9 replies to this topic

[Tartarrsauce]

I am on the wait list for a Takahashi e-130D. I ordered here in Japan from a company called K-ASTEC and it included tube rings, dovetail, and all the adapters to attach a camera, oag, EFW (see image below). While I wait I'm trying to get all of the accessories (i.e. Filters) ready for when the scope finally arrives.

 

I have, however, ran into some issues when trying to figure out backspacing from the corrector. In the white-papers Takahashi states that the backspacing should be 56.2mm from the corrector. Of course, you have to consider your filter thickness when calculating back spacing. The adapter that ships with the telescope allows me to rotate the camera, EFW, OAG, without moving the focuser itself. However, K-ASTEC, has made the adapter to have 57.2mm -0.2mm/+0.8mm of backspacing. That means they assume that you will be using filters with 3mm of thickness.

 

I'd like to avoid Chroma/Astrodon at the moment and I don't really want to go with Baader (bad experiences with them in the past). I've considered Antlia 50mm (3mm thick) filters, but their QC is not the best and I got burned by them on my last purchase.

 

I really want to go with Astronomik filters, especially their MaxFR filters for fast optics. But the filters are 1.1mm thick, as are all Astronomik filters. 

 

How critical is this backspacing for these scopes? I know most correctors have a tolerance of -1mm/+1mm, but I want to make sure before I actually make a purchase. 

 

 

[Dean]

It's very picky about the backspacing. With the ASI6200, I would recommend getting within at least 0.5mm and even then, being 0.5mm off will have some distortion in the corners - it depends on how much you can tolerate.

Maybe get rid of the K-ASTEC adapter and use a thinner non-roating adapter? The diagram says it is 7mm thick, I'm sure you can get something a few mm thinner. You would still be able to rotate the focuser itself. That's a bit of a pain because if you aren't very careful, rotating an Epsilon focuser can change the tilt and cause issues.

[Tartarrsauce]

It's very picky about the backspacing. With the ASI6200, I would recommend getting within at least 0.5mm and even then, being 0.5mm off will have some distortion in the corners - it depends on how much you can tolerate.

Maybe get rid of the K-ASTEC adapter and use a thinner non-roating adapter? The diagram says it is 7mm thick, I'm sure you can get something a few mm thinner. You would still be able to rotate the focuser itself. That's a bit of a pain because if you aren't very careful, rotating an Epsilon focuser can change the tilt and cause issues.

I’m actually planning on using the ASI2600 so I think I have a little more room for error. 
 

About removing the adapter: The point you made about the focuser is the exact reason the adapter/rotator is included in the first place. I would like to keep it on to avoid introducing any tilt into the system. 

[Dean]

I made a typo, I meant the APS-C sized 2600. I have an E-180 that I use with the QHY equivalent 268. While at f/2.8 it's pickier than the E-130 at f/3.3, the corner stars are pretty bad for me if the BS is off by 0.5mm, so while the E-130 wouldn't be as bad, it still wouldn't be very good. Also you say the adapter thickness is between -0.2mm and 0.8mm - that's quite a bit of variance especially since it's only 7mm thick

[Tartarrsauce]

Also you say the adapter thickness is between -0.2mm and 0.8mm - that's quite a bit of variance especially since it's only 7mm thick

The adapter basically replaces the tilt plate and doubles as both a rotator and a tilt plate for the camera. It comes set at 7.2mm, but can be adjusted to 7mm~8mm with the push pull screws. 

[Dean]

The adapter basically replaces the tilt plate and doubles as both a rotator and a tilt plate for the camera. It comes set at 7.2mm, but can be adjusted to 7mm~8mm with the push pull screws. 

Ah, OK that makes sense.

 

With the Astronomik filters, your going to be off by around 0.7mm. There will be a fair amount of distortion in the corners. You may be fine with it - depends on wow picky you are.

 

I have the Astronomik 36mm filters and generally I like them, especially for the price. I'm not sure what size you are looking at, but 36mm would be the minimum size. They work very well with my other scopes, but on the E-180 I have a few issues that have me thinking of using other filters

 

1) They are mounted so they are a little smaller than 36mm. With my E-180, I get pretty strong vignetting in the corners that calibrates out abut half the time. The reason I think is my QHY filter wheel isn't all that accurate in positioning the filters making flats problematic with the E-180. Your filter wheel may be more accurate and may not have a problem and the E-130 being a little slower will have a little less vignetting. Anyway, if you are going to use 36mm filters, you may want to consider unmounted which would rule out the  Astronomik.

 

2) The aren't all quite parafocal enough for the E-180. For mine, the blue filter is off the most - by 32 microns. That's outside the CFZ of both the E-180 and E-130. I autofocus and use filter offsets, so this isn't really a problem for me.

 

3) I get strong halos around very bight stars with the RGB filters. I don't know if that's the filters or reflection off the corrector. It's only around the very brightest stars so I couls live with it.

 

I'm not that familiar with the other filters, so I don't how they are with being parafocal enough for the E-130 (might be hit and miss) or if they would do any better with the halos (some I have read may be worse). The biggest issue for me is the vignetting which has me considering unmounted filters. If your filter wheel is more accurate than my QHY, you should be fine with mounted 36mm filters.

Edited by Dean, 29 May 2022 - 11:10 AM.

[andysea]

I don’t know what the tolerance is but I can tell you that with the 180, I tried to go a whole mm less than the 56mm spacing and I lost sharpness across the field. I would try to get within .1mm at least.
Strangely the manuals that came with my 130 and 180 say 56mm and not 56.2.

There is another piece to the puzzle: some camera manufacturers specify a back focus +\- 0.5mm. That is a 1mm tolerance that can definitely throw things off.

[Dean]

There is another piece to the puzzle: some camera manufacturers specify a back focus +\- 0.5mm. That is a 1mm tolerance that can definitely throw things off.

Yep. The more I think about it, I wouldn''t go with the   Astronomik filters & that rotating adapter. What I would do is replace the focuser with an Optec Leo and ditch the rotaing adapter. Then go with the Astronomik filters. The savings by going with the Astrnomik instead of Chroma filters would help effect the cost of the Leo. The Leo would allow rotation and get rid of the tilt issue. On top of that, you would be able to auto focus too, which IMO is a big plus.

[Tartarrsauce]

Strangely the manuals that came with my 130 and 180 say 56mm and not 56.2.

That is very strange indeed. On Takahashi's website, under telescope specifications, the metal back distance for all three Epsilon scopes is listed as 56.2mm. 

 

What I would do is replace the focuser with an Optec Leo and ditch the rotaing adapter.

As appealing as the Optec Leo is, there are no sellers in Japan and I really hate dealing with importing products when I don't have to. For the price of importing the Optec Leo I could just buy Chroma filters, so it doesn't really save me any money. 

[Dean]

As appealing as the Optec Leo is, there are no sellers in Japan and I really hate dealing with importing products when I don't have to. For the price of importing the Optec Leo I could just buy Chroma filters, so it doesn't really save me any money. 

I understand the import situation. It's really bad if you need it serviced and have to ship it overseas.

 

I think you would be disappointed with the Astronomik filters using that rotating adapter. With 3mm filters, you have worry about the accuracy of the specs for the camera, filter wheel, OAG and the adapter since you have no room to spare.

 

Maybe you could find a replacement focuser that has a lower profile than the stock one with a seller in Japan. Or maybe one that is less expensive than the Leo where you would consider importing, like a Moonlite perhaps. That would be less than half the cost of the Leo. I don't know if they have anything that has a lower profile than the stock focuser though.

Edited by Dean, 29 May 2022 - 11:08 PM.