80mm vs 250mm

[Thefatkitty]

Friday night (Oct 4) was fairly clear here. Also the temps are dropping, which makes being outside comfortable. Nice to finally wear track pants and a t-shirt without sweating into my eyepieces and getting eaten by skeeters

I went outside at 8:30PM, dressed in the above, with the temperature a balmy 54F. Nice. I could take a whole summer of that. I brought out my 80mm on its CG4, 10" dob, and a little Towa 3" I picked up some time ago. The Towa was amazing for what it is. I put a 1.25 focuser on it, and with my Fujiyama KK's the view was just stunning, albeit a bit small.

I did a tour of the eastern sky with all three scopes as the Moon moved out from behind the tree to the south. At about 10PM and 50F, I tried for some pics of the rock with my 80mm. Those came out pretty good, then I looked over at my 10". I wonder....
Well of course it won't come to focus with my DSLR, at least not with the 2" extension tube and the 2" to 1.25 adapter on top of that. However, I found that if I took the extension tube and adapter out, I could just lay the camera on the focuser (holding onto it, of course), line up the camera with the secondary, and by turning the focuser out a bit, the Moon snapped into focus.
I should probably take pics to better explain what I mean; if you want I can provide some

Anyways, I took about 20 pics like that. I didn't have the remote shutter connected to the camera at that point, which would really have been the idea. And holding the camera still wasn't easy as well, it kept sliding around. By now the temp was 48F, and I was thinking it might be time to swap the T for a sweater. Ah, it was damp out as well, and at midnight it started raining.

So here is my point. I thought the pics with the 10" would be a lot larger than the ones in my 80mm. But I forgot about focal ratio's. The 80mm is f/11.4, whereas the 10" is f/4.9.
I used the same processing settings for both in PIPP, making the output frame size to be 2400 X 2400.

This is the 80mm:

80th_ImPPG.png

And this is the 10":

10.png

Not a huge difference, size wise, and the frac has more detail, but the camera was solidly mounted to it. Not so the 10"....

I did the math, and I'd have to cut 2 inches off the back of the 10" to move the mirror up for DSLR focus. Eyepiece focus wouldn't be an issue as I have another 2" extension tube. The scope is a bit of a rat as well, so I'm certainly not worried about looks.
Those of you on here that know me well also know I'd have no issues about doing this either

The question is: Should I? It would take about an hour to do, and I have everything I need waiting to go. But I'm wondering if I'm overlooking anything regarding using it after for visual. Would an extra 2 inches on the focuser narrow the field of view dramatically? I don't need it to be as narrow as my Towa 3"...

Any advice would be greatly appreciated!


All the best,

[Juno16]

Hi Mark,

No idea what you should do with the 10”, but the image from the 80mm sure looks nice.

I love your temps! We had a ‘cool’ 70 degrees yesterday morning that felt good.

Thanks,
Jim

[bladekeeper]

The only thing that comes to my mind would be if moving the primary closer by 2" would make the light cone a bit too wide for the secondary, causing a loss of light.

Not sure that would be the case, but might be something to consider. Not even sure how one could determine that. It may be a non-issue.

[yobbo89]

You'll need a bigger secondary.

I would say there are other methodes if you are a tad away from focus you can do without modifying the ota,

Get a low profile focuser, buy long collimation bolts for the primary, use mirror less dslr or mini c mount style cmos camera.

[JayTee]

Here, give this secondary mirror size calculator a shot and see what you can come up with. Personally, if you have the tools and the gumption make the mod.
http://www.bbastrodesigns.com/diagonal.htm

Cheers,
JT

[redhat]

Hi Thefatkitty,

Check this one out:

https://youtu.be/9d0292TBMHo?t=1211

[Don Quixote]

Cool comparison, Mark.
I am glad you could get out there and enjoy the sky !

[Bigzmey]

NIce shot from 80mm Mark! I would investigate the issue more before taking your 10" to the saw. I feel you will introduce undesired vignetting and loss of light by messing with light train this way. Low profile focuser is better way to go.

[SkyHiker]

If that 10" does not track, the only thing you can image is the Moon. So why not use a 2x Barlow and stitch with ICE? That will save you the trouble of cutting off a piece of tube for something that you may only do a few times.

Otherwise, how about replacing the screws with longer ones? I did that and was able to get focus with my mirror-less DSLR. Not sure how much you need.