Author Topic: Reducing focal plane by moving primary mirror back?  (Read 198 times)

Nathan Sorgaard

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Reducing focal plane by moving primary mirror back?
« on: December 28, 2017, 11:48:51 PM »
Hi all question i have a 8in dob a xt8 i picked up a nice cradle mount for it i just installed antares 1/30th wave secondary i want to get as close to the secondary with the eyepiece as i can right now it focuses about 12inch from the secondary to the eyepiece with a 9mm nagler t6 now if i move the primary mirror back is every inch i move the primary back going to reduce the focal plane by the same amount?

Thanks everyone 😊



Larry Hopkins

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Re: Reducing focal plane by moving primary mirror back?
« Reply #1 on: January 08, 2018, 01:44:35 AM »
That is correct. The focal length of the primary does not change so you have to maintain the same total distance to the image. Move the mirror back, lower the focal position.

dan

jumphindnore

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Re: Reducing focal plane by moving primary mirror back?
« Reply #2 on: January 09, 2018, 07:49:21 PM »
You have to watch that the focuser tube, when your eyepieces are in focus, does not go down so low as to enter the field of view. Many focuser tubes are long and will do so. The added diffraction and to an extent blockage of light can be significant.

safrioheartli

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Re: Reducing focal plane by moving primary mirror back?
« Reply #3 on: January 10, 2018, 03:07:02 AM »
Ok cool thanks I'm probably going to get a 2 speed moonlight for it at some point i just want to get it closer than it is right now its way out there lol the way they designed these scopes there's really no way to move the mirror back or move the spider and secondary mirror and focuser forward so I'm going to get a little ingenious I have another 8inch tube it's approx 3/8" smaller diameter I am going to hack a foot of the tube off the mirror cell end then I am going to put felt on the outside of the tube to use as a shim then I can simply Slide the tube back and forth inside the other tube to get it where I want it does that sound doable?

Thanks Stan

falkwinsliche

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Re: Reducing focal plane by moving primary mirror back?
« Reply #4 on: January 11, 2018, 03:18:40 AM »
It sounds like it would work for adjusting the position of the focal plane.

You would want a way to lock it in place somehow or the collimation of the scope may shift if the tube with the mirror is not held tightly.

Just having the felt as a support might give you problems maintaining collimation of the optics.

It isn't an easy thing to change the way they are built. I definitely agree with that!

Ken Kamkoff

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Re: Reducing focal plane by moving primary mirror back?
« Reply #5 on: January 12, 2018, 08:26:23 AM »
That’s doable. I made a ‘telescoping’ OTA for a 6” using two carefully sized tubes -- slide apart to working length and slide together for compact storage/travel. Your shims should not be soft. Cork might be better than felt. You want to keep the two tubes rigidly aligned for the sake of holding collimation. If you screw the two tubes together, you don’t need much overlap, but I’d suggest about 4" or 5” overlap in any case to give you plenty of fiddle room. Make sure you don’t set the focal plane so low that you don’t have enough available in-travel on your focuser to accommodate all eyepieces and a barlow.

Gregory Plummer

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Re: Reducing focal plane by moving primary mirror back?
« Reply #6 on: January 12, 2018, 08:29:39 PM »
Maybe once i get it where i want it i can drill holes in the outside tube and use some teflon screws to sock it down i was going to extend the other end of the tube with the spider and secondary and focuser but thought that's where I'm going to be grabing the tube to move it while I'm veiwing right?

Thanks Stan

Christian Shim

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Re: Reducing focal plane by moving primary mirror back?
« Reply #7 on: January 16, 2018, 02:53:20 AM »
Ok yeah it's definitely been a challenge but starting to pay off last night i got a few peeks at Jupiter with the new 1/30th wave secondary definitely made a difference sharpened it right up
Thanks Stan

John Trujillo

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Re: Reducing focal plane by moving primary mirror back?
« Reply #8 on: January 17, 2018, 01:21:50 AM »
Quote
Maybe once i get it where i want it i can drill holes in the outside tube and use some teflon screws to sock it down i was going to extend the other end of the tube with the spider and secondary and focuser but thought that's where I'm going to be grabing the tube to move it while I'm veiwing right?

Thanks Stan


Modifying the primary end is less work -- just one component to move and a few new holes for screws to hold the tube extension in place. Also, most commercial OTA’s don’t have much available room ahead of the spider (they’d benefit from longer tubes to act a dew/light shields but that’s a different project).
You could use screws just to apply pressure to your sliding extension, but the sheet metal of the tube isn’t thick enough to thread. You’d need to make a collar or braze/weld posts for threading. Much easier would be to just drill straight through both tubes and use screws/nuts to keep the assembly together.

Here are two pics of the 6” f/5 I made with nesting phenolic tubes: in use and "packed" The friction fit is tight -- no screws needed. But if I didn’t want to preserve the slide-to feature, I’d screw it down and be done with it.

Jeremy Gambel

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Re: Reducing focal plane by moving primary mirror back?
« Reply #9 on: January 21, 2018, 01:19:56 AM »
Nice my sts10" teeter is going to be completed soon it's going to be nice this one here is just one of my tinker projects but the veiws trough it last night were very promising i want to get closer to the secondary mirror brighter on DSOs

exmartata

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Re: Reducing focal plane by moving primary mirror back?
« Reply #10 on: January 21, 2018, 02:08:33 AM »
Well i cut the tube this evening going to attach it to the tube tomorrow i might use two zip ties to hold the ext tube in place until I get the focal plane where I want it to be

tenpaseper

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Re: Reducing focal plane by moving primary mirror back?
« Reply #11 on: January 21, 2018, 04:31:40 AM »
As long as the current positioning of the secondary is large enough to provide a reasonable size fully illuminated field, I am not sure what this effort will accomplish. The usual motivation for a lower profile focuser and moving the primary is to allow the use of a smaller secondary but you already have the secondary. I this situation, the gain is in the size of the fully illuminated field, a concern for deep sky but not for viewing the planets as long as the secondary is not currently too small.

Jon

Joe Mallard

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Re: Reducing focal plane by moving primary mirror back?
« Reply #12 on: January 21, 2018, 06:21:23 AM »
Does getting the focal plane closer to the secondary mirror increase illumination? Right now I have a 1.83 inch secondary mirror stock was 2"

cardcudeflee

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Re: Reducing focal plane by moving primary mirror back?
« Reply #13 on: January 23, 2018, 01:31:20 AM »
Quote
Does getting the focal plane closer to the secondary mirror increase illumination? Right now I have a 1.83 inch secondary mirror stock was 2"


Moving the focal plane closer to the secondary mirror increases the diameter of the fully illuminated field of view.

Inside that field of view, the entire primary mirror is seen at the focal plane, outside that field the entire primary is not visible and the illumination begins to drop off.

This is primarily a concern for deep space observation since that is where one uses wide fields of view. Typically Newtonians are setup with 0.3-0.5 magnitudes drop off at the edge of the widest possible field of view, a 31 Nagler or 41 Panoptic.

Planetary observers typically sacrifice the diameter of the fully illuminated field because they are operating at high magnifications and the smaller illuminated field allows for a smaller CO.

But the visual effect of the smaller CO even at the widest fields of view is small so there's not much reason to reengineer the scope for that and for planetary, since you are using the same diagonal, there is little reason to make the effort, the CO does not change.

Jon

James Bradmon

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Re: Reducing focal plane by moving primary mirror back?
« Reply #14 on: January 23, 2018, 06:33:15 AM »
Ok cool thanks i moved the primary Back 2 and 1/4 inches I couldn't see the whole primary in the secondary at the focal plane now I can so what I did was necessary right?