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Takahashi FSQ-85EDX Reducer question

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#1 lakerunr

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Posted 24 May 2022 - 12:16 PM

I've done quite a bit of investigation on this subject, and it appears that the QE 0.73x reducer is not a very good choice. I have one, and can verify this by experience. Anything over a 1" sensor (ASI183) is a waste. Terrible star elongation in all corners, as well as the bright star vignetting. I've tried +/- up to 4mm of the manufacturer's metal back spec. (72.2mm) in 0.2mm increments. It improves some at closer spacing (even though pattern suggests it is already too close at 72.2). On larger format sensors (ASI294, 2600) there is obvious tilt, also. I have tried loosening the grub screws in collar, aligning and re-tightening carefully. Tilt changes a bit, but never goes away. I am ready to give up on it. I have none of these issues on APS-C and full frame with the 1.01x flattener. Clear nights are sparse here, can't afford to waste them experimenting with this.

 

My question concerns the other Tak reducer that is supposedly usable on the Baby Q, the CR 0.73 reducer. Has anyone used this with the Baby Q with good results?

 

I have looked at a few images on Astrobin that were done with the CR, but they are mostly highly cropped. Those that are not, seem to show some star distortion in corners and the bright star vignetting (light-house beam effect).

 

These Tak reducers are expensive, and I don't want to shell out more big bucks for the CR if it is not foolproof. I can live without wide field, no problem with mosaics, but the theoretic reduced imaging time would be nice.

 

I'm guessing the problem is with the configuration of the Petzval Baby Q. I have a TSA-120 triplet which I use with the Tak TOA 35 0.7x reducer (also expensive) with none of these issues, round stars in corners out to APS-C (have not tried full frame). I also read that some people use the QE on the FSQ-106, but no plans to get one of these.

 

Thanks!



#2 Jose_Salcedo

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Posted 25 May 2022 - 12:57 PM

I have a TSA-120 with the CA-35 0.7x reducer and with the 2600 camera all stars are perfectly round. I just got a FSQ-85 EDP (without reducer) with the 1.01x flattener and will be testing it in the next few days. From what I studied on this issue, I expect perfectly round stars with the flattener but not with the reducer, and so I do not plan to buy this reducer.


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#3 James Peirce

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Posted 25 May 2022 - 01:26 PM

As the sensor gets larger and the focal ratio gets faster, smaller inconsistencies in backfocus and tilt are going to be greatly amplified. This setup moves into a range where build inconsistency between astronomy cameras, tilt in spacers (e.g. the ZWO spacers that came with my 2600 were bad enough that they introduced noticeable tilt when used with demanding optics), sag, etc. start to have a noticeable impact on the field rendered while imaging. Assuming no collimation issue (the FSQ- telescopes are sensitive to abuse, such as a heavy hit in shipping) care will need to be taken to make sure none of those other issues exist, backfocus is very accurate, and a tilt adapter may need to be used for the best performance.



#4 lakerunr

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Posted 25 May 2022 - 03:35 PM

I have a TSA-120 with the CA-35 0.7x reducer and with the 2600 camera all stars are perfectly round. I just got a FSQ-85 EDP (without reducer) with the 1.01x flattener and will be testing it in the next few days. From what I studied on this issue, I expect perfectly round stars with the flattener but not with the reducer, and so I do not plan to buy this reducer.

I think you have a good plan. 

 

As the sensor gets larger and the focal ratio gets faster, smaller inconsistencies in backfocus and tilt are going to be greatly amplified. This setup moves into a range where build inconsistency between astronomy cameras, tilt in spacers (e.g. the ZWO spacers that came with my 2600 were bad enough that they introduced noticeable tilt when used with demanding optics), sag, etc. start to have a noticeable impact on the field rendered while imaging. Assuming no collimation issue (the FSQ- telescopes are sensitive to abuse, such as a heavy hit in shipping) care will need to be taken to make sure none of those other issues exist, backfocus is very accurate, and a tilt adapter may need to be used for the best performance.

Yeah, funny I get little to no tilt with the 1.01x flattener with same spacers and camera, just not the Tak reducer and spacer... however it is f/3.8 vs f/5.3.

 

I think I'll just return the reducer. If I really need wide field and speed I guess I'll just use my Samyang 135 (round stars across APS-C at f/2.8, no tilt). Can't afford to waste the limited clear skies I've had here lately, messing around with this.

 

I'm guessing the CR reducer would be similar with maybe less tilt, since it has no grub screws/adjustable collar. 


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#5 James Peirce

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Posted 26 May 2022 - 04:38 PM

Yeah, funny I get little to no tilt with the 1.01x flattener with same spacers and camera, just not the Tak reducer and spacer... however it is f/3.8 vs f/5.3.

 

I think I'll just return the reducer. If I really need wide field and speed I guess I'll just use my Samyang 135 (round stars across APS-C at f/2.8, no tilt). Can't afford to waste the limited clear skies I've had here lately, messing around with this.

