinite focal length, the optimum distance is 54.5 mm. That's close enough to 55 mm. "Infinite focal length" is anything larger than 900 mm in practice.

When ZWO do the calculations for the optimum distance for their filter wheels and cameras, they use a back focus of 56 mm, having added 1 mm for the added optical path increase due to the filters. But Göran has an osc, so that shouldn't apply. I'm out of ideas.

 

Edited November 1, 2019 by wimvb

[Adam J]

2 hours ago, gorann said:

Sorry for getting late back into my thread that apparently got a life of its own😉. I assume we all are or have been fighting this spacing issue quite a few times. I have to say that the dedicated SW flatteners for my two  Esprits have worked perfectly at the distances given, but that TS reduser provides a challenge.

So, Adam, you are my hope in the astrodarkness (if it arrives - only clouds now), and I will try 52.9 mm, or as close to that as possible. By the way, I, and Wim @wimvb, and my mathematically talented wife, all tried to make a curve fit to that TS table and were all a bit uncertain about the result.

I am an analyst by profession and a physicist by degree. I did not try to fit to the entire curveas looking at the data it appears that the last few points are tending towards linear anyway, the last point in the series (800mm) was an outlier for the model which otherwise had very low residuals so I removed it.