Extension tube for Skywatcher Esprit 100 & ZWO ASI 1600 MM cooled

[lrt75914]

Hey all,

I recently bought a Skywatcher Esprit 100 and I’m generally quite happy with the quality of the telescope. 

Since I’m located in Germany I tried to order a couple of extension tubes and an adapter from a German

retailer to fit my ZWO ASI 1600 MM Cooled camera to my new telescope. 

As it transpired, however, the German retailer was blissfully unaware that their Chinese manufacturer does not

seem to exercise any quality control whatsoever, making any of the ‚precisely machined‘ adapters absolutely 

useless.

 

I'm now thinking of ordering a custom machined extension tube because I had it with ordering dozens of

different adapters that never fit as well as they should. Before I do, however, I wanted to make sure that

I have the correct optical length for the costume adapter. According to Skywatcher the backfocus for 

The Skywatcher Esprit 100 Field Flattener measures 63mm from the M66x1 thread to the camera sensor.

I use the ZWO EFWMini filter wheel with an eff. optical length of 20mm and the 1600 MM camera with a

back focus distance of 6.5mm. That means that there are 63mm - 20mm - 6.5mm = 36.5mm left that I

need to bridge with the adapter. Furthermore, since I use a monochromatic sensor, I also need to account

for the effect that the filters have on the overall back focus distance. If I'm not mistaken this means that I need

to add 1/3rd of the filter thickness to the 36.5 mm. I'm currently using ZWO ASI's RGB-L Filter set which

are 1.9mm thick. I'm also using the Baader H-alpha 7nm CCD Narrowband-Filter which is 2mm thick. 

Thus, the effective optical length of the adapter should be 36.5mm + 0.66mm ≈ 37.17mm. In the future, 

however, I may want to buy a set of Astrodon filters which have a thickness of 3mm. This means that

the adapter should have a length of 37.5mm. My idea was to go with the 'future proof' option and order

the adapter with an overall length of 37.5mm. Does anyone here know if this would result in a noticeable

degradation of the image quality when using the ZWO ASI Filter Set - i.e. noticeable field curvature/aberrations

at the edge of the frame?

 

Furthermore, I was told to subtract 1mm from the optical length of the adapter to allow for fine tuning using

spacer rings. I'm not a big fan of these spacer rings so my second question is whether you guys reckon that

this is an advisable thing to do?

 

Cheers,

 

Patrick

[Uplooker]

I use one of these, makes things a lot easier to get the correct distance.

https://www.altairastro.com/m48-17-29mm-variable-spacer-set-3-part.html

[lrt75914]

I used one of those variable spacers before and wasn't a big fan of it. It wasn't very sturdy and seemed to introduce a slight tilt in the optical path. Thank you for your suggestion though :).

[Filroden]

Double check the back focus requirement of the flattener as Skywatcher printed the wrong figure in the Esprit 80 manual. I think they updated the PDF version online.

[John78]

So you should make sure you are under the backfocus distance with your tubes and then use shims, you can't get closer if you are too far away - so you want the 1/3 filter of the thinnest filter and then a little less again due to screwing things up tight and not being able to undo them.

My Chinese 10mm tube arrived today off the slow boat, seems close enough to me....

[RayD]

As noted by @Filrodencheck your distances.  I'm not sure what reducer/adaptor you have on the rear of the FF, but with the one that came with my Esprit 100 FF from Flo, it left me 55mm to make up, and certainly for me with my Atik 16200, which has a pretty big APS-H sensor, the distance was critical.

I would say it was sound advice to leave short and make up with delrins for a few reasons:

1. There is no guarantee that your sensor is spot on what is noted.  They can often be up to +- 0.5mm, so fine tuning is possible
2. They really do help things to come apart again if necessary and help prevent the items locking together
3. If you change filters in the future it gives you some movement

It took me a while to get mine right due to the incorrect noted dimensions (these are now right on the OTA information on FLO's site) but once you do it's a little cracker.

[lrt75914]

42 minutes ago, Filroden said:

Double check the back focus requirement of the flattener as Skywatcher printed the wrong figure in the Esprit 80 manual. I think they updated the PDF version online.

 

24 minutes ago, RayD said:

As noted by @Filrodencheck your distances.  I'm not sure what reducer/adaptor you have on the rear of the FF, but with the one that came with my Esprit 100 FF from Flo, it left me 55mm to make up, and certainly for me with my Atik 16200, which has a pretty big APS-H sensor, the distance was critical.

I would say it was sound advice to leave short and make up with delrins for a few reasons:

1. There is no guarantee that your sensor is spot on what is noted.  They can often be up to +- 0.5mm, so fine tuning is possible
2. They really do help things to come apart again if necessary and help prevent the items locking together
3. If you change filters in the future it gives you some movement

It took me a while to get mine right due to the incorrect noted dimensions (these are now right on the OTA information on FLO's site) but once you do it's a little cracker.

Thank you for the advice. I did check all the manuals that are available to me and so far they've all said the same thing. I don't now if the design of the flattener changed but the one that I've got has a M66x1 thread on it's backside which can be stepped down to a M48x0,75 thread with a preinstalled 8mm extension tube. Therefore the flattener has 55mm of back focus with and 63mm of back focus without the preinstalled extension tube. At least that's the information I've been supplied with. I will, however, check with my telescope supplier to see if he can confirm these measurements. 

 

39 minutes ago, John78 said:

So you should make sure you are under the backfocus distance with your tubes and then use shims, you can't get closer if you are too far away - so you want the 1/3 filter of the thinnest filter and then a little less again due to screwing things up tight and not being able to undo them.

My Chinese 10mm tube arrived today off the slow boat, seems close enough to me....

Lucky you

[RayD]

1 minute ago, lrt75914 said:

Thank you for the advice. I did check all the manuals that are available to me and so far they've all said the same thing. I don't now if the design of the flattener changed but the one that I've got has a M66x1 thread on it's backside which can be stepped down to a M48x0,75 thread with a preinstalled 8mm extension tube. Therefore the flattener has 55mm of back focus with and 63mm of back focus without the preinstalled extension tube. At least that's the information I've been supplied with. I will, however, check with my telescope supplier to see if he can confirm these measurements. 

That looks like you have the right info.  55mm from the adaptor/reducer is spot on but, as noted, best to leave spacers a little short and make up the fine tuning with delrins.