 

I'm guessing the CR reducer would be similar with maybe less tilt, since it has no grub screws/adjustable collar. 

That's not too unusual. The demands of the reducer are going to be much greater on any concerns (e.g. bad spacers, small deviation in sensor alignment). Is the Samyang 135 mounted to an interchangeable lens camera? That helps a lot with a lot of this fuss as the sensor in those cameras tends to be mounted far more reliably, and the camera mount removes a lot of room for concern. They can become a bigger fuss to mount to an astronomy camera, but even there, it's just a matter of having quality adapters and very precise backspacing.



#6 jamesNewmanF125

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Posted 26 May 2022 - 05:00 PM

I have one, and can verify this by experience. Anything over a 1" sensor (ASI183) is a waste. Terrible star elongation in all corners, as well as the bright star vignetting. I've tried +/- up to 4mm of the manufacturer's metal back spec. (72.2mm) in 0.2mm increments. It improves some at closer spacing (even though pattern suggests it is already too close at 72.2). On larger format sensors (ASI294, 2600) there is obvious tilt, also. I have tried loosening the grub screws in collar, aligning and re-tightening carefully. Tilt changes a bit, but never goes away. I am ready to give up on it. 

 

 

That's weird.  Sara Wager would use a reducer at times with a KAF8300, and I never saw any heavy issues in her images.  

 

Do have any examples of the deformation?  I'll be using the QE reducer soon with an APS-H sensor, so I'd like to see if I have similar results as yours (should I have issues).



#7 mmalik

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Posted 26 May 2022 - 06:05 PM

My question concerns the other Tak reducer that is supposedly usable on the Baby Q, the CR 0.73 reducer. Has anyone used this with the Baby Q with good results?

Why reducer? Why not a flattener...? Regards


Edited by mmalik, 26 May 2022 - 06:07 PM.


#8 lakerunr

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Posted 27 May 2022 - 01:16 PM

That's weird.  Sara Wager would use a reducer at times with a KAF8300, and I never saw any heavy issues in her images.  

 

Do have any examples of the deformation?  I'll be using the QE reducer soon with an APS-H sensor, so I'd like to see if I have similar results as yours (should I have issues).

I have put files in Google drive at link below. I can't attach FITS, or XISF, or TIFF here. JPEG is useless at reduce resolution.

There is a raw FITS frame, same frame calibrated, and a stack of 18 frames (Lum.XISF). Hope it helps.

 

https://drive.google...DnH?usp=sharing

 

These images were captured with ASI2600MM, gain 100 offset 50, bin 1x1.

 

BTW, I believe Wager's KAF8300s are 4/3 (QSI683) and 1" (QSI690) sensor configurations and I don't know how much cropping is done.


Edited by lakerunr, 27 May 2022 - 02:16 PM.


#9 lakerunr

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Posted 27 May 2022 - 01:19 PM

Why reducer? Why not a flattener...? Regards

I already have the x1.01 flattener. Was going to user reducer (supposed to give flat image to 44mm dia) for wider FOV and shorter imaging times (0.73x focal length).



#10 James Peirce

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Posted 27 May 2022 - 03:56 PM

I have put files in Google drive at link below. I can't attach FITS, or XISF, or TIFF here. JPEG is useless at reduce resolution.

There is a raw FITS frame, same frame calibrated, and a stack of 18 frames (Lum.XISF). Hope it helps.

 

https://drive.google...DnH?usp=sharing

 

These images were captured with ASI2600MM, gain 100 offset 50, bin 1x1.

 

BTW, I believe Wager's KAF8300s are 4/3 (QSI683) and 1" (QSI690) sensor configurations and I don't know how much cropping is done.

There is tilt. The camera sensor, adapters (likely none of Takahashi’s), or something else. The camera could be rotated independently of the telescope to see if the star deformation remains constant or follows the camera’s orientation as a useful diagnostic step (if it rotates, the tilt is associated with the optical train after the rotator, if it doesn't, the issue prior). It can help to slew to a different direction, such as overhead, to see if sag is an issue. Any adapters which aren’t good should be replaced (micrometer is helpful) and if there is tilt on the sensor it should be adjusted accordingly.

 

Widefield, with this fast focal ratio, and a relatively large sensor, all of those factors are able to play a very significant role in how flat the focal plane is, even if the telescope is doing an excellent job.



#11 jamesNewmanF125

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Posted 27 May 2022 - 04:59 PM

 

 

BTW, I believe Wager's KAF8300s are 4/3 (QSI683) and 1" (QSI690) sensor configurations and I don't know how much cropping is done.

KAF8300 is 22.5 diagonal.  Was just going off what you were saying that anything bigger than an ASI183 (tiny sensor, 15.9 diagonal) would show deformation.




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