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Cannot reach focus with Newtonian F/5 150mm with filter wheel on Venus

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#1 astroflak88

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Posted 10 June 2021 - 11:58 PM

Hi all,

 

I recently picked up a 150mm F/5 newtonian and my objective was to image venus with my filter wheels using the Uvenus, R, G, B and L filters with my ASI174mm. This will keep me busy until I get my 16" F/5 in October 2021, which I ordered back in March. I used an equatorial mount with tracking for the reflector and was able to lock in on Venus. I installed the filter wheel and I had the focuser all the way in and it still can't focus on venus, i guess the filter wheel needs to be shorter but I have no way to do that since it's fixed. Is there a device I can buy to install along the light path to make the light path shorter? I need to image with this filter wheel because its much easier, if i do it manually one by one and un-screwing and screwing in the filters individually each time, it will really take out the center focus of venus because of all the fidgeting and movement caused by me removing the eyepieces and accessories to screw in the filters taking the telescope off center view of the planet. Anyone have any recommendations? Will i be having this same problem with the 16" F/5 since both have the same F ratio?


Edited by astroflak88, 10 June 2021 - 11:59 PM.


#2 Bill Weir

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Posted 11 June 2021 - 12:29 AM

So can you reach focus with just the camera? I’m guessing not if it’s a normally configured newt. There is not enough out-focus. The filter will will complicate things even more.

 

Bill



#3 astroflak88

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Posted 11 June 2021 - 12:50 AM

yes I can reach focus with just the camera but not being able to use the filter wheel will defeat the purpose of me imaging. So I am thinking will this be the same issue when I get my 16" F/5 Goto in October ?



#4 Bill Weir

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Posted 11 June 2021 - 02:38 AM

Have you tried adding a Barlow to the front end?

 

Bill


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#5 ABQJeff

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Posted 11 June 2021 - 02:51 AM

For newts and dobs, because of limited focus travel, many will do a filter slider inside the tube.

 

See this thread:  https://www.cloudyni...on-tubed-scope/

 

Or in this case outside the tube:

 

https://www.cloudyni...-filter-slides/

 

Astrocrumb and Moonlite make filter sliders tailored for reflector tubes.



#6 happylimpet

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Posted 11 June 2021 - 03:08 AM

Have you tried adding a Barlow to the front end?

 

Bill

What I was going to suggest. Typically you'll want a barlow anyway to get a sensible image scale, so if you put that at the start of the camera train it should extend focus significantly, and enable focus. Fingers crossed.....



#7 kathyastro

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Posted 11 June 2021 - 05:40 AM

It is not the focal ratio.  It is the reach of the focuser.  You need to ensure that the focuser on your new scope gives you more back focus than the one you currently have.  Back focus is not dependent on focal ratio.  It is determined by the length of the tube and the length of the focuser.


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#8 iantaylor2uk

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Posted 11 June 2021 - 05:57 AM

I think it is more likely an in-focus issue (i.e you can’t move the focusser in enough). You could try moving the mirror up the tube by a few mm by using the collimation screws.


Edited by iantaylor2uk, 11 June 2021 - 05:57 AM.

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#9 sixela

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Posted 11 June 2021 - 09:21 AM

A barlow will be needed, so placing it in front of the filter wheel as a nosepiece that slides far into the focuser will probably fix this (the question is how much distance and thus barlow factor will be needed to get in focus, and hopefully the barlow won't intrude in the light bundles.)


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#10 astroflak88

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Posted 11 June 2021 - 09:52 AM

Thanks for all your suggestions I looked at the slider filter and looks like alot of modifying especially the external version. I'll try the Barlow in front of filter wheel tonight.

#11 astroflak88

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Posted 11 June 2021 - 12:44 PM

I think it is more likely an in-focus issue (i.e you can’t move the focusser in enough). You could try moving the mirror up the tube by a few mm by using the collimation screws.

Yes i can't in-focus it enough to reach focus w/o removing the filter wheel. 



#12 Mirzam

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Posted 11 June 2021 - 05:54 PM

 You might be able to get a lower profile (shorter) focuser.  Or maybe the Barlow lens will give you a bit more backfocus.  However, focal plane position is a basic design parameter of all Newtonian telescopes.  The secondary is typically minimized in size to reduce obstruction while bringing the focal plane to an accessible position.  Imaging Newtonians are designed with larger secondaries necessary to provide a wider illuminated field and more backfocus.  It may be possible to add some spacers, perhaps while using longer collimating bolts and springs to push the primary cell and the focal plane forward a bit.  But then you run the risk of having an undersized secondary.

