16 replies to this topic

[rsliva]

I recently acquired a used C11 SCT after doing astrophotography with a ES ED127, an Orion 60mm f/4 guide scope, an ES field flattener, and a Nikon D750 camera. I like to shoot galaxies, and I thought the longer focal length would be useful for smaller galaxies. However the coma on the SCT is ridiculous (can’t afford an EdgeHD). The scope came with the Celestron 6.3 reducer but the images were only marginally improved with the reducer. The ES field flattener is for f/5 to f/7 short refractors, but I figured what the hay. I added that to my train right after the 6.3 reducer, and ASTAP showed my new fl was f/4.9. My images are much improved with reduced coma. Guiding is also improved since I don’t use OAG (although I keep the exposures at 180s or less). Attached is a photo of M101 done at f/4.9. The odd star shapes on brighter stars shows up whenever the 6.3 reducer is in the train. This is 70x60s at ISO 200, stacked with Siril and post-processed with Pixelmator Pro.

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[Tapio]

Interesting combination.
Is this a full frame image?

[hyiger]

Cool. I've been thinking of something like that for my C11. I was shooting at f/7 and would definitely like to go faster if I can with taking the Hyperstar route. 

[MarMax]

I recently acquired a used C11 SCT after doing astrophotography with a ES ED127, an Orion 60mm f/4 guide scope, an ES field flattener, and a Nikon D750 camera. I like to shoot galaxies, and I thought the longer focal length would be useful for smaller galaxies. However the coma on the SCT is ridiculous (can’t afford an EdgeHD). The scope came with the Celestron 6.3 reducer but the images were only marginally improved with the reducer. The ES field flattener is for f/5 to f/7 short refractors, but I figured what the hay. I added that to my train right after the 6.3 reducer, and ASTAP showed my new fl was f/4.9. My images are much improved with reduced coma. Guiding is also improved since I don’t use OAG (although I keep the exposures at 180s or less). Attached is a photo of M101 done at f/4.9. The odd star shapes on brighter stars shows up whenever the 6.3 reducer is in the train. This is 70x60s at ISO 200, stacked with Siril and post-processed with Pixelmator Pro.

Thanks for sharing this information. It's interesting you are getting a 0.78x reduction using only a "flattener" with the Celestron f/6.3 reducer. Are you sure it's not a 0.8x reducer/flattener? It's possible to get to f/4.9 with just a flattener if you are using 140mm +- of backspacing.

[Oddyse]

  Galaxy/nebulae itself is exceptional. 

[randcpoll]

How interesting, and great photo! I have the same question as MarMax above though.

Edited by randcpoll, 22 March 2023 - 02:32 PM.

[rsliva]

Interesting combination.
Is this a full frame image?

This image has been cropped and scaled. I'm having some networking issues, but when I can I'll post a raw stacked image to get an idea of what the actual scale is.

[rsliva]

Thanks for sharing this information. It's interesting you are getting a 0.78x reduction using only a "flattener" with the Celestron f/6.3 reducer. Are you sure it's not a 0.8x reducer/flattener? It's possible to get to f/4.9 with just a flattener if you are using 140mm +- of backspacing.

I'm using the Explore Scientific Field Flattener (https://explorescientificusa.com/products/explore-scientific-field-flattener-f-5-to-f-7) which according to their specs "Causes no change to the telescope focal length". The flattener is about 85mm, so with the 55mm to the D750 sensor, that's 140mm from the end of the 6.3 reducer.

[randcpoll]

Great stuff! I'll have to try it on my C8 since I have a flattener.

[rsliva]

Here's a minimally processed version of the stacked image, uncropped and at full scale, to get an idea of the FOV of the scope at f/4.9.

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[MarMax]

Since I'm sure there are others interested in exploring what you have done it's important to have some very specific information. I don't have the ES field flattener but I'd like to update my C11 notes with your findings.

 

If you don't mind, a picture would be very helpful. It will show the rear cell an how you are attaching the f/6.3 reducer. Also the specifics of the SCT to T2 or DSLR adapter you are using.

 

And here is a handy calculator for SCT's and reducers. This is what I used to come up with the 140mm.

[rsliva]

Since I'm sure there are others interested in exploring what you have done it's important to have some very specific information. I don't have the ES field flattener but I'd like to update my C11 notes with your findings.

If you don't mind, a picture would be very helpful. It will show the rear cell an how you are attaching the f/6.3 reducer. Also the specifics of the SCT to T2 or DSLR adapter you are using.

And here is a handy calculator for SCT's and reducers. This is what I used to come up with the 140mm.

[rsliva]

Attached is a photo of the train starting with the 3-2 inch reducer plate, the Celestron 6.3 reducer, a TPO 2 inch SCT adapter, the ES flattener, Nikon T-ring, and camera. The TPO adapter is usually used for a 2 inch diagonal or eyepiece, but is need for the flattener which has no threads on the forward side.

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[MarMax]

Thank you for the photo. Now for more questions . . . lol.

 

What is the length of the TPO 2" SCT adapter and what is the length of the 2" nose of the ES field flattener. Also, how far is the first lens from the nose of the ES field flattener. And what is the distance from the rear mounting shoulder of the ES field flattener to the camera sensor.

 

Thanks in advance.

[rsliva]

Thank you for the photo. Now for more questions . . . lol.

 

What is the length of the TPO 2" SCT adapter and what is the length of the 2" nose of the ES field flattener. Also, how far is the first lens from the nose of the ES field flattener. And what is the distance from the rear mounting shoulder of the ES field flattener to the camera sensor.

 

Thanks in advance.

These are approximate. No worries on the questions.

SCT Adapter: 47mm

Flattener nose: 34mm

First lens to end of nose: 66mm

Flattener shoulder to sensor: 55mm

[Tapio]

Maybe there's more reduction now because there's more distance now from f/6.3 reducer to camera chip (than the nominal 105mm) ?

If I'm calculating right.

[MarMax]

I'm thinking you are even past 140mm of backfocus.

 

 

Assuming you can somewhat scale off the image. If the TPO 2" SCT adapter is 47mm then the flattener is about the same and the adapter is about 16mm. The flange focal distance for the D750 is 42.5mm so this adds up to a bit more than 150mm.

 

What you really need to know if the optical path of the flattener because it may be different than the physical path. Since you've plate solved to determine f/4.9 we'll just have to assume you are somewhere between 140-150mm which somewhat jives with the calculator.

 

Still a very interesting discovery.