aplantic coma corrector Skywatcher 2", F4 aplanatic coma corrector and APS-C sensor

[OJ87]

Hello everyone, 

I recently bought this coma corrector for my Skywatcher Quattro 10S F4 , 1000mm primary for imaging with my unmodified canon 77D (APS-C sensor ) . Unfortunately I couldn't test it till now and I still have two days return guaranty. 

I read in one article that it's made for full-frame sensors , but there is no tests at all with APS C sensor, could someone (who has this CC and tested it on such sensor ) tell me if it would "cut off" a lot of FOV? Or it may face focusing (or any other) problems?

also I want to ask if anyone tried it with light pollution filter, specifically the Hutech IDAS LPS D1, D2 or P2? My question here is: could I attach a 2" or 58mm filter on the telescope side of CC (or between the camera and CC) without facing focus problems?

I know there is a Clipfilter for EOS  cams, which will fit just inside the cam, but then I can't use it with EF-S lenses. 

Best regards

Edited March 11, 2019 by OJ87

[alacant]

5 hours ago, OJ87 said:

could someone (who has this CC and tested it on such sensor ) tell me if it would "cut off" a lot of FOV?

Hi. We use it with aps-c and an ES f3.9. It covers the whole fov but with the small secondary on the quatrro, you'll see a lot of vignetting. HTH,

Edited March 11, 2019 by alacant

[alacant]

5 hours ago, OJ87 said:

could I attach a 2" or 58mm filter on the telescope side of CC (or between the camera and CC) without facing focus problems?

Hi again. No. The filter must go either on the nosepiece of the cc on the telescope side. If you want it between camera and cc you'll need a clip in type.

**You could attach a 48mm filter to the male thread on the camera side of the cc but hey, spacing is critical;)

HTH

[OJ87]

1 hour ago, alacant said:

Hi. We use it with aps-c and an ES f3.9. It covers the whole fov but with the small secondary on the quatrro, you'll see a lot of vignetting. HTH,

Hi alacant, 

thanks for answering my questions. I thought the secondary mirror of Quattro is not that small (82mm), anyway would that vignetting be acceptable/ correctable with flats or would you advise me to buy another CC and return this back?

[alacant]

32 minutes ago, OJ87 said:

vignetting be acceptable/ correctable

Hi. Yes. No problem. I mentioned it only because you may think the vignetting excessive (if you've tried say a slower reflector) and blame it on the cc.

For reference, the secondary fitted to our 8" is 85mm. 

Cheers and HTH.

[OJ87]

thanks., that was a lot of help. 

best regards

[Louis D]

4 hours ago, alacant said:

Hi. Yes. No problem. I mentioned it only because you may think the vignetting excessive (if you've tried say a slower reflector) and blame it on the cc.

For reference, the secondary fitted to our 8" is 85mm. 

Cheers and HTH.

Am I reading that right?  A 42% obstruction by diameter?  WOW! ? I guess compromises must be made for reflector astrophotography.  By comparison, for strictly visual use, I use an 18% obstruction in my 8".

[AKB]

1 hour ago, Louis D said:

A 42% obstruction by diameter?

This really isn't as bad as it sounds.

8.5cm diameter (57 cm^2 area) with 20.0 cm diameter (314 cm^2 area) gives an unobstructed area of 257 cm^2, which is what you would get from a 18.1 cm unobstructed diameter.

Of course, there are other optical factors at play other than simply light-gathering area.

[Louis D]

19 minutes ago, AKB said:

This really isn't as bad as it sounds.

8.5cm diameter (57 cm^2 area) with 20.0 cm diameter (314 cm^2 area) gives an unobstructed area of 257 cm^2, which is what you would get from a 18.1 cm unobstructed diameter.

Of course, there are other optical factors at play other than simply light-gathering area.

It's probably not a big deal on high contrast objects.  It probably wouldn't be a good choice for imaging low contrast objects like Jupiter or Saturn, though.

[alacant]

1 hour ago, Louis D said:

I guess compromises must be made for reflector astrophotography.

It can be quite alarming when you look at the diameter but it's really the area you should be comparing.

What is even more alarming is how much light is lost by bypassing small secondaries altogether. We found out by accident by pointing a reflector we were upgrading at the sun. You could burn paper held at the open end of the tube by the light that was missing the secondary altogether! There's an article on it here.

I don't see why photographic newtonian manufacturers insist on having the focal plane so far out from the tube wall. Having a low profile focuser with a smaller secondary would be far better. Yeah, I know TS do it, but at €silly.

Cheers 

[GTom]

On 11/03/2019 at 07:08, alacant said:

Hi. We use it with aps-c and an ES f3.9. It covers the whole fov but with the small secondary on the quatrro, you'll see a lot of vignetting. HTH,

I was looking in to this, considering picking up a TS Photon 10" piece with an Aplanatic (GPU?) corrector, that one has a large, 88mm secondary. Perfect match so far. However, no free lunch I am afraid: the 2" focuser is too tight in my books. The alternative was the StellaLyra with 3" focuser. But that one has a small, 70mm secondary -> &#@&--bang. 

The theory: the corrector has a 44mm diameter (allow for internal 42?), length is 108mm. The systems backfocus is 55mm, placing the sensor a total of 163mm away from the "42mm neck". If the corrector was a simple 42mm ID tube, it would yield a 42-(163/4) ~=1mm clear, fully illuminated field. Fortunately it is not just a tube, the actual light path within the corrector may help. Still, even 4/3 sensors show some vignetting in their corners, afraid the situation with my 35mm diagonal sensor would be far worse.

Edited August 1, 2023 by GTom

[alacant]

On 01/08/2023 at 12:52, GTom said:

70mm secondary

Hi

I don't think the gso/ts OTA diameter is excessive, so with decent flat frames, you should be OK.

Cheers

[GTom]

Sure, you can "flat out" anything, but what's the point of having an 8" scope if it works like (collects light, resolves) an 6" should?

[Louis D]

3 hours ago, GTom said:

Sure, you can "flat out" anything, but what's the point of having an 8" scope if it works like (collects light, resolves) an 6" should?

It will still have the spatial resolution of an 8" which can be important for fine details.  A central obstruction does not change this.  Think about very long baseline interferometry.  It has a huge central obstruction, and yet achieves very high spatial resolution.

[GTom]

If you do planetary lucky imaging, sure. For DSO-s in real life seeing and guiding will limit your resolution long before the 6" will.