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Hotech 2" SCA field flattener

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#1 Der_Pit

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Posted 16 June 2019 - 06:23 AM

Maybe a question especially to Stelios, as he recommends it quite often - but likely more people here are using it, and can give some info there.

 

I'm currently using the TSFLAT2 from Teleskop-Service for my Astro-Physics Stowaway (old).

While it does a good job (I think) in flattening the field, it does introduce chromatism, I do have to adjust focus when changing filters.  This is a bit nasty, because I use an OAG, so my guide star gets defocused when doing that :(

 

So I started looking at alternatives, most notably the Hotech one (I don't want a reducer, only a flattener).  So the main question would be whether you have such issues with the Hotech when using it on an initially color-clean APO refractor.

 

The other question came up while looking at its specs:  It has a 'self-centering' adapter that compresses rubber rings to clamp itself into the tube of the focuser.  Is that indeed stable enough to support a full image gear (EFW, OAG, ASI1600 main cam and ASI290MMmini guide cam)?

 



#2 Apothegary

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Posted 16 June 2019 - 07:13 AM

I have the Hotech flattener with the exact gear attached that you describe (ZWO brand). The field is flat edge to edge and the self-centering rings hold everything secure. I had the same concern initially, but should be no worries.


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#3 Der_Pit

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Posted 16 June 2019 - 09:47 AM

The field is flat edge to edge and the self-centering rings hold everything secure.

That is with the ES127 from your signature?  That is also a high-quality APO, is it? Do you notice focus differences for your various filters?  That would be my main concern, as the TSFLAT2 also delivers a flat field..



#4 Apothegary

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Posted 16 June 2019 - 10:02 AM

Yes, I use the ES127 with that flattener. Images seem on-par with my Stellarvue, so I would say the quality is adequate. I looked at my at-focus postions for the ZWO LRGB for my last time out and they all seemed parfocal. Focus numbers should be accurate, as they were through the autofocus routine within SGP and stars were sharp.



#5 Stelios

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Posted 16 June 2019 - 01:19 PM

Maybe a question especially to Stelios, as he recommends it quite often - but likely more people here are using it, and can give some info there.

 

I'm currently using the TSFLAT2 from Teleskop-Service for my Astro-Physics Stowaway (old).

While it does a good job (I think) in flattening the field, it does introduce chromatism, I do have to adjust focus when changing filters.  This is a bit nasty, because I use an OAG, so my guide star gets defocused when doing that frown.gif

 

So I started looking at alternatives, most notably the Hotech one (I don't want a reducer, only a flattener).  So the main question would be whether you have such issues with the Hotech when using it on an initially color-clean APO refractor.

 

The other question came up while looking at its specs:  It has a 'self-centering' adapter that compresses rubber rings to clamp itself into the tube of the focuser.  Is that indeed stable enough to support a full image gear (EFW, OAG, ASI1600 main cam and ASI290MMmini guide cam)?

I don't quite understand the "introduce chromatism" part. How can you have chromatism with a mono filter? 

Are you saying that you need to refocus *more* when you are using the flattener than without it, when changing filters? I have always refocused between filters (although the Astronomiks are nearly parfocal) and have never had any issues with the OAG. The difference in focus is very small. 

 

In any case, the Hotech doesn't exaggerate the difference in any noticeable way. I have not tried to measure it (as it never occurred to me as an issue) but I also image with an OAG on all my scopes (F/L's 600mm, 805mm, 1421mm) and not had any issues with refocusing on different filters. (I have used the Hotech only on the 600mm and 805mm scopes). 



#6 Der_Pit

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Posted 16 June 2019 - 01:44 PM

I don't quite understand the "introduce chromatism" part. How can you have chromatism with a mono filter? 

Are you saying that you need to refocus *more* when you are using the flattener than without it, when changing filters? I have always refocused between filters (although the Astronomiks are nearly parfocal) and have never had any issues with the OAG. The difference in focus is very small.


Yes, that's what I refer to. Without the flattener, filters are basically parfocal.  But the flattener, as it is a refractive element like the refractor itself, can be of better or lower quality, and of course also change focus differently, depending on wavelength.  And that is what I suspect the TSFLAT2 does.

 

Edit: And it's not the filters themselves.  The images of the L filter are substantially less sharp than the RGB ones...
 

In any case, the Hotech doesn't exaggerate the difference in any noticeable way. I have not tried to measure it (as it never occurred to me as an issue) but I also image with an OAG on all my scopes (F/L's 600mm, 805mm, 1421mm) and not had any issues with refocusing on different filters. (I have used the Hotech only on the 600mm and 805mm scopes).

 Well, my AP is an f/4.9, so it's focal depth isn't too large.  IIRC, I had a step size around 4μ in my focusser, and focus is some 100 units off for R/B relative to G, so 0.4mm.  That's enough to clearly distort the stars for the guide cam.  Not enough that PHD2 cannot guide, but my impression is guiding is better with the focused stars...
 

Yes, I use the ES127 with that flattener. Images seem on-par with my Stellarvue, so I would say the quality is adequate. I looked at my at-focus postions for the ZWO LRGB for my last time out and they all seemed parfocal. Focus numbers should be accurate, as they were through the autofocus routine within SGP and stars were sharp.

So that makes 2 positive votes for the Hotechs quality already.  I'm tempted to give it a try....


Edited by Der_Pit, 16 June 2019 - 01:47 PM.



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