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Help setting up Backfocus on Epsilon-180 with ZWO OAG and EFW

Takahashi Epsilon-180ED ZWO OAG-L Equipment CMOS
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Deepan Vishal avatar
Deepan Vishal avatar
Hi,

I am looking to dial the right backfocus for my setup. I am using ZWO ASI2600MM on my epsilon with ZWO EFW and OAG-L. The entire setup comes to around 56.7.

Here’s the connection and respective spacing in mm.

Camera (12.5) -> EFW(20) -> OAG-L(17.5) -> M54 tilt plate (5) -> M54 male to M54 male adapter(1.7) 

Totally around 56.7mm

I am looking to reduce the distance in the male to male m54 adapter(1.7) and ZWO tilt plate (5)

I remember some small business/seller selling a ZWO Tilt plate to M54 male adapter. 

Can someone point me where to find this adapter? 
I know precise parts can make me one, is there anyone else making this adapter?

Folks using ZWO EFW and OAG setup, how are you arriving at the required backfocus?

Is there an OAG that’s thinner than ZWO OAG-L and compatible with ZWO EFW 2” EFW(the bolt on version?
Wayne Malkin avatar
Wayne Malkin avatar
Hi Deepan,

I am curious if you are seeing anything in your images that would lead you here. The ideal backfocus is 56.2mm, so you are only 0.5mm out. My understanding is that the optics of the E-180 tolerate additional backfocus much better than reduced backfocus, and the APS-C sensor size should be extra-tolerant. The 56.7 should be pretty good.

BTW my backfocus is 57.75, with the ASI2400 (full frame) and the 1.5x Takahashi extender, which puts my E-180 at about 750mm (f/4.2). This arrangement seems to be much more tolerant than the native f/2.8.

Wayne
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Deepan Vishal avatar
Deepan Vishal avatar
Wayne Malkin:
Hi Deepan,

I am curious if you are seeing anything in your images that would lead you here. The ideal backfocus is 56.2mm, so you are only 0.5mm out. My understanding is that the optics of the E-180 tolerate additional backfocus much better than reduced backfocus, and the APS-C sensor size should be extra-tolerant. The 56.7 should be pretty good.

BTW my backfocus is 57.75, with the ASI2400 (full frame) and the 1.5x Takahashi extender, which puts my E-180 at about 750mm (f/4.2). This arrangement seems to be much more tolerant than the native f/2.8.

Wayne

Hi Wayne, 

Thanks for the message. 

Are you guiding with your epsilon? With OAG or Guidescope?

In my case, the stars shapes and pattern in edge of the images indicate the sensor if far away from the corrector (>56.2mm). 
And it is not really bad and I can live with the blurXterminator. 

But my concern is, the stars in the OAG camera appears really bad and I suspect that this backfocus and tilt is causing bad guiding, despite having the mount well balanced and having accurate polar alignment. 

Before going after tilt, I am thinking of fixing the backfocus first. 

I evaluated the following options - 
1. Switching back to guide scope
2. Consider buying Pro Duo
3. Shaving some back focus. 

Guide scope is practical buy my image train & mount is well balanced and I don't want to disturb the position of the camera on image train. 2600MM pro duo looks not practical when it comes to narrowband imaging. I also have a slower telescope, which will suffer more than the Epsilon.

Considering these, I am left with shaving some back focus of the OAG and tilt plate. 

Looks like at this time to shave some distance I have couple of choices - 
1. Pegasus Indigo OAG
2. Precise Parts Adapter. 
3. Find that seller who advertised selling a custom adapter for ZWO OAG-L to M54 male.

Unless I find that custom adapter, I am inclined to go towards the Pegasus indigo OAG.
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Robert Sahari avatar
Robert Sahari avatar
I don't know why you need that M54 male to M54 male adapter. Does the Epsilons coma corrector come with a female M54 thread? That's awkward as hell.

EDIT: You could probably use a thread adapter like this one ->https://agenaastro.com/baader-m54x0-75-inverter-ring-m54-male-to-m54-male-thread-adapter-m54-inve-2458061.html

It wouldn't add any optical length, as the 8mm of threads copletely disappear, and you can add spacers to get the backfocus just right.
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Wayne Malkin avatar
Wayne Malkin avatar
Hi Deepan,

I have the AP Mach2GTO mount with absolute encoders and usually shoot unguided up to about 2 minute subs (1" per pixel). I also have the advantage of living under crummy skies 360 days of the year, so many sins go unnoticed due to seeing, or because I shoot at 1.5-2.5" per pixel (also due to seeing limitations). When I do guide, it is with a small guide scope, not OAG. So I am afraid I can't offer much relevant experience on that end.

My mount is outdoors and covered and maintains good polar alignment for a long time, but the imaging trains (3 of them) are indoors and carried out each night. The compromises I have made are oriented to getting the best results I can under urban skies with the least amount of pain (ie. anything more than 5 minutes setup, drifty collimation, etc.).

The Epsilon Extender 1.5x is my best accessory. 750mm matches well with the 2600 (~1") and 2400 (~1.5") sensors (and the 6200 if I go there some day), f/4.2 is still pretty fast, and collimation is more resilient.

Wayne
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Peter Knappert avatar
Peter Knappert avatar
Hi Deepan,

I used my Epsilon 180-ED for astrofotography  in two ways:

1. Guidescope with 205mm  focal length and a ASI290mm guidecam
2. With TS-OAG and a ASI290mm guidecam

I tested very long with the image train before I came to the final configuration 
My configuration with OAG is now the following:

QHY268m(12.5mm) + QHYUSFilterwheel(17.0)mm) + M54Spacer(5mm) + M54 QHYEOSBajonett(10mm) + TSOAG(10.7mm) + M48-M54-Adapter (Selfmade)(1.4mm) = 56,6mm.

The backfocus of my Epsilon is exact 56.2mm . Because I used astronomik filters with a thickness of 1mm I must add 1mm/3 = 0.3 mm to the Epsilon backfokus and came to 56.5mm.

Lights are corrected now perfect with pinpoint stars in APS-C and fullformat with my EOS 6da.

You can see the configuration on the added pictures



My other working on the Epslon is with  a small guidescope:
Here I connected the QHY268m with a WideMount EOS Adapter (TATKA00233). This Adapter has a thickness of 12.2mm and gives in my configuration a total backfocus distance
of 56.4 mm. The results are the same 

Peter

My Astrofoto Homepage
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Deepan Vishal avatar
Deepan Vishal avatar
Thank you so much everyone.

i went with pegasus indigo OAG and  successfully freed up about 8.5mm. Used blue fir ball adapters to arrive at exactly 56.5.

Here’s my image train 

zwo 2600mm (12.5mm) -> zwo EFW (20mm) -> Pegasus indigo OAG + M54 plate (14mm) -> m54 male to m54 male adapter (2mm) + 8mm spacer. 

Total 56.5 mm.

I haven’t looked at the stars yet. But I am glad I have enough room to tune backfocus now.
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