 

JimC



#13 sixela

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Posted 12 June 2021 - 04:17 AM

Just for clarity: don’t use a barlow with eyepiece holder (these eyepiece holders that do not insert into the focuser serve to _ avoid_ having to make the focuser travel out when you insert the barlow) but screw a barlow element in front of the nose piece to your filter wheel.

#14 bokemon

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Posted 12 June 2021 - 04:56 AM

What thread-in barlow do you guys suggest?



#15 astroflak88

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Posted 12 June 2021 - 10:26 AM

I put an RMA on my newtonian and returning it. It's too much of a hassle. Will I be having this same problem with a 16" goto f/4.4 dobsonian?

#16 astroflak88

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Posted 12 June 2021 - 10:31 AM

Just for clarity: don’t use a barlow with eyepiece holder (these eyepiece holders that do not insert into the focuser serve to _ avoid_ having to make the focuser travel out when you insert the barlow) but screw a barlow element in front of the nose piece to your filter wheel.


I have powermates Barlow lenses which I think are different in design than your standard Barlow. So I think I just can't unscrew the powermates to install on the nosepiece? There's more than one element to the powermate when you unscrew them

#17 sixela

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Posted 12 June 2021 - 03:05 PM

Yup — those are less effective.
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#18 kathyastro

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Posted 12 June 2021 - 03:13 PM

I put an RMA on my newtonian and returning it. It's too much of a hassle. Will I be having this same problem with a 16" goto f/4.4 dobsonian?

Almost certainly, yes.  The focusers on Dobs and most other Newtonians are designed to accommodate a typical range of eyepieces.  They are not designed to accommodate any other equipment.  You could look for an astrograph Newtonian, which is designed with more back focus, but it would not come with a Dob mount.


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#19 belliott4488

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Posted 13 June 2021 - 06:45 PM

You don't happen to have a coma corrector, do you? They often have a small amount of magnification, which gives you a little extra back-focus. 

 

I can't focus my DSLR if I mount it directly to my 8" F3.9 Newt - not enough in-travel on the focuser - but when I put my CC in the train it works fine.


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#20 astroflak88

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Posted 13 June 2021 - 10:01 PM

Do you guys think that I should ditch the 16" goto dob and just get a c14 edge for planetary imaging? I already have the equatorial mount (Losmandy G11) and the necessary counterweights to support it.


Edited by astroflak88, 13 June 2021 - 10:01 PM.


#21 bokemon

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Posted 13 June 2021 - 10:02 PM

No, it's too heavy for your G11



#22 Bill Weir

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Posted 13 June 2021 - 10:04 PM

Yes.

 

Bill



#23 sixela

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Posted 14 June 2021 - 02:37 AM

Ditching the 16” Dob for this reason is like changing cars because the ashtray is full, frankly.

Since you need image scale you’ll need a barlow element, and that will probably fix your problem. That you current focal extender won’t work is a detail.

Trying it will be ‘slightly’ simpler than learning to use a c14. Heck, even installing a filter slide will.

Take a look at https://www.astrokraai.nl/ . He’s used a 16” Dob with a black and white camera and rGB filters for all his recent images except the ones on 1M telescopes.

Edited by sixela, 14 June 2021 - 02:42 AM.


#24 astroflak88

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Posted 14 June 2021 - 04:22 PM

No, it's too heavy for your G11


I saw multiple imagers using a c14 on a G11

#25 astroflak88

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Posted 14 June 2021 - 04:22 PM

Ditching the 16” Dob for this reason is like changing cars because the ashtray is full, frankly.

Thanks for replying I'll look at that website

Since you need image scale you’ll need a barlow element, and that will probably fix your problem. That you current focal extender won’t work is a detail.

Trying it will be ‘slightly’ simpler than learning to use a c14. Heck, even installing a filter slide will.

Take a look at https://www.astrokraai.nl/ . He’s used a 16” Dob with a black and white camera and rGB filters for all his recent images except the ones on 1M telescopes.


Thanks I'll look into that website you posted

Edited by astroflak88, 14 June 2021 - 04:23 PM.



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