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Novice Challenge

My EAA Adventure continues...


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Hi All, My EAA adventure continues...
I had a short window last night so got my scope out and did a bit of live stacking using my 0.63x focal reducer for the first time.
The reducer was attached directly to the scope back, my 2" mirror diagonal screwed directly to the reducer, and my camera fitted directly to the diagonal (ie. minimum light path)
Immediately it was obvious that the reduction was way more than 0.63x, which was subsequently confirmed by checking some of the stacked images on astrometry.net. The actual reduction turns out to be 0.415x.
My preference was to use the diagonal to ensure there is no risk of the camera impinging on the mount at high elevations, however it is clear this is not an option...
I brought the scope in, removed the diagonal, and fitted some m42 spacers to try and get closer to the 105mm back focus specified by Celestron, but by then the clouds had arrived...
so I have been unable to try again to take some frames and calculate the corrected focal length, and by trial and error get close to the specified 0.63x reduction. 
I assume that this back focus error created the much larger field of view and distorted star shapes (look like hearts - at least to my eyes). 
I am looking forward to the next clear night and next trial!
I attach a few of the nights images for info.

M51 Stack_96frames_810s_WithDisplayStretch.png

M51 Stack_101frames_853s_WithDisplayStretch.png

M81 Stack_41frames_328s_WithDisplayStretch.png

NGC 4565 Stack_55frames_439s_WithDisplayStretch.png

NGC2392 Stack_52frames_462s_WithDisplayStretch.png

NGC2392 Stack_59frames_524s_WithDisplayStretch.png

M1 Stack_173frames_1473s_WithDisplayStretch.png

M42 Stack_9frames_75s_WithDisplayStretch.png

M42 Stack_18frames_150s_WithDisplayStretch.png

M44 Stack_11frames_98s_WithDisplayStretch.png

M44 Stack_39frames_346s_WithDisplayStretch.png

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It is important to get the back focus distance correct with Reducers / Barlows / Field Flatteners and putting a 2" diagonal in the way will prevent this, and it adds unnecessary glass to the light path. I'd recommend using spacer rings and aiming to get the back focus specified by the manufacturer. With my x0.6 Reducer the back focus needs to be correct to within 1mm or I get distorted stars around the edges. With Barlows I always measure the actual magnification as it is usually different from what is specified (my Baader Classic Q is specified at x1.3 without its extension tube but I actually get x1.7 when I use it with my camera).

Your live stack snapshots look great by the way. Your camera has the same sensor as mine (IMX585) and with the Reducer giving x0.415 you have an effective focal length of 975mm (2350mm x 0.415) which is about the same as I get with my Explorer 150 PDS (actually 750mm). The snapshots you posted are at least as good as those I'm getting, with the exception of M42 which is not well positioned at the moment. Next year I'm sure you'll be able to see M42 in all its glory using EAA, and it is truly glorious!

It would be good to know how many frames you stacked when you post snapshots as this makes a big difference. Here is M51 from the 3rd April.

M51_Clear_4.0s_x400_247frames_D03_04_2023_T22_39_03_WithDisplayStretch_Cropped.thumb.png.4fb3b3d937982808da1cc3a3d143c2c8.png

I was using the 72mm F6.0 Apo so less magnification than you and less light grab, but this is 247 frames at 4s exposure and x400 gain, with no filter. This is just the snapshot from the live stack on the night. I don't normally wait for this many frames but I had to download a new version of Stellarium and so just let the frames stack up.

 

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Thanks Peter and Jonathan, sorry I didn't post the individual image exposures, however they were all taken around the Sharpcap recommendations for optimum exposure (smart histogram) of around 295 gain, between 5 - 8 second exposure time and between 5 and 80 stacked images.

I have already (mechanically with spacers) set up the back focus to approx 105mm - however it looks like a few days before the weather is likely to co-operate!  I will post an update once I have made some progress!

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2 hours ago, PeterC65 said:

Your camera has the same sensor as mine (IMX585) and with the Reducer giving x0.415 you have an effective focal length of 975mm

Peter how does a 0.63x reducer give 965mm focal length? I have always worked it out as 2350 x 0.63 = 1480mm. Am I missing something?

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28 minutes ago, bosun21 said:

Peter how does a 0.63x reducer give 965mm focal length? I have always worked it out as 2350 x 0.63 = 1480mm. Am I missing something?

Ian said he was getting x0.415 from his reducer as he had a 2" diagonal between it and the camera, and 2350 x 0.415 = 965mm. The images look about right for that focal length too.

 

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1 hour ago, PeterC65 said:

Ian said he was getting x0.415 from his reducer as he had a 2" diagonal between it and the camera, and 2350 x 0.415 = 965mm. The images look about right for that focal length too.

 

My apologies Peter, I must have not read back far enough. I need to stop that 😂